Urban Ecology

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Garden Plants Gone Wild: The Periwinkles

In a garden setting, a successful groundcover is a plant that is durable and adaptable, spreads readily, and fills in space thouroughly. The point of planting a groundcover is to cover exposed soil and create a sort of living mulch. In fact, groundcovers provide similar benefits to mulch. They prevent erosion, help retain soil moisture, and prevent weeds. It should come as no surprise then, that a plant that fulfills all of these requirements has the potential to become a weed, especially if given the opportunity to escape and establish itself outside of its intended location.

This isn’t a hypothetical. This exact scenario has played out numerous times. A good example of this are the periwinkles: Vinca major and Vinca minor. Both have been popular garden plants for centuries. Their introduction to U.S. gardens came as early as the 1700’s. Today, both species (including cultivars of each) can be found for sale in nurseries throughout the country, even while escaped periwinkles proceed to spread across natural areas and uncultivated spaces. Even in gardens where periwinkles have been intentionally planted, they can be deemed no longer welcome due to their aggressive nature. Eliminating them, however, is a formidable task.

greater periwinkle (Vinca major)

Periwinkles are relatively easy to identify, yet telling the two apart can sometimes pose a challenge. Knowing what to look for can make this a fairly simple task. Vinca major (greater periwinkle) is the larger of the two. Its stems are tough and sprawl up to 5 feet long, rooting adventitiously when in contact with the ground. Vegetative spread also occurs via stolons and rhizomes, modified stems that spread horizontally both above and below ground and whose main purpose is to produce new roots along their length. This extensive rooting creates dense mats of stems and foliage, precisely what you’d want from an effective groundcover. Leaves are arranged oppositely and are semi-evergreen with fine hairs along their margins. They are thick, glossy, simple, dark green, and ovate to ovate-lanceolate in shape. They have a relatively long petiole, and some leaves can even appear heart-shaped.

The flowers of V. major are blue to purple and fused at the base to form a tube, separating into five distinct lobes and creating a shape similar to a pinwheel. They are borne on a long stalk in the axils of leaves and measure about two inches wide. Their sepals are long, slender, pointed, and lined with bristly hairs. Fruits rarely form, but when they do, they are narrow follicles.

comparing the petals and sepals of Vinca major (left) and Vinca minor (right)

Vinca minor (lesser periwinkle) is a slightly smaller plant with a similar habit, spreading vegetatively in all the same ways as V. major. Stems are slender and smooth and leaves are evergreen. Compared to V. major, the leaves of V. minor are smaller, narrower, and have hairless margins and short petioles, otherwise they are very similar. Same goes for the flowers, which look identical on both species, except that those of V. minor are slightly smaller (about one inch wide) and borne on shorter stalks. Sepals are shorter, broader, more rounded at the tips, and lack the bristly hairs of V. major.

Because periwinkles only rarely produce seed, their main method of getting around is vegetatively. Fragments of roots or rhizomes hide in soil and are moved from one location to another inadvertently. Periwinkles are often used in hanging baskets and containers, and when these things are cast aside at the end of a season, the perennial roots of periwinkles may continue to grow, spreading out beyond the potting mix and into the soil.

Dump soil, yard waste, and improperly disposed of containers are the main ways that periwinkles find their way into natural areas. Both species can be found in the understories and edge habitats of woodlands, as well as along roadsides and pathways, and in vacant lots and old homesites. They can also be found in riparian areas, where waterways can carry fragments of plants to new locations. The Invasive Plant Atlas compiles reports of both V. major and V. minor growing outside of cultivation and tracks them on a map. They also track which states include them in noxious weeds lists or laws.

lesser periwinkle (Vinca minor)

The best way to keep periwinkles from continuing to spread outside of cultivation is to refrain from growing them. If you choose to have them in your yard, dispose of plant parts properly. If you keep them in containers, send those containers to the landfill when you are done with them. If your property is adjacent to natural areas, the risk may be too great and you may want to consider a different groundcover. Depending on where you live, alternatives vary. In the Intermountain West, potential substitutes include wild strawberry (Fragaria virginiana), woodland strawberry (F. vesca), kinnikinnik (Arctostaphylos uva-ursi), and wild ginger (Asarum caudatum). Each of these are low growing, evergreen to semi-evergreen, spreading plants that do well in shade and can handle some degree of drought.

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Horticulture’s Weedy Introductions in a Changing Climate

In case I need a reminder that the horticulture industry has a history of introducing weedy plants to natural areas, I get one each time I bike to work. Riding along the Boise River Greenbelt, a trail that for much of its length is flanked by cultivated landscapes on one side and a highly modified but largely naturalized river bank on the other, I see a mixture of both native and introduced plants. Of the introduced plants, many are horticultural species that have escaped cultivation and established themselves on the bank of the river. There are catalpa and black locust trees brought in from the other side of the country, St. John’s wort and chicory from Eurasia, honeysuckles primarily from Asia, and a few different cherry species and hybrids with varied provenances. And this is just a small sample of what can be found along my three and a half mile bike ride.

