When Urban Pollinator Gardens Meet Native Plant Communities

Public concern about the state of bees and other pollinating insects has led to increased interest in pollinator gardens. Planting a pollinator garden is often promoted as an excellent way for the average person to help protect pollinators. And it is! However, as with anything in life, there can be downsides.

In many urban areas, populations of native plants remain on undeveloped or abandoned land, in parks or reserves, or simply as part of the developed landscape. Urban areas may also share borders with natural areas, the edges of which are particularly prone to invasions by non-native plants. Due to human activity and habitat fragmentation, many native plant populations are now threatened. Urban areas are home to the last remaining populations of some of these plants.

Concern for native plant populations in and around urban areas prompted researchers at University of Pittsburgh to review some of the impacts that urban pollinator gardens may have and to develop a “roadmap for research” going forward. Their report was published earlier this year in New Phytologist.

Planting a wildflower seed mix is a simple way to establish a pollinator garden, and such mixes are sold commercially for this purpose. Governmental and non-governmental organizations also issue recommendations for wildflower, pollinator, or meadow seed mixes. With this in mind, the researchers selected 30 seed mixes “targeted for urban settings in the northeastern or mid-Atlantic USA” to determine what species are being recommended for or commonly planted in pollinator gardens in this region. They also developed a “species impact index” to assess “the likelihood a species would impact remnant wild urban plant populations.”

A total of 230 species were represented in the 30 seed mixes. The researchers selected the 45 most common species for evaluation. Most of these species (75%) have generalized pollination systems, suggesting that there is potential for sharing pollinators with remnant native plants. Two-thirds of the species had native ranges that overlapped with the targeted region; however, the remaining one-third originated from Europe or western North America. The native species all had “generalized pollination systems, strong dispersal and colonization ability, and broad environmental tolerances,” all traits that could have “high impacts” either directly or indirectly on remnant native plants. Other species were found to have either high dispersal ability but low chance of survival or low dispersal ability but high chance of survival.

This led the researchers to conclude that “the majority of planted wildflower species have a high potential to interact with native species via pollinators but also have the ability to disperse and survive outside of the garden.” Sharing pollinators is especially likely due to super-generalists like the honeybee, which “utilizes flowers from many habitat types.” Considering this, the researchers outlined “four pollinator-mediated interactions that can affect remnant native plants and their communities,” including how these interactions can be exacerbated when wildflower species escape gardens and invade remnant plant communities.

photo credit: wikimedia commons

The first interaction involves the quantity of pollinator visits. The concern is that native plants may be “outcompeted for pollinators” due to the “dense, high-resource displays” of pollinator gardens. Whether pollinator visits will increase or decrease depends on many things, including the location of the gardens and their proximity to native plant communities. Pollinator sharing between the two has been observed; however, “the consequences of this for effective pollination of natives are not yet understood.”

The second interaction involves the quality of pollinator visits. Because pollinators are shared between native plant communities and pollinator gardens, there is a risk that the pollen from one species will be transferred to another species. High quantities of this “heterospecific pollen” can result in reduced seed production. “Low-quality pollination in terms of heterospecific pollen from wildflower plantings may be especially detrimental for wild remnant species.”

The third interaction involves gene flow between pollinator gardens and native plant communities. Pollen that is transferred from closely related species (or even individuals of the same species but from a different location) can have undesired consequences. In some cases, it can increase genetic variation and help address problems associated with inbreeding depression. In other cases, it can introduce traits that are detrimental to native plant populations, particularly traits that disrupt adaptations that are beneficial to surviving in urban environments, like seed dispersal and flowering time. Whether gene flow between the two groups will be positive or negative is difficult to predict, and “the likelihood of genetic extinction versus genetic rescue will depend on remnant population size, genetic diversity, and degree of urban adaptation relative to the planted wildflowers.”

The fourth interaction involves pathogen transmission via shared pollinators. “Both bacterial and viral pathogens can be transmitted via pollen, and bacterial pathogens can be passed from one pollinator to another.” In this way, pollinators can act as “hubs for pathogen exchange,” which is especially concerning when the diseases being transmitted are ones for which the native plants have not adapted defenses.

photo credit: wikimedia commons

All of these interactions become more direct once wildflowers escape gardens and establish themselves among the native plants. And because the species in wildflower seed mixes are selected for their tolerance of urban conditions, “they may be particularly strong competitors with wild remnant populations,” outcompeting them for space and resources. On the other hand, the authors note that, depending on the species, they may also “provide biotic resistance to more noxious invaders.”

All of these interactions require further investigation. In their conclusion, the authors affirm, “While there is a clear potential for positive effects of urban wildflower plantings on remnant plant biodiversity, there is also a strong likelihood for unintended consequences.” They then suggest future research topics that will help us answer many of these questions. In the meantime, pollinator gardens should not be discouraged, but the plants (and their origins) should be carefully considered. One place to start is with wildflower seed mixes, which can be ‘fine-tuned’ so that they benefit our urban pollinators as well as our remnant native plants. Read more about plant selection for pollinators here.

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Year of Pollination: Pollinator Walk at Earthly Delights Farm

Last week I had the privilege of attending a pollinator walk with a local entomologist at Earthly Delights Farm, a small, urban farm in Boise, Idaho. The entomologist was Dr. Karen Strickler, an adjunct instructor at College of Western Idaho and the owner of Pollinator Paradise. A small group of us spent a couple of hours wandering through the farm looking for pollinators and discussing whatever pollinator or non-pollinator related topic that arose. Earthly Delights Farm, along with growing and selling produce using a subscription-based model, is a seed producing farm (and part of a larger seed growing operation called Snake River Seed Cooperative), so there were several crops flowering on the farm that would typically be removed at other farms before reaching that stage, such as lettuce and carrots. The farm also shares property with Draggin’ Wing High Desert Nursery, a nursery specializing in water efficient plants for the Intermountain West, which has a large demonstration area full of flowering plants. Thus, pollinators were present in abundance.

A series of isolation tents over various crops to help prevent cross pollination between varieties.

A series of isolation tents placed over various crops to help prevent cross pollination between varieties – an important component of seed saving.

While many groups of pollinators were discussed, including leafcutter bees, bumblebees, honeybees, sweat bees, hummingbirds, and beetles, much of our conversation and search was focused on syrphid flies. Flies are an often underappreciated and overlooked group of pollinators. While not all of the 120,000 species of flies in the world are pollinators, many of them are. The book Attracting Native Pollinators by the Xerces Society has this to say about flies: “With their reputation as generalist foragers, no nests to provision, and sometimes sparsely haired bodies, flies don’t get much credit as significant pollinators. Despite this reputation, they are often important pollinators in natural ecosystems for specific plants, and occasionally for human food plants.” They are especially important pollinators in the Arctic and in alpine regions, because unlike bees, they do not maintain nests, which means they use less energy and require less nectar, making them more fit for colder climates.

One food crop that flies are particularly efficient at pollinating is carrots. According the Xerces Society, carrot flowers are “not a favorite of managed honeybees.” Most flies do not have long tubular, sucking mouthparts, so they search for nectar in small, shallow flowers that appear in clusters, such as plants in the mint, carrot, and brassica families. Flower-visiting flies come in search of nectar and sometimes pollen for energy and reproduction. While acquiring these meals they can at times inadvertently collect pollen on their bodies and transfer it to adjacent flowers. They are generally not as efficient at moving pollen as other pollinators are, but they can get the job done.

Blister beetle on carrot flowers (a preferred food source of flies). Beetles can be important pollinators, even despite chewing on the flowers as they proceed.

Blister beetle on carrot flowers (a preferred food source of flies). Beetles can be effective pollinators as well, even despite chewing on the flowers as they proceed.

During the pollinator walk, we were specifically observing flies in the family Syrphidae, which are commonly known as flower flies, hoverflies, or syrphid flies. Many flies in this family mimic the coloring of bees and wasps, and thus are easily confused as such. Appearing as a bee or wasp is a form of protection from predators, who typically steer clear from these insects to avoid being stung. The larvae of syrphid flies often feed on insects, a trait that can be an added benefit for farmers and gardeners, particularly when their prey includes pest insects like aphids. Other families of flies that are important pollinators include Bombyliidae (bee flies), Acroceridae (small-headed flies), Muscidae (house flies), and Tachinidae (tachinid flies).

Common banded hoverfly (Syrphus ribesii) - one species of hundreds in the syrphid fly family, a common and diverse family of flower visiting flies (photo credit: www.eol.org)

Common banded hoverfly (Syrphus ribesii) – one species of thousands in the syrphid fly family, a common and diverse family of flower-visiting flies (photo credit: www.eol.org)

Because many species of flies visit flowers and because those flies commonly mimic the appearance of bees and wasps, it can be difficult to tell these insects apart. Observing the following features will help you determine what you are looking at.

  • Wings – flies have two; bees have four (look closely though because the forewings and hindwings of bees are attached with a series of hooks called hamuli making them appear as one)
  • Hairs – flies are generally less hairy than bees
  • Eyes – the eyes of flies are usually quite large and in the front of their heads; the eyes of bees are more towards the sides of their heads
  • Antennae – flies have shorter, stubbier antennae compared to bees; the antennae of flies also have bristles at the tips
  • Bees, unlike flies, have features on their legs and abdomens designed for collecting pollen; however, some flies have mimics of these features
Bumblebee on Echinacea sp.

Bumblebee visiting Echinacea sp.

Another interesting topic that Dr. Strickler addressed was the growing popularity of insect hotels – structures big and small that are fashioned out of a variety of natural materials and intended to house a variety of insects including pollinators. There is a concern that many insect hotels, while functioning nicely as a piece of garden artwork, often offer little in the way of habitat for beneficial insects and instead house pest insects such as earwigs. Also, insect hotels that are inhabited by bees and other pollinators may actually become breeding grounds for pests and diseases that harm these insects. It is advised that these houses be cleaned or replaced regularly to avoid the build up of such issues. Learn more about the proper construction and maintenance of insect hotels in this article from Pacific Horticulture.

A row of onions setting seed at Earthly Delights Farm. Onions are another crop that is commonly pollinated by flies.

A row of onions setting seed at Earthly Delights Farm. Onions are another crop that is commonly pollinated by flies.

Year of Pollination: An Argentinian Cactus and Its Unlikely Pollinator

A few weeks ago I wrote about pollination syndromes – sets of floral triats that are said to attract specific groups of pollinators. In that post I discussed how pollination syndromes have largely fallen out of favor as a reliable method of predicting the pollinators that will visit particular flowers. In this post I review a recent study involving a species of cactus in Argentina that, as the authors state in their abstract, “adds another example to the growing body of mismatches between floral syndrome and observed pollinator.”

Denmoza rhodacantha is one of many species of cacti found in Argentina. It is the only species in its genus, and it is widely distributed across the east slopes and foothills of the Andes. It is a slow growing cactus, maintaining a globulous (globe-shaped) form through its juvenile phase and developing a columnar form as it reaches maturity. D. rhodacantha can reach up to 4 meters tall and can live beyond 100 years of age. Individual plants can begin flowering in their juvenile stage. Flowers are red, nectar rich, scentless, and tubular. The stigma is lobed and is surrounded by a dense grouping of stamens. Both male and female reproductive organs are extended above the corolla. The flowers have been described by multiple sources as being hummingbird pollinated, not based on direct observation of hummingbirds visiting the flowers, but rather due to the floral traits of the species.

Denmoza rhodacantha illustration - image credit: www.eol.org

Denmoza rhodacantha illustration  (image credit: www.eol.org)

In a paper entitled, Flowering phenology and observations on the pollination biology of South American cacti – Denmoza rhodacantha, which was published in volume 20 of Haseltonia (the yearbook of the Cactus and Succulent Society of America), Urs Eggli and Mario Giorgetta discuss their findings after making detailed observations of a population of D. rhodacantha in early 2013 and late 2013 – early 2014. The population consisted of about 30 individuals (both juveniles and adults) located in the Calchaqui Valley near the village of Angastaco, Argentina. At least three other species with “hummingbird-syndrome flowers” were noted in the area, and three species of hummingbirds were observed during the study periods. Over 100 observation hours were logged, and during that time “the studied plants, their flowering phenology, and flower and fruit visitors were documented by means of photographs and video.”

The flowers of D. rhodacantha only persist for a few short days, and in that time their sexual organs are only receptive for about 24 hours. The flowers are self-sterile and so require a pollinator to cross pollinate them. Despite their red, tubular shape and abundant nectar, no hummingbirds were observed visiting the flowers. One individual hummingbird approached but quickly turned away. Hummingbirds were, however, observed visiting the flowers of an associated species, Tecoma fulva ssp. garrocha. Instead, a species of halictid bee (possibly in the genus Dialictus) was regularly observed visiting the flowers of D. rhodacantha. The bees collected pollen on their hind legs and abdomen and were seen crawling across the lobes of the stigma. None of them were found feeding on the nectar. In one observation, a flower was visited by a bee that was “already heavily loaded with the typical violet-coloured pollen of Denmoza,” suggesting that this particular bee species was seeking out these flowers for their pollen. Small, unidentified beetles and ants were seen entering the flowers to consume nectar, however they didn’t appear to be capable of offering a pollination service.

D. rhodacantha populations have been observed in many cases to produce few fruits, suggesting that pollination success is minimal. The authors witnessed “very low fruit set” in the population that they were studying, which was “in marked contrast to the almost 100% fruit set rates of the sympatric cactus species at the study site.” This observation wasn’t of great concern to the authors though, because juvenile plants are present in observed populations, so recruitment appears to be occurring. In this study, dehisced fruits were “rapidly visited by several unidentified species of ants of different sizes.” The “scant pulp” was harvested by smaller ants, and larger ants carried away the seeds after “cleaning them from adhering pulp.”

The authors propose at least two reasons why hummingbirds avoid the flowers of D. rhodacantha. The first being that the native hummingbirds have bills that are too short to reach the nectar inside the long tubular flowers, and often the flowers barely extend beyond the spines of the cactus which may deter the hummingbirds from approaching. The second reason is that other plants in the area flower during the same period and have nectar that is easier to gather. The authors acknowledge that this is just speculation, but it could help explain why the flowers are pollinated instead by an insect (the opportunist, generalist halictid bee species) for whom the flowers “could be considered to be ill adapted.” The authors go on to say, “it should be kept in mind, however, that adaptions do not have to be perfect, as long as they work sufficiently well.”

Patagona gigas (giant hummingbird) was observed approaching the flower of a Denmoza rhodacantha but quickly turned away (photo credit: www.eol.org)

Patagona gigas (giant hummingbird) was observed approaching the flower of a Denmoza rhodacantha but quickly turned away (photo credit: www.eol.org)

More Year of Pollination posts on Awkward Botany:

Year of Pollination: Pollination Syndromes and Beyond

A discussion of pollination syndromes should begin with the caveat that they are a largely outdated way to categorize plant-pollinator interactions. Still, they are important to be aware of because they have informed so much of our understanding about pollination biology, and they continue to be an impetus for ongoing research. The concept of pollination syndromes exists in part because we are a pattern seeking species, endeavoring to place things in neat little boxes in order to make sense of them. This is relatively easy to do in a hypothetical or controlled environment where the parameters are selected and closely monitored and efforts are made to eliminate noise. However, the real world is considerably more dynamic than a controlled experiment and does not conform to black and white ways of thinking. Patterns are harder to unveil, and it takes great effort to ensure that observed patterns are genuine and not simply imposed by our pattern seeking brains.

That being said, what are pollination syndromes?  Pollination syndromes are sets of floral traits that are thought to attract specific types of pollinators. The floral traits are considered to have evolved in order to appeal to a particular group of pollinators – or in other words, selective pressures led to adaptations resulting in mutualistic relationships between plants and pollinators. Pollination syndromes are examples of convergent evolution because distantly related plant species have developed similar floral traits, presumably due to similar selection pressures. Pollination syndromes were first described by Italian botanist, Federico Delpino, in the last half of the 19th century. Over several decades his rudimentary ideas were fleshed out by other botanists, resulting in the method of categorization described (albeit briefly) below.

Honey bee on bee's friend (Phacelia tanacetifolia)

A honey bee getting friendly with bee’s friend (Phacelia tanacetifolia)

Pollination by bees (melittophily) – Flowers are blue, purple, yellow, or white and usually have nectar guides. Flowers are open and shallow with a landing platform. Some are non-symmetrical and tubular like pea flowers. Nectar is present, and flowers give off a mild (sometimes strong) sweet scent.

Pollination by butterflies (psychophily) – Flowers are pink, purple, red, blue, yellow, or white and often have nectar guides. They are typically large with a wide landing pad. Nectar is inside a long, narrow tube (or spur), and flowers have a sweet scent.

Pollination by hawkmoths and moths (sphingophily and phalaenophily) – Moth pollinated flowers open at night, have no nectar guides, and emit a strong, sweet scent. Flowers pollinated by hawkmoths are often white, cream, or dull violet and are large and tubular with lots of nectar. Those pollinated by other moths are smaller, not as nectar rich, and are white or pale shades of green, yellow, red, purple, or pink.

Pollination by flies (myophily or sapromyophily) – Flowers are shaped like a basin, saucer, or kettle and are brown, brown-red, purple, green, yellow, white, or blue.  Some have patterns of dots and stripes. If nectar is available, it is easily accessible. Their scent is usually putrid. A sapromyophile is an organism that is attracted to carcasses and dung. Flies that fall into this category visit flowers that are very foul smelling, offer no nectar reward, and essentially trick the fly into performing a pollination service.

Pollination by birds (ornithophily) –  Flowers are usually large, tubular, and red, orange, white, blue, or yellow. They are typically without nectar guides and are odorless since birds don’t respond to scent. Nectar is abundant and found at various depths within the flower.

Pollination by bats (chiropterophily) – Flowers are large, tubular or bell shaped, and white or cream colored with no nectar guides. They open at night, have abundant nectar and pollen, and have scents that vary from musty to fruity to foul.

Pollination by beetles (cantharophily) – Flowers are large and bowl shaped and green or white. There are no nectar guides and usually no nectar. The scent is strong and can be fruity, spicy, or putrid. Like flies, some beetles are sapromyophiles.

Locust borer meets rubber rabbitbrush (Ericameria nauseosa)

A locust borer meets rubber rabbitbrush (Ericameria nauseosa)

In addition to biotic pollination syndromes, there are two abiotic pollination syndromes:

Pollination by wind (anemophily) – Flowers are miniscule and brown or green. They produce abundant pollen but no nectar or odor. The pollen grains are very small, and the stigmas protrude from the flower in order to capture the windborne pollen.

Pollination by water (hydrophily) –  Most aquatic plants are insect-pollinated, but some have tiny flowers that release their pollen into the water, which is picked up by the stigmas of flowers in a similar manner to plants with windborne pollen.

This is, of course, a quick look at the major pollination syndromes. More complete descriptions can be found elsewhere, and they will differ slightly depending on the source. It’s probably obvious just by reading a brief overview that there is some overlap in the floral traits and that, for example, a flower being visited by a bee could also be visited by a butterfly or a bird. Such an observation explains, in part, why this method of categorizing plant-pollinator interactions has fallen out of favor. Studies have been demonstrating that this is not a reliable method of predicting which species of pollinators will pollinate certain flowers. A close observation of floral visitors also reveals insects that visit flowers to obtain nectar, pollen, and other items, but do not assist in pollination. These are called robbers. On the other hand, a plant species may receive some floral visitors that are considerably more effective and reliable pollinators than others. What is a plant to do?

Pollination syndromes imply specialization, however field observations reveal that specialization is quite rare, and that most flowering plants are generalists, employing all available pollinators in assisting them in their reproduction efforts. This is smart, considering that populations of pollinators fluctuate from year to year, so if a plant species is relying on a particular pollinator (or taxonomic group of pollinators) to aid in its reproduction, it may find itself out of luck. Considering that a flower may receive many types of visitors on even a semi-regular basis suggests that the selective pressures on floral traits may not solely include the most efficient pollinators, but could also include all other pollinating visitors and, yes, even robbers. This is an area where much more research is needed, and questions like this are a reason why pollination biology is a vibrant and robust field of research.

A bumble bee hugs Mojave sage (Salvia pachyphylla)

A bumble bee hugs the flower of a blue sage (Salvia pachyphylla)

Interactions between plants and pollinators is something that interests me greatly. Questions regarding specialization and generalization are an important part of these interactions. To help satiate my curiosity, I will be reading through a book put out a few years ago by the University of Chicago Press entitled, Plant-Pollinator Interactions: From Specialization to Generalization, edited by Nickolas M. Waser and Jeff Ollerton. You can expect future posts on this subject as I read through the book. To pique your interest, here is a short excerpt from Waser’s introductory chapter:

Much of pollination biology over the past few centuries logically focused on a single plant or pollinator species and its mutualistic partners, whereas a focus at the level of entire communities was uncommon. Recently we see a revival of community studies, encouraged largely by new tools borrowed from the theory of food webs that allow us to characterize and analyze the resulting patterns. For example, pollination networks show asymmetry – most specialist insects visit generalist plants, and most specialist plants are visited by generalist insects. This is a striking departure from the traditional implication of coevolved specialists!

References:

Year of Pollination: The Anatomy of a Bee

A greater appreciation for pollinators can be had by learning to identify them – being able to tell one from another and calling them by name. Anyone can tell a butterfly from a bee, but how about telling a sweat bee from a leafcutter bee? Or one species of sweat bee from another species of sweat bee? That takes more training. This is where knowing the parts of a bee becomes important.

I am new to learning the names of pollinators. I’ve been learning the names of plants for many years now (and I still have a long way to go), but my knowledge of insect identification is largely limited to one entomology course I took in college and the occasional reading about insects in books and magazines. So, this post is just as much for me as it is for anybody else. It also explains why it is brief and basic. It’s for beginners.

This first illustration is found in the book Pollinators of Native Plants by Heather Holm. The book starts with brief overviews of pollination, pollinators, and pollinator conservation, but then spends nearly 200 pages profiling specific plants and describing the particular species of pollinating insects that visit them. The photos of the insects are great and should be very useful in helping to identify pollinators.

bee anatomy_pollinators of native plants book

This next illustration is from the book California Bees and Blooms by Gordon W. Frankie, et al. The title is a bit deceptive because much of what is found in this book is just as applicable to people outside of California as it is to people within. There is some discussion about plants and pollinators specific to California and the western states, but there is also a lot of great information about bees, flowers, and pollination in general, including some great advice on learning to identify bees. The book includes this basic diagram, but it also provides several other more detailed illustrations that help further describe things like mouth parts, wings, and legs.

bee anatomy_california bees and blooms book

As part of their discussion on identifying bees, the authors of California Bees and Blooms offer these encouraging and helpful words to beginners like me: “Even trained taxonomists must examine most bees under a microscope to identify them to species level, but knowing the characteristics to look for can give you a pretty good idea of the major groups and families of bees that are visiting your garden. These include size, color, and features of the head, thorax, wings, and abdomen.”

If you would like to know more about the pollinators found in your region, including their names, life history, and the plants they visit, books like the aforementioned are a good start. Also, find yourself a copy of a field guide for the insects in your area and a good hand lens. Then spend some time outside closely and quietly observing the busy lives of the tiny things around you. I plan to do more of this sort of thing, and I am excited see what I might find. Let me know what you find.

Here are a few online resources for learning more about bee anatomy and bee identification:

Other “Year of Pollination” Posts:

Year of Pollination: Hellstrip Pollinator Garden

This month I have been reading and reviewing Evelyn Hadden’s book, Hellstrip Gardening, and I have arrived at the fourth and final section, “Curbside-Worthy Plants.” As the title suggests, this section is a list of plants that Hadden has deemed worthy of appearing in a curbside garden. It’s not exhaustive, of course, but with over 100 plants, it’s a great start. Photos and short descriptions accompany each plant name, and the plants are organized into four groups: showy flowers, showy foliage, culinary and medicinal use, and four-season structure.

This list is useful and fun to read through, but there isn’t much more to say about it beyond that. So I have decided to write this month’s Year of Pollination post about creating a hellstrip pollinator garden using some of the plants on Hadden’s list. Last year around this time I wrote about planting for pollinators where I listed some basic tips for creating a pollinator garden in your yard. It’s a fairly simple endeavor – choose a sunny location, plant a variety of flowering plants that bloom throughout the season, and provide nesting sites and a water source. If this sounds like something you would like to do with your hellstrip, consider planting some of the following plants.

Spring Flowers

Spring flowering plants are an important food source for pollinators as they emerge from hibernation and prepare to reproduce. There are several spring flowering trees and shrubs on Hadden’s list. Here are three of them:

  • Amelanchier laevis (Allegheny serviceberry) – A multi-trunked tree or large shrub that flowers early in the spring. Other small trees or shrubs in the genus Amelanchier may also be suitable.
  • Cercis canadensis (eastern redbud) – A small tree that is covered in tiny, vibrant, purple-pink flowers in early spring.
  • Ribes odoratum (clove currant) – A medium sized shrub that flowers in late spring. Try other species of Ribes as well, including one of my favorites, Ribes cereum (wax currant).

There aren’t many spring flowering herbaceous plants on Hadden’s list, but two that stood out to me are Amsonia hubrichtii (bluestar) and Polemonium reptans (creeping Jacob’s ladder).

Creeping Jacob's ladder (Polemonium reptens) is native to eastern North America and attracts native bees with its mid-spring flowers. (photo credit: www.eol.org)

Creeping Jacob’s ladder (Polemonium reptens) is native to eastern North America and attracts native bees with its mid-spring flowers. (photo credit: www.eol.org)

Summer Flowers

There is no shortage of summer flowering plants, and Hadden’s list reflects that. When planting a pollinator garden, be sure to include flowers of different shapes, sizes, and colors in order to attract the greatest diversity of pollinators. Here are a few of my favorite summer flowering plants from Hadden’s list:

  • Amorpha canescens (leadplant) – A “good bee plant” and also a nitrogen fixer.
  • Asclepias tuberosa (butterfly weed) – “Valuable pollinator plant and larval host for monarch, gray hairstreak, and queen butterflies.” I love the tight clusters of deep orange flowers on this plant.
  • Coreopsis verticillata (threadleaf coreopsis) – I really like coreopsis (also known as tickseed). Try other species in the genus as well.
  • Penstemon pinifolius (pineleaf penstemon) – North America is bursting with penstemon species, especially the western states. All are great pollinator plants. Pineleaf penstemon is widely available and great for attracting hummingbirds.
  • Salvia pachyphylla (Mojave sage) – A very drought-tolerant plant with beautiful pink to purple to blue inflorescences. Salvia is another genus with lots of species to choose from.
  • Scutellaria suffratescens  (cherry skullcap) – A good ground cover plant with red-pink flowers that occur from late spring into the fall.
The flowers of butterfly weed (Asclepias tuberosa). Milkweed species (Asclepias spp.) are essential to monarch butterflies as they are the sole host plant of their larvae.

The flowers of butterfly weed (Asclepias tuberosa). Milkweed species (Asclepias spp.) are essential to the survival of monarch butterflies as they are the sole host plant of their larvae.

Fall Flowers

Fall flowering plants are essential to pollinators as they prepare to migrate and/or hibernate. Many of the plants on Hadden’s list start flowering in the summer and continue into the fall. A few are late summer/fall bloomers. Here are some of my favorites:

  • Epilobium canum (California fuchsia) – “Profuse orange-red tubular flowers late summer into fall furnish late-season nectar, fueling hummingbird migration.”
  • Liatris punctata (dotted blazing star) – Drought-tolerant plant with tall spikes of purple-pink flowers. “Nectar fuels migrating monarchs.”
  • Symphyotrichum oblongifolium (aromatic aster) – Loaded with lavender-blue flowers in the fall. It’s a spreading plant, so prune it back to keep it in check. Hadden recommends it for sloped beds.
  • Agastache rupestris (sunset hyssop) – Spikes of “small tubular flowers in sunset hues attract hummingbirds, butterflies, and bees midsummer to fall.” Try other species in the Agastache genus as well.
  • Monarda fistulosa (wild bergamot) – The unique flower heads are like magnets to a wide variety of pollinators. Also consider other Monarda species.
Lemon beebalm (Monarda citriodora), an annual plant that attracts an array of pollinators.

Lemon beebalm (Monarda citriodora), an annual plant that attracts an array of pollinators.

As with any other garden, your hardiness zone, soil conditions, water availability, and other environmental factors must be considered when selecting plants for your hellstrip pollinator garden. Groups like Pollinator Partnership and The Xerces Society have guides that will help you select pollinator friendly plants that are suitable for your region. Additionally, two plans for “boulevard pollinator gardens” complete with plant lists are included in the book Pollinators of Native Plants by Heather Holm – one plan is for sunny and dry spots and the other is for shady and wet spots (pgs. 268-269). Once your pollinator garden is complete, consider getting it certified as a pollinator friendly habitat. There are various organizations that do this, such as the Environmental Education Alliance of Georgia. If you are interested in such a thing, the public nature of your hellstrip garden makes it an ideal place to install a sign (like the one sold in The Xerces Society store) announcing your pollinator garden and educating passersby about the importance of pollinator conservation.

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Other “Year of Pollination” Posts

Ground Nesting Bees in the Garden

Earlier this year I wrote about planting for pollinators. In that post I briefly introduced various things that people can do to encourage pollinator activity in their yards and gardens. One thing that I mentioned was the importance of providing nesting sites. Most pollinators are insects and insects are small, so the distance that they are able to travel in search of food is relatively limited. According to the Xerces Society, the smallest bees can only fly a few hundred feet from their nests. Providing nesting sites in close proximity to foraging sites is incredibly important.

Roughly 70% of native bee species in North America are ground nesting bees, so chances are pretty good that if you are providing forage for bees in your yard, a good number of the bees that visit will be ground nesting bees. In order to ensure the survival of these bees, consider providing nesting habitat for them on your property.

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Lasioglossum leucozonium – a North American ground nesting bee (also known as a sweat bee) – photo credit: www.eol.org

Here are a few things to keep in mind when developing nesting habitat for ground nesting bees:

Create and Maintain Undisturbed Bare Ground: You may already have ground nesting bees living in your yard and you don’t even know it. Obvious evidence of nests is difficult to spot. If you can find tunnel entrances, they will look like small ant mounds. If you find a series of small “ant mounds”, watch for bee activity during sunny times of the day. Activity can be quite ephemeral though, so it is difficult to know if bees have just moved in or if they have moved on. Avoid tilling up soil and walking through areas where you suspect or intend for bee activity. Leave patches of bare ground unplanted and unmulched in order to encourage bees to nest there.

Sunny and South Facing: Bees are most active when the sun is shining and temperatures are warm. For this reason they tend to build their nests in warm, sunny spots. However, warm, sunny spots are also the best locations for many plants. Consider sharing these sites with ground nesting bees. Avoid putting down mulch in these areas and keep vegetation sparse and minimal.

Avoid Pesticides: When encouraging pollinator activity in your yard and garden, it is best to avoid using pesticides as much as possible. Herbicides kill potential food sources. Insecticides can kill pollinating insects along with pest insects. And soil fumigants can harm ground nesting bees.

Provide Some Accommodations: Due to the diversity of ground nesting bees, it is difficult to provide nesting habitat for all potential species. Some prefer loose, sandy soil while others prefer smooth, packed ground. Some bees will nest on level ground, while others prefer sloped ground. The habitat you are able to provide will depend on the conditions present on your property. Some modifications can be made, but this all depends on the resources available to you and how particular you want to get. Apart from maintaining a patch of undisturbed, unmulched, south facing ground, there are three additional things that you can offer ground nesting bees to make them feel more at home on your property: food (in the form of diverse flowers blooming throughout the growing season), a water source (in the form of a birdbath or something similar), and a few rocks for the bees to perch on and warm their tiny bodies.

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The tunnel entrance of a ground nesting bee.

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Tunnel entrances are often found in groups in areas of bare ground mixed with patchy vegetation.