Summer of Weeds: Common Mullein

The fuzzy, gray-green leaves of common mullein are familiar and friendly enough that it can be hard to think of this plant as a weed. Verbascum thapsus is a member of the figwort family and is known by dozens of common names, including great mullein, Aaron’s rod, candlewick, velvet dock, blanket leaf, feltwort, and flannel plant. Its woolly leaves are warm and inviting and have a history of being used as added padding and insulation, tucked inside of clothing and shoes. In Wild Edible and Useful Plants of Idaho, Ray Vizgirdas writes, “the dried stalks are ideal for use as hand-drills to start fires; the flowers and leaves produce yellow dye; as a toilet paper substitute, the large fresh leaves are choice.”

Common mullein is a biennial that was introduced to eastern North America from Eurasia in the 1700’s as a medicinal plant and fish poison. By the late 1800’s it had reached the other side of the continent. In its first year it forms a rosette of woolly, oblong and/or lance-shaped leaves. After overwintering it produces a single flower stalk up to 6 feet tall. The woolly leaves continue along the flower stalk, gradually getting smaller in size until they reach the inflorescence, which is a long, dense, cylindrical spike. Sometimes the stalk branches out to form multiple inflorescences.

First year seedlings of common mullein (Verbascum thapsus)

The inflorescence doesn’t flower all at once; instead, a handful of flowers open at a time starting at the bottom of the spike and moving up in an irregular pattern. The process takes several weeks to complete. The flowers are about an inch wide and sulfur yellow with five petals. They have both female and male sex parts but are protogynous, meaning the female organs mature before the male organs. This encourages cross-pollination by insects. However, if pollination isn’t successful by the end of the day, the flowers self-pollinate as the petals close. Each flower produces a capsule full of a few hundred seeds, and each plant can produce up to 180,000 seeds. The seeds can remain viable for over 100 years, sitting in the soil waiting for just the right moment to sprout.

Common mullein is a friend of bare, recently disturbed soil. It is rare to see this plant growing in thickly vegetated areas. As an early successional plant, its populations can be abundant immediately after a disturbance, but they do not persist once other plants have filled in the gaps. Instead they wait in seed form for the next disturbance that will give them the opportunity to rise again. They can be a pest in gardens and farm fields due to regular soil disturbance, and are often abundant in pastures and rangelands because livestock avoid eating their hairy leaves. Because of its ephemeral nature, it is generally not considered a major weed; however, it is on Colorado’s noxious weed list.

Several features make common mullein a great example of a drought-adapted plant. Its fleshy, branching taproot can reach deep into the soil to find moisture, the thick hairs on the leaves help reduce water loss via transpiration, and the way the leaves are arranged and angled on the stalk can help direct rain water down toward the roots.

Common mullein has an extensive history of ethnobotanical uses. Medicinally it has been used internally to treat coughs, colds, asthma, bronchitis, and kidney infections; and as a poultice to treat warts, slivers, and swelling. The dried flower stalks have been used to make torches, and the fuzzy leaves have been used as tinder for fire-making and wicks in lamps.

The hairy leafscape of common mullein (Verbascum thapsus)

More Resources:

Quote of the Week:

From Gaia’s Garden by Toby Hemenway

Here’s why opportunistic plants are so successful. When we clear land or carve a forest into fragments, we’re creating lots of open niches. All that sunny space and bare soil is just crying out to be colongized by light- and fertlity-absorbing green matter. Nature will quickly conjure up as much biomass as possible to capture the bounty, by seeding low-growing ‘weeds’ into a clearing or, better yet, sprouting a tall thicket stretching into all three dimensions to more effectively absorb light and develop deep roots. … When humans make a clearing, nature leaps in, working furiously to rebuild an intact humus and fungal layer, harvest energy, and reconstruct all the cycles and connections that have been severed. A thicket of fast-growing pioneer plants, packing a lot of biomass into a small space, is a very effective way to do this. … And [nature] doesn’t care if a nitrogen fixer or a soil-stabilizing plant arrived via continental drift or a bulldozer’s treads, as long as it can quickly stitch a functioning ecosystem together.