St. John’s wort (Hypericum perforatum) on the banks of the Boise River

This is certainly not a new concern. We have been aware of the role that horticulture plays in introducing invasive species for quite some time now. Several years back, while doing a deep dive into the topic of invasive species, I wrote about this issue right here on this very blog. According to a study published in Frontiers in Ecology and the Environment (2021), out of 1285 plant species identified as invasive, 61% are currently sold in nurseries. If that’s not concern enough, an additional factor to consider is climate change. Plants that were less likely to escape cultivation and head for the wild, may take the opportunity to do so in a changing climate. Plus, horticultural plants that are already problems in certain areas could expand their range as climates become more favorable in new locations, especially if these plants continue to be sold in nearby nurseries.

These concerns and more are the topic of a paper published in BioScience (2023). Evelyn M. Beaury, et al. looked at nurseries across the United States and the plants they sell in order to determine where invasive plants are still being sold in regions where they are invasive. Additionally, they looked at plants known to be invasive but that are not currently invasive in the regions they are being sold. Using climate models, they predicted whether or not these plants could become invasive under changing climates.

Plants are being moved around with a lot more ease than they once were, and the sales of problematic plants are increasingly difficult to regulate. For one thing, plants prohibited for sale in one state can be purchased at nurseries in neighboring states and brought back to be planted in regions where those plants are invasive. And while mail order has existed for a long time, online ordering makes the process even simpler; and many online plant vendors are not liscensed nurseries, making them much more difficult to regulate. But even regulation is typically a response to something that has already become a problem, rather than a proactive measure to prevent plants from escaping into natural areas.

Beaury, et al. identified 672 nurseries across the United States, both online and traditional retailers. Each of these nurseries were selling one or more of the 89 plant species that became the focus of their research. These are plant species that are either on federal or state noxious weed lists or that have been identified as invasive by Invasive Plant Atlas. The reach of each nursery was determined by using customer reviews to compute distances that plants might travel after being purchased at nurseries or from online stores. Obviously, not every customer that purchases a plant leaves a review, but this is a good way to get a general idea how far away customers are from nurseries without having access to more detailed records. These geotagged reviews can also be cross-referenced with known distributions of invasive plants. Using climate models and environmental predictor variables, the researchers determined areas of current and potential invasion for each of the 89 plants.

tansy (Tanacetum vulgare) – one of the 89 plant species looked at in the study

The first question was about proximity to current records of plant invasions. Results showed that “49 of the 89 ornamental invasives were sold within 21 kilometers (13 miles) of an observed record of invasion.” When invasive plants are sold and planted near locations where they are already known to be invasive, it gives them the opportunity to add new plants to existing or developing invasions. In ecology, this is known as propagule pressure. When it comes to current and future climate, most species in the study are being sold by nurseries where the climate is either currently favorable for range expansion or may eventually become favorable. Specifically for future climate, 40 of the 89 plants are being sold in regions that are currently suitable for invasion and will continue to be suitable as the climate changes, and 25 of the 89 plants are being sold in regions where the climate is currently unsuitable but will become suitable as temperatures warm.

Particularly for plants being sold in areas that are not yet suitable for invasion, there is time to educate both the nursery industry and the general public and to look for alternatives to these plants. However, as the researchers point out, their analysis “only examined about 10% of the larger pool of U.S. ornamental plants known to be invasive,” and they “sampled only a subset of the nurseries that could be selling invasive species in the United States.” It is highly likely that the results of this study are an underestimation of the problem. Clearly the work of education and finding alternatives to problematic plants is monumental. The hope is that studies like this can help with education and can assist with working out ways to regulate sales of invasive plants.

coltsfoot (Tussilago farfara) – another one of the 89 plant species looked at in the study

Regulating the sale of plants is beyond most of our control, and how much regulation we should be enforcing on nurseries in the first place is a debate we should be having. Outside of those questions, there is a responsibility that we should take as gardeners and as residents of the planet. If we choose to grow plants, it is crucial that we get to know them. We should be taking the time to observe the degree to which they spread and how they are being dispersed. When they do move around our yards, where are they going, and are they able to grow outside of our care? Are they leaving our properties and coming up elsewhere? If we choose to plant non-native species, we should be mindful of how they might affect nearby, wild landscapes if they were to escape our yards and establish themselves in these locations. We should also be aware of where we live in the city. If our gardens are in the middle of a dense urban landscape, perhaps there is less concern that our plants will move beyond the borders of our gardens. But if we garden near natural areas, we should be significantly more selective about the things we plant, and we ought to be more observant as to what those plants are up to.

Nurseries generally sell the plants that gardeners want to buy, which means we can choose not to buy problematic plants and instead demand alternatives to these plants. Seeking out nurseries that sell the types of plants that are better suited for our regions and do not exhibit invasive behaviors can send a message to other growers that they should phase out certain plants and start growing the plants that gardeners are asking for. This may be a simplistic take, and as with most things, it’s complicated. While one of the goals of this research is to help influence regulators, another goal is simply to “[share] information about high-risk ornamental invaders across states and regions, and [work] with horticulture and community members to reduce the escape of ornamental species into natural areas.” This is precisely the area where gardeners can make a difference.

On that note, I will be starting a new series of posts to discuss some of the ornamental species that have gone weedy. By getting to know the plants that find themselves in this predicament, we can be better situated to make informed decisions about what to do about them.

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Winter Trees and Shrubs: Box Elder

Box elder is a maple that doesn’t often get credit for being a maple. Moreso, it is a tree that is not thought highly of, and it may not even be welcome in certain discussions around maples. You could even say that box elder is a “rogue maple,” as Arthur Plotnick deems it in The Urban Tree Book. It should come as no surprise, but if people are going to talk about a plant this way, it’s only going to make me like it more.

First off, the leaves of Acer negundo are palmately compound, compared to the simple, palmately lobed leaves of other maples. A box elder leaf is generally composed of three leaflets, sometimes five or seven. The center leaflet is on a longer stalk than the side leaflets, which are oppositely arranged. Sometimes the leaves are confused with ash tree leaves (Fraxinus spp.), which is the reason for another common name, ash-leaved maple. It’s also not uncommon for box elder leaves to be mistaken for poison ivy, which is maybe not a plant you’d want to be confused with. In the fall, unlike the beautiful reds, oranges, purples, and yellows of so many other maples, the leaves of box elder turn a drab yellow before dropping, a trait that Plotnick calls “an embarrassment.”

leaf of box elder (Acer negundo)

It’s not really its leaf, however, that bothers people. Box elder is a fast-growing and prolific tree. Basically, it’s weedy, and because it grows so quickly, its wood is generally weak and its structure unpredictable. It falls apart easily, and it doesn’t always grow in typical tree fashion. Sometimes its more of a shrub, and sometimes it leans so far over that it practically lies on the ground. It sprouts and suckers in abundance, and as John Eastman puts it in The Book of Forest and Thicket, it “generally ignore(s) human standards for an attractive tree.”

Maybe before people knew better, or during a time when people just needed a fast growing tree to plant in their developing cities, box elders were widely planted across North America and beyond. That and their ability to spread readily helps to obscure their native distribution. Box elder is naturalized across North America, but its original distribution (whatever that means) appears to be more midwestern and eastern. Box elder grows in a variety of conditions, and is actually quite drought tolerant, which has allowed it to feel right at home in the American west, whether it is truly native here or not.

Identifying box elder in the winter can be pretty easy, especially if you come across a seed-bearing individual. Box elders are dioecious, meaning male and female flowers occur on separate individuals (another thing that sets them apart from other maples). A samara-bearing tree is a female tree, and the samaras often hang from the branches in great numbers. Like other maples, the fruits are paired together, each seed with a prominent wing. The samaras of boxelder point downward and hang from long stalks, persisting on the tree throughout the winter.

persistent samaras of box elder

The twigs of box elder are glaucous (covered with a thin, powdery bloom) and can be either green, bluish-white, or reddish-purple. They have tiny, white lenticles, and their buds are oppositely arranged. Leaf scars are a broad U-shape, and scars opposite of each other meet at the tops of the U’s, coming to a raised point. At least three bundle traces (sometimes more) are visible in the scar. Leaf buds are protected by two scales that are covered in dense, white hairs. The scales are similar in color to the twigs, but appear more white due to the fuzz. Lateral buds are 1/8 inch long or longer. Slicing open the twigs lengthwise reveals solid, white pith in the center that blends in easily with the wood.

winter twigs of box elder
winter twigs of box elder, pith exposed

The bark of young box elders has slight vertical ridges and is light brown or grey-brown, with dull orange, raised, wart-like lenticels. It is finely scaly. The scales and ridges become more rough and blocky as the tree ages. Mature bark is light brown to grey and heavily furrowed, with blocky segments breaking apart horizontally.

maturing bark of box elder

A few years ago I was introduced to a massive box elder in Boise, Idaho. It quickly became my favorite tree in the area, and I visit it frequently. For all the hate that box elder can receive, seeing its potential to create an experience that few other trees in the area offer can send all of that negative talk out the window. Visit something like it yourself sometime, and see what it can do for you.

massive box elder

More Winter Trees and Shrubs:

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Weeds of Boise Takes a GIS Course

Why has this blog been so quiet lately? There are plenty of excuses for that. It doesn’t really matter either way, but since we’re on the subject, one thing that has kept me occupied recently is being back in school. I’m working on a certificate in GIS, and I’m hoping to make some cool maps. More on that later perhaps.

For now, I thought I’d share one of my final projects. I figured it was a good excuse to use something I’ve already been working on – namely, Weeds of Boise – and apply it to one of my GIS classes. If you’ve been following Weeds of Boise, not much here will be new. Except the interactive maps!

Using observations from iNaturalist, I created three different maps of weeds found around Boise. I published them, using ArcGIS StoryMaps, alongside more of my usual pontificating about weeds and urban areas, etc. The maps aren’t perfect, but they opened up some interesting possibilities for what Weeds of Boise might become. Something to explore further in the future.

Until then, here is a link to the project if you’d like to take a look: Wild Urban Flora of Boise Idaho.

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Weeds of Boise: Neglected Parking Lot Islands at Post Office on 13th Street

Boise’s summers are decidedly hot and dry. Months can pass without any measurable precipitation, meanwhile temperatures regularly peak in the 90’s. In the heat of the summer, multiple days in a row above 100 degrees Fahrenheit is not unheard of. Under these conditions, irrigation is a must if you intend to keep plants alive, particularly plants not adapted to our climate. Skipping out on watering or having something go awry with the irrigation system quickly becomes noticeable as the soil goes bone dry and plants begin to wither away. If this goes on for too long, many plants will not recover, including established trees and shrubs. However, the toughest plants among us, particularly the weeds, will move in to take their place.

You can see an example of this at the United States Postal Service building at 13th Street and Shoreline Lane in Boise, Idaho. The islands in the parking lot are not being watered, which has clearly been the case for quite some time because even the trees and shrubs are dying off. Apart from occasional mowing, very little maintenance is occurring, and our wild urban flora is about all that remains.

parking lot island at the corner of 13 Street and Shoreline Lane

Parking lots are not ideal locations for growing plants. Asphalt and cement dominate – two major contributors to the urban heat island effect – and automobile pollution is concentrated on account of all the cars coming and going on a regular basis. Many parking lots include islands where plants (often poorly maintained), along with other features like signs and lights, are placed. In general, these islands are far too small for trees, but trees are planted nonetheless in a desperate attempt to shade these formidable environments. In locations where snow is common, the snow from parking lots is often piled up on these islands to clear room for cars, while road salts and other ice melts are heavily applied in order to keep people and cars from sliding around. Parking lot plants have to endure all this and more, so it’s no surprise that they usually look pretty rough.

The stresses of added heat, pollution, trampling, and poor care are enough for plants to put up with. Cut off their irrigation supply, and parking lot plants are sure to give up the ghost. A situation like this is an excellent place to familiarize yourself with your wild urban flora. Many weeds seemingly have no problem tolerating these conditions. To demonstrate this, I inventoried the weeds found in neglected parking lot islands at the post office on 13th Street and Shoreline Lane. What follows are a few photos and a list of the weeds I’ve identified so far. Like all posts in the Weeds of Boise series, this list may be updated as I continue to check back in on this location.

spring draba (Draba verna)
shepherd’s purse (Capsella bursa-pastoris)
dandelion (Taraxacum officinale)
black medic (Medicago lupulina)
crabgrass (Digitaria sanguinalis)
annual sow thistle (Sonchus oleraceus)
snag (aka standing dead tree) at post office on 13th Street
  • Anthriscus caucalis (bur chervil)
  • Bromus tectorum (cheatgrass)
  • Capsella bursa-pastoris (shepherd’s purse)
  • Ceratocephala testiculata (bur buttercup)
  • Chondrilla juncea (rush skeletonweed)
  • Claytonia perfoliata (miner’s lettuce)
  • Convolvulus arvensis (field bindweed)
  • Conyza canadensis (horseweed)
  • Digitaria sanguinalis (crabgrass)
  • Draba verna (spring draba)
  • Elymus repens (quackgrass)
  • Epilobium ciliatum (willowherb)
  • Erodium cicutarium (redstem filare)
  • Euphorbia maculata (spotted spurge)
  • Holosteum umbellatum (jagged chickweed)
  • Hordeum jubatum (foxtail barley)
  • Lactuca serriola (prickly lettuce)
  • Lamium amplexicaule (henbit)
  • Malva neglecta (common mallow)
  • Medicago lupulina (black medic)
  • Parthenocissus quinquefolia (Virginia creeper)
  • Poa annua (annual bluegrass)
  • Polygonum aviculare (prostrate knotweed)
  • Portulaca oleracea (purslane)
  • Senecio vulgaris (common groundsel)
  • Sonchus oleraceus (annual sow thistle)
  • Taraxacum officinale (dandelion)
  • Tragopogon dubius (salsify)
  • Tribulus terrestris (puncturevine)
  • Trifolium sp. (clover)

Where there are parking lot islands, there are bound to be weeds whether the islands are being maintained or not. What have you found growing in the parking lot islands in your city? Feel free to share in the comment section below.

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Book Review: Wild Wasatch Front

If it isn’t clear by now from my Weeds of Boise series and countless other posts, I happen to be interested in the flora and fauna of urban areas. Urban ecology is a fascinating field of study, and I’m not sure that it gets the attention it deserves. Nature is not some far away place, and you shouldn’t have to leave city limits to go in search of it. Remarkably, nature exists right outside your front door, even if you live in the middle of a massive city. It may be a different sort of nature than the one you might find in a national forest or a state park, and it may be composed of species introduced from all corners of the world, but it is still a collection of living organisms interacting with each other and the surrounding environment in unique and important ways. The question is, can you grow to appreciate nearby nature and recognize that the ecological interactions that exist within the context of a city are just as valid as those you’ll find outside of our built environments?

Luckily, there are resources that can help you with that, including a recent book compiled by Lisa Thompson and others at the Natural History Museum of Utah (NHMU). It’s called Wild Wasatch Front, and it’s of particular interest to me because it covers a region that’s relatively close by, and our two locations share a number of similarities. Plus, I played a small role in reviewing some of the plants (specifically the weeds) that ended up in the book (Sierra would insist that I mention this, so there you go). Similar books exist for other regions across North America and elsewhere, so I encourage you to seek out a book that applies to your hometown.

The Wasatch Front is a metropolitan region in north-central Utah that spans the western side of the Wasatch Mountains and includes a long string of cities and towns extending for many miles in all directions. Included in that list of cities is Salt Lake City, the state’s capitol and largest city in the state. The idea for a book about urban nature in the Wasatch Front was inspired by an exhibit at NHMU called “Nature All Around Us.” The exhibit and resulting book offer a new perspective for those insisting that “nature and cities cannot coexist” or that the nature found in cities is influenced by humans and therefore shouldn’t be considered “real.” Hundreds of organisms making a life for themselves within the boundaries of our cities might argue otherwise.

Wild Wasatch Front is divided into three main sections, with each section being worth the price of the book on its own. First there are a series of essays about urban nature and ecology. Names you might recognize, including Emma Marris and Riley Black, contributed to the book, as well as several other people that live and work in the western U.S. and have an interest in nature and environmental issues, especially as they relate to cities. Novel ecosystems is a reoccurring theme, not just in the essays but throughout the book. In her essay, Sarah Jack Hinners writes, “urban nature is a mixture of the intentional and the unintentional,” adding that “for every tree or rosebush or lawn that we plant and carefully nurture, there are multitudes of other plants and animals that grow and thrive uninvited and unnurtured by us.”

The largest section in the book is a field guide, profiling 127 plus species that call the Wasatch Front home, some native and some transplants. This section is divided into subsections that include birds, invertebrates, fungi and lichen, mammals, reptiles and amphibians, street trees, and wild plants. The entry for each species includes a brief description, a few interesting facts, and details on how and where to find them, accompanied by images. With the variety of creatures covered, you are sure to find something that interests you and a reason to go out looking for your favorites. You may even learn something new about a species you’ve been seeing for years, such as house finches. It turns out that the colorful patches on a male house finch are the result of the plants they eat. These patches can be red, orange, or yellow. The redder the better though, because female house finches seek out mates with this coloration.

Naturally, my focus was mainly aimed at the plants covered in this section. I appreciated the mixture of native and introduced plants, even the inclusion of plants considered to be invasive. Instead of vilifying these species, there is an attempt to understand them and find value in them, even in spite of the concerns and negative opinions held about them. Box elder (Acer negundo) is an example of a plant that has both native and introduced populations. Once widely planted in yards and on farms, this tree has “fallen out of favor.” Its weak wood (a result of growing so quickly), can result in a messy, unattractive tree, making it a poor choice for a street tree. However, it propagates itself readily and shows up in vacant lots and other urban locations that receive minimal management and human attention. In the Wasatch Front, you can find box elders that are native, naturalized, and cultivated, an unlikely scenario unique to urban areas.

massive box elder (Acer negundo) in Boise, Idaho

The third and final section of the book is a guide to 21 different hikes and field trips in and around the Wasatch Front. Each field trip features a hand-drawn map and some basic notes about the hike. Details about what can be seen along the way are included in the descriptions, which are sure to entice you into visiting. Whether or not you think you’ll ever make it out to any of these spots, this section is still worth reading if only for the ongoing discussions about urban ecology. For example, in the entry for Gib’s Loop, abrupt changes in land ownership and land use (a common experience when hiking in urban areas) is addressed: “Human impacts in the foothills…don’t end at backyard fences, and many animals use resources in both habitats. It’s more interesting to think of cities and the surrounding foothills as part of an interconnected system rather than separate and distinct.”

The field trip section is also used as a teaching opportunity to describe more of the species you’ll find in the Wasatch Front. In the entry for Creekside Park, learn how to identify creeping mahonia (Berberis repens), with its low growing habit and matte leaves, and compare it to Oregon grape (B. aquifolium), with its more upright habit and shinier leaves.

Berberis aquifolium (on the left) compared to Berberis repens (on the right)

Last year, in anticipation of Wild Wasatch Front, I came across another book with a similar focus. This book was put out by a group called The Urban Field Naturalist Project, headquartered in Australia. Their book, A Guide to the Creatures in Your Neighbourhood, encourages its readers to become urban naturalists and offers resources to help them get started. Just like Wild Wasatch Front, the bulk of the book is a field guide to species found in and around urban areas (in Australia, of course). In place of a guide to hikes and field trips, there are instructions on how to start nature journaling, which is a key component of becoming an urban field naturalist. Getting outside and learning to recognize nearby nature is step one, documenting what you see and sharing those observations with others is step two. Taken together, these two books will help you gain a better appreciation for urban nature and will hopefully inspire you to work to conserve what is there and make room for more.

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Winter Trees and Shrubs: Sweetgum

Sweetgum is a distinctive tree at any time of year. It stands out among most other deciduous trees with its excurrent growth habit, which gives it a narrowly conical or pyramidal shape. Only in its advanced age does it begin to develop a more rounded and irregular form. Its leaves are star-shaped with either five or seven lobes and span between four and seven inches. Their glossy green color gives way to a wide range of colors in the fall, including yellow, orange, red, and maroon, remaining on the tree for several weeks depending on weather conditions. The fruits are particularly distinct, especially in the winter after the leaves have dropped. Woody balls made up of a series of capsules with pointed beaks hang from long stalks throughout the winter, windborne seeds having been released from small openings in the fall. Without even bothering to look at the twigs, seeing these durable, spiky balls hanging from branches (or feeling them under your feet) is a quick indication that you are looking at a sweetgum.

one of many fall colors found on sweetgum (Liquidambar styraciflua)

Liquidamabar styraciflua (one of the funnest botanical names to say) was previously placed in the family Hamamelidaceae but is now one of the few members of Altingiaceae. Its natural distribution is broad, covering a large portion of the eastern United States and west into Missouri then down into Texas, Mexico, and much of Central America. Outside of its natural range it has been widely planted as an ornamental, and there are several popular sweetgum cultivars currently in cultivation. Both the common and botanical names for this tree refer to the resin found in its bark, which historically has had many uses.

The winter twigs of sweetgum are stout, round, smooth, and yellow-brown to green or olive-green to brown-purple in color. They can also be glossy and feature a few scattered lenticels. Older twigs (or branchlets) are brown at first and then light grey with dark grey lenticels. They are occasionally adorned with corky wings similar to those of bur oak. Because the wings aren’t always present, it can be a fun thing to encounter when you are out looking at twigs.

corky wings on the branchlets of sweetgum

The winter buds of sweetgum twigs are egg-shaped and made up of bud scales with acute tips and ciliate margins. They are green to orange-brown or reddish in color and occasionally sticky. Lateral buds are alternately arranged, are much smaller than terminal buds, and are stalkless and flattened against the twig. They sit above a slightly raised leaf scar that is half-elliptical to triangular in shape and has three distinct vascular bundle traces. The pith of sweetgum twigs is solid, continuous, brownish, and irregularly shaped.

winter twig of sweetgum tree

Sweetgum bark is light to dark grey and is made up of a series of rough, vertically arranged, scaly ridges that become deeply furrowed with age. The mace-like fruits of sweetgum are about one inch wide and, at a glance, are similar in appearance to the seed balls of sycamore trees; however, sycamore seed balls easily break apart when compressed, while the pointed, woody capsules that make up a sweetgum ball are held firmly together and can hold their shape for long periods of time. When these “gumballs” collect on the ground below, they can become a hazard, especially where there is lots of foot traffic. Speaking from experience, they are also obnoxious when operating a mower. This polarizing feature has resulted in bad opinions of the sweetgum tree. Luckily, some people are out there defending it.

sweetgum bark
the persistent fruits of sweetgum

More Winter Trees and Shrubs:

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Weeds of Boise: Boise State University Campus, part two

In part one of this two part series, I introduced you to the Boise State University campus, located in the heart of Boise, Idaho. I’ve been spending the past year walking the campus and cataloging the weeds that I find there. Boise has a fairly mild climate compared to the rest of Idaho, so weeds are generally easy to find just about any time of year. What weeds are present depends on what time of year it is. To get a complete picture of the suite of weeds that can be found on a site, it’s important to make observations throughout the year. Weeds can also come and go, with certain species becoming more abundant in some years than others, so making observations over multiple years also helps. This is why I try to update posts that are part of the Weeds of Boise series as I make return visits and encounter additional weed species.

What follows is the second half of the list of weeds I’ve documented so far at Boise State University. I’m including a photograph for each month of the year (July – December), as well as a list of what I’ve encountered up to this point. I’m also including a list of weeds that I didn’t come across but that are documented on iNaturalist.

birdsfoot trefoil (Lotus corniculatus) at BSU in July 2023
yellow nutsedge (Cyperus esculentus) at BSU in August 2023
velvetleaf (Abutilon theophrasti) at BSU in September 2023
chicory (Cichorium intybus) at BSU on October 2023
puncturevine (Tribulus terrestris) at BSU in November 2023
bull thistle (Cirsium vulgare) at BSU in December 2023

Additional weeds found on the BSU campus from July – December 2023:

  • Abutilon theophrasti (velvetleaf)
  • Cichorium intybus (chicory)
  • Cirsium vulgare (bull thistle)
  • Cyperus esculentus (yellow nutsedge)
  • Eragrostis cilianensis (stinking lovegrass)
  • Lotus corniculatus (birdsfoot trefoil)
  • Medicago sativa (alfalfa)
  • Melilotus alba (white sweetclover)
  • Solanum nigrum (black nightshade)
  • Sonchus asper (prickly sowthistle)
  • Tribulus terrestris (puncturevine)

Additional weeds observed on the BSU campus by iNaturalist users as of December 2023:

  • Aegilops cylindrica (jointed goatgrass)
  • Bromus diandrus (ripgut brome)
  • Cerastium nutans (nodding chickweed)
  • Chorispora tenella (blue mustard)
  • Elymus repens (quackgrass)
  • Hypericum perforatum (St. John’s wort)
  • Lepidium perfoliatum (clasping pepperweed)
  • Matricaria discoidea (pineappleweed)
  • Ornithogalum umbellatum (star-of-Bethlehem)
  • Vicia tetrasperma (four-seeded vetch)
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Weeds of Boise: Boise State University Campus, part one

If you live in a major city (or even a minor one), there is a good chance it is home to a college or university (perhaps several). Universities tend to take up a lot of space, which means there is often a plethora of landscaping accompanying their buildings, hardscaping, and other impervious surfaces. Among all the turf, flower beds, tree wells, and other greenspaces, there is bound to be a fair share of weeds. In spite of how hard the groundskeepers may work, the campus is not likely to ever be completely weed-free. Lucky for us, this means that institutions of higher learning are excellent places to familiarize ourselves with many of the weed species that occur in our cities, particularly weeds that are common in garden beds and turfgrass.

Near downtown Boise, on the southside of the Boise River, you will find the ever-expanding campus of Boise State University, home of the Broncos and their famous blue turf. According to the internet’s favorite encyclopedia, the campus is 285 acres in size, plenty of space for weeds to grow and abudant opportunities to hunt them out. Tallying the number of weed species in a place like this takes time. The benefit of botanizing for weeds is that you can find them at just about any time of year. While some species only show up in certain seasons, others can be seen practically year-round.

In order to document the weeds of Boise State University, I’m spending the entire year walking the campus listing and photographing the weeds I find. What follows is the first half of what’s been documented so far. I’m including a photograph for each month of the year, as well as a list of what I’ve encountered. In part two, I’ll share a list of any additional weeds found throughout the remainder of the year. While you’re waiting for that, check out the other posts in the Weeds of Boise series.

common groundsel (Senecio vulgaris) at BSU in January 2023
chickweed (Stellaria media) at BSU in February 2023
hairy bittercress (Cardamine hirsuta) at BSU in March 2023
ivyleaf speedwell (Veronica hederifolia) at BSU in April 2023
black medic (Medicago lupulina) at BSU in May 2023
creeping thistle (Cirsium arvense) at BSU in June 2023

List of weeds found on the campus of Boise State University as of June 2023:

  • Ailanthus altissima (tree of heaven)
  • Anthriscus caucalis (bur chervil)
  • Amaranthus retroflexus (redroot pigweed)
  • Arctium minus (lesser burdock)
  • Bassia scoparia (kochia)
  • Bromus tectorum (cheatgrass)
  • Capsella bursa-pastoris (shepherd’s purse)
  • Cardamine hirsuta (hairy bittercress)
  • Ceratocephala testiculata (bur buttercup)
  • Chenopodium album (lamb’s quarters)
  • Chondrilla juncea (rush skeletonweed)
  • Cirsium arvense (creeping thistle)
  • Claytonia perfoliata (miner’s lettuce)
  • Convolvulus arvensis (field bindweed)
  • Conyza canadensis (horseweed)
  • Descurainia sophia (flixweed)
  • Digitaria sanguinalis (crabgrass)
  • Draba verna (spring draba)
  • Epilobium ciliatum (willowherb)
  • Erodium cicutarium (redstem filare)
  • Euphorbia maculata (spotted spurge)
  • Galium aparine (cleavers)
  • Geum urbanum (herb Bennet)
  • Holosteum umbellatum (jagged chickweed)
  • Hordeum jubatum (foxtail barley)
  • Lactuca serriola (prickly lettuce)
  • Lamium purpureum (purple deadnettle)
  • Lepidium sp. (whitetop)
  • Malva neglecta (common mallow)
  • Medicago lupulina (black medic)
  • Oxalis corniculata (creeping woodsorrel)
  • Parthenocissus quinquefolia (Virginia creeper)
  • Plantago lanceolata (narrowleaf plantain)
  • Plantago major (broadleaf plantain)
  • Poa annua (annua bluegrass)
  • Poa bulbosa (bulbous bluegrass)
  • Polygonum aviculare (prostrate knotweed)
  • Portulaca oleracea (purslane)
  • Prunella vulgaris (self-heal)
  • Ranunculus repens (creeping buttercup)
  • Senecio vulgaris (common groundsel)
  • Sonchus sp. (sow thistle)
  • Stellaria media (chickweed)
  • Taraxacum officinale (dandelion)
  • Tragopogon dubius (salsify)
  • Trifolium repens (white clover)
  • Ulmus pumila (Siberian elm)
  • Veronica hederifolia (ivyleaf speedwell)
  • Vulpia myuros (rat’s tail fescue)

Do you frequent the BSU campus? Have you seen anything not on my list? Comment below or send me a message and let me know what you’ve seen and where.

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My Review of Decurrent Trees

Just because it’s winter doesn’t mean there aren’t plenty of plants to look at. Deciduous trees and shrubs become particularly interesting during the winter months with their exposed branches and their growth habits made more obvious. The beauty of a tree’s “skeletal” structure is revealed when it’s stripped of its leaves and set against a winter sky. Winter is also a great time to prune certain trees and shrubs (when appropriate), partly because their branches are so easily viewed and “problem” areas readily reveal themselves. Whether you’re observing a tree’s branching structure simply for enjoyment-sake or because you plan to prune, you may find yourself noticing distinct differences in the growth habits of trees. Distinct growth habits can help you identify trees. They can also tell you something about a tree’s environment or growing conditions.

In the book, The Tree, Colin Tudge defines a tree as “a big plant with a stick up the middle.” Sometimes this “stick” runs straight up from the ground to the top of the tree without interruption and is the tallest portion of the plant. Other times, the “stick” reaches a certain height and branches out into multiple “sticks,” each one reaching out in a different direction – some heading more outward, while others continue to reach for the sky. This is the difference between excurrent and decurrent growth.

excurrent growth habit: dawn redwood (Taxodium distichum)

A tree with an excurrent growth habit has one central leader – or single trunk – that reaches all the way to the top of the tree. Side branches occur along the length of the trunk and generally get shorter as they move up the tree, producing a pyramidal or conical shape. Think of a typical Christmas tree. Many conifers exhibit excurrent growth, as do several deciduous trees such as sweetgum and pin oak, as well as aspens and other poplars. When a tree divides part way up the trunk, splitting into several large branches – none of which could be considered the dominant branch – it is exhibiting a decurrent growth habit. Trees that generally fall into this category include elms, maples, oaks, and ashes. The growth habit of a tree is largely a result of its genetics, but plants are known for their plasticity, taking on a wide variety of forms depending on their parentage and their circumstances. Trying to identify a tree based only its growth habit, isn’t likely to yield great results.

decurrent growth habit: oak (Quercus sp.)

The environment that a plant is growing in can have noticeable effects on the form the plant takes. A tree growing up in a forest thick with other trees will typically grow straight up in search of sunlight and will branch out very little until it can get up high enough to do so. That same species of tree growing in an open field might instead branch out extensively at a much lower height, taking advantage of the generous amount of space to stretch its branches out wide. As Tudge puts it in The Tree, “one form for the forest, another for the open ground.” Additionally, things can happen in a tree’s life that will drastically alter its form. If, for example, a storm comes through and breaks off a tree’s central leader, several side branches might grow out and upward to take its place, giving an otherwise excurrent tree a decurrent form. The pruning that humans often do (sometimes unwisely) to trees and shrubs, particularly in urban settings, can also alter a plant’s natural growth habit considerably. These are important considerations to make when assessing the forms of trees.

decurrent growth habit: golden rain tree (Koelreuteria paniculata)

I give decurrent growth habits five stars. No shade on excurrent trees. They’re also beautiful. But while trees with excurrent growth habits have otherwise predictable forms, decurrent trees are full of surprises. Their broad and rounded forms provided by their deliquescent branching structures are endlessly interesting, and their capacious canopies ensure that no two trees are alike.

decurrent growth habit: Malus sp. (I presume)