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Love and Hate – The Story of Purple Loosestrife

In the early 1800’s, seeds of purple loosestrife found their way to North America. They arrived from Europe several times by various means – accidentally embedded in the ballast of ships, inadvertently tucked in sheep’s wool, and purposely carried in the hands of humans. Native to much of Europe and parts of Asia and commonly found growing in wetlands and other riparian areas, purple loosestrife’s appealing spikes of magenta flowers, sturdy, upright growth habit, and ease of propagation made it a prized ornamental; its abundant nectar made it a favorite of beekeepers.

During its first 150 years or so in North America, purple loosestrife became naturalized in ditches, wet meadows, and the banks of streams, rivers, lakes, and ponds while also enjoying a place in our gardens. Concern about its spread was raised in the first half of the twentieth century, but it wasn’t until the 1980’s after an extensive survey was done and a special report was issued by the U.S. Fish and Wildlife Service that attitudes about purple loosestrife shifted dramatically. At that point, it was no longer a benign invader and welcome garden companion. It was, instead, a biological menace that needed to be destroyed.

Lytrhrum salicaria – commonly known as purple loosestrife, spiked willow-herb, long purples, rainbow weed, etc. – is an herbaceous perennial in the family Lythraceae. It reaches up to two meters tall; has square or angular stems with lance-shaped, stalkless leaves up to ten centimeters long; and ends in dense, towering spikes of pink-purple, 5-7 petaled flowers. The flowers attract a wide variety of pollinating insects – mostly bees – and afterwards produce small capsules full of tiny, red-brown seeds. Charles Darwin thoroughly studied the flowers of purple loosestrife; he was intrigued by the plant for many reasons, including its heterostyly (a topic for another post).

Lythrum salicaria (purple loosestrife) – image credit: wikimedia commons

Purple loosestrife seeds remain viable for up to 20 years and are transported by wind, water, and in mud stuck to the feet of birds. Apart from seeds, populations expand clonally as root crowns grow larger each year and produce increasingly more stems. Broken stem pieces also take root in mud, creating new plants. Purple loosestrife’s ability to form expansive populations in a quick manner, pushing other plants aside and forming what appears to be a dense monoculture, is part of the reason it has earned itself a place among the International Union for Conservation of Nature’s list of 100 World’s Worst Invasive Alien Species.

But is this ranking justified? In a paper published in Biological Invasions in 2010, Claude Lavoie compares news reports about purple loosestrife around the turn of the century with data presented in scientific papers and finds that the reports largely exaggerate the evidence. Purple loosestrife was being accused of all manner of crimes against nature and was being condemned before there was sound evidence to justify such actions.

It began with the U.S. Fish and Wildlife Service’s special report published in 1987. According to Lavoie, “a long list of the impacts of the species on wetland flora and fauna [was] presented,” but the claims were not supported by observational or experimental data – “the impacts [were] only suspected.” Regardless, wetland managers began campaigns against purple loosestrife in order to convince the public that it was a Beautiful Killer. News outlets were quick to spread the word about this “killer” plant. When biological control programs began in the 1990’s, news outlets reported on their success. Little empirical evidence had been published on either topic, and debates about purple loosestrife’s impacts remained unsettled in the scientific community.

The flowers of purple loosestrife (Lythrum salicaria) – photo credit: wikimedia commons

Around this time, five reviews were published examining the evidence against purple loosestrife. Lavoie reports that all but one of them “rely on a relatively high number of sources that have not been published in peer-reviewed journals.” After examining the reviews, Lavoie concludes: “although each review provided valuable information on purple loosestrife, most were somewhat biased and relied on a substantial amount of information that was anecdotal or not screened by reviewers during a formal evaluation process. Only one review was impartial, and this one painted an inconclusive picture of the species.”

Research has continued regarding the impacts of purple loosestrife, and so Lavoie examined 34 studies that were published during the 2000’s in search of conclusive evidence that the plant is as destructive to wetlands and wildlife as has been claimed. Upon examination he concludes that “stating that this plant has ‘large negative impacts’ on wetlands is probably exaggerated.” The most common accusation – that purple loosestrife crowds out native plants and forms a monoculture – “is controversial and has not been observed in nature (with maybe one exception).” Lavoie finds that there is “certainly no evidence that purple loosestrife ‘kills wetlands’ or ‘creates biological deserts,'” and “there are no published studies [in peer-reviewed journals] demonstrating that purple loosestrife has an impact on waterfowl or fishes.” All other negative claims against purple loosestrife “have not been the object of a study,” except for its impact on amphibians, which had at that time only been tested on two species, one “reacting negatively.” Certain claims – such as purple loosestrife’s impact on wetland hydrology – should be studied more in depth “considering the apparent public consensus on the detrimental effects of purple loosestrife” on wetland ecosystems.

Lavoie agrees that it is reasonable to control purple loosestrife when the intention is to reduce additional pressures on an ecosystem that is already highly threatened. However, he warns that “focusing on purple loosestrife instead of on other invasive species or on wetland losses to agriculture or urban sprawl could divert the attention of environmental managers from more urgent protection needs.” There is mounting evidence that purple loosestrife invasions are disturbance-dependent and are “an indicator of anthropogenic disturbances.” In order to protect our wetlands, we must first protect them against undue disturbance. Lavoie supports using the Precautionary Principle when dealing with introduced species; however, he finds the approach “much more valuable for newcomers than for invaders coexisting with native species for more than a century.”

A field of purple loosestrife in Massachusetts – photo credit: wikimedia commons

Purple loosestrife has found its way to nearly every state in America and most of the Canadian provinces. Peter Del Tredici writes in Wild Urban Plants of the Northeast, “Conservationists despise purple loosestrife, despite its beauty, and it is listed as an invasive species in most of the states where it grows.” By listing a plant as a noxious weed, landowners are obligated to remove it. Care must be taken though, as removal of purple loosestrife can result in a secondary invasion by noxious weeds with an even worse track record, such as common reed or reed canary grass. “Hardly a gain from the biodiversity point of view,” quips Lavoie.

Claude Lavoie’s paper and the papers he references are definitely worth reading. It is important that we continue to study purple loosestrife and species like it in order to fully understand the impact that introduced species are having on natural areas, especially since it is unlikely that we will ever completely eliminate them. On that note, I’ll leave you with this passage from The Book of Swamp and Bog by John Eastman:

The situation is easy for environmentalists to deplore. This plant, like few others, stirs our alien prejudice. Our native cattails, for example, are almost as rudely aggressive and competitive in many wetland areas as purple loosestrife. Yet, because cattails obvioulsy ‘belong here,’ they seldom evoke the same outraged feelings against their existence. … With the spread of purple loosestrife, we have new opportunities to witness the phases of an ever-recurring ecological process. We can watch it affect, change, adapt, and refit both its own elements and those of invaded communities into new arrangements of energy efficiency. The point is that we might as well study this process rather than simply deplore it; we have few alternatives.

Campaigns Against Invasive Species, part one

I have been posting almost exclusively about invasive species for four months now. If you have made it this far, I salute you. It is neither the most exciting nor the most encouraging topic, but it is the journey I am on (for whatever reason), and I am pleased to have you along.

In the battle against invasive species, citizen awareness and participation is imperative. The public and private sectors can try as they may, but if individual citizens are acting in ways that help introduce or spread invasives, then much of this effort can be for naught. Thus, campaigns to educate the public are regularly launched.

One popular way to spread the word is through video. Often, the goal of these videos is to both educate and entertain. Some achieve this better than others, while some are downright dull or simply baffling. Speculating on the effectiveness of these videos is not the purpose of this post. Rather, I just thought I would take a break from the usual text heavy posts and share a few videos that I found interesting and/or entertaining. If you have a favorite invasive species video, please share it in the comment section below.

Invasive species explained:

Introducing Bob Noxious from Invasive Species of Idaho:

And here is the particularly creepy, Vin Vasive, from USDA APHIS:

Invaders! in British Columbia:

In Namibia, “Cacti must die!”:

Eco Sapien and the story of Japanese knotweed in the UK:

What happened when American minks, brought to Europe for the fur trade, escaped into natural areas?:

Michigan’s Department of Environmental Quality explains how invasive species spread:

Pennsylvania’s Wild Resource Conservation Program teaches kids about invasive species:

MinuteEarth‘s take on invasive species:

Also, check out these five TEDx talks:

Screening for Invasive Plants at Botanical Gardens and Arboreta

As discussed in last week’s post, many of the invasive species that we find in our natural areas were first introduced to North America via the horticulture trade. As awareness of this phenomenon grows, steps are being taken by the horticulture industry to address this issue. The concluding remarks by Sarah Reichard and Peter White in their 2001 article in BioScience describe some recommended actions. One of them involves the leadership role that botanical gardens can play by both stopping the introduction and spread of invasive species and by presenting or promoting public education programs.

Reichard and White offer North Carolina Botanical Garden as an example, citing their “Chapel Hill Challenge,” which urges botanical gardens to “do no harm to plant diversity and natural areas.” Reichard and White also encourage botanical gardens and nurseries to adopt a code of conservation ethics addressing invasive species and other conservation issues. Codes of conduct for invasive species have since been developed for the botanical garden community and are endorsed by the American Public Gardens Association.

 

Botanical gardens that adopt this code have a number of responsibilities, one of which is to “establish an invasive plant assessment procedure,” preferably one that predicts the risks of plant species that are new to the gardens. In other words, botanical gardens are encouraged to screen the plants that are currently in their collections, as well as plants that are being added, to determine whether these plants currently exhibit invasive behavior or have the potential to become invasive. Many botanical gardens now have such programs in place, and while they may not be able to predict all invasions, they are a step in the right direction.

In an article published in Weed Technology (2004), staff members at Chicago Botanic Garden (CBG) describe the process they went through to determine a screening process that would work for them. CBG has an active plant exploration program, collecting plants in Asia, Europe, and other parts of North America. Apart from adding plants to their collection, one of the goals of this program is to find plants with horticulture potential and, through their Ornamental Plant Development department, prepare these plants to be introduced to the nursery industry in the Chicago region. As their concern about invasive species has grown, CBG (guided by a robust Invasive Plant Policy) has expanded and strengthened its screening process.

In order to do this, CBG first evaluated three common weed risk assessment models. The models were modified slightly in order to adapt them to the Chicago region. Forty exotic species (20 known invasives and 20 known non-invasives) were selected for testing. Each invasive was matched with a noninvasive from the same genus, family, or growth form in order to “minimize ‘noise’ associated with phylogenetic differences.” The selected species also included an even distribution of forbs, vines, shrubs, and trees.

Weed risk assessment models are used to quickly determine the potential of a plant species to become invasive by asking a series of questions about the plant’s attributes and life history traits, as well as its native climate and geography. A plant species can be accepted, rejected, or require further evaluation depending on how the questions are answered. For example, if a plant is known to be invasive elsewhere and/or if it displays traits commonly found in other invasive species, it receives a high score and is either rejected or evaluated further. Such models offer a quick and affordable way to weed out incoming invasives; however, they are not likely to spot every potential invasive species, and they may also lead to the rejection of species that ultimately would not have become invasive.

After testing the three models, CBG settled on the IOWA-modified Reichard and Hamilton model “because it was extensively tested in a climatic zone reasonably analogous to … Illinois,” and because it is easy to use and limits the possibility of a plant being falsely accepted or rejected. The selected model was then tested on 208 plants that were collected in the Republic of Georgia. Because few details were known about some of the plants, many of the questions posed by the model could not be answered. This lead CBG to modify their model to allow for such plants to be grown out in quarantined garden plots. This way pertinent information can be gathered, such as “duration to maturity; self-compatibility; fruit type and potential methods of seed or fruit dispersal; seed production, viability, and longevity in the field; and vegetative spread.” CBG believes that evaluations such as this will help them modify their model over time and give them more confidence in their screening efforts.

More about botanical gardens and invasive species: Botanic Gardens Conservation International – Invasive Alien Species

More about weed risk assessment models: Weed Risk Assessment – A way forward or a waste of time? by Philip E. Hulme

Alien Plant Invasions and the Extinction Trajectory

One of the concerns about introduced species becoming invasive is that they threaten to reduce the biodiversity of the ecosystems they have invaded. They do this by spreading rampantly, using up resources and space, altering ecosystem functions, and ultimately pushing other species out. In the case of certain invasive animals, species may be eliminated via predation; but plants don’t eat each other (generally), so if one plant species is to snuff out another plant species it must use other means. Presently, we have no evidence that a native plant species has been rendered extinct solely as a result of an invasive plant species. That does not mean, however, that invasive plants are not doing harm.

In a paper published in AoB Plants in August 2016, Paul O. Downey and David M. Richardson argue that, when it comes to plants, focusing our attention on extinctions masks the real impact that invasive species can have. In general, plants go extinct more slowly than animals, and it is difficult to determine that a plant species has truly gone extinct. Some plants are very long-lived, so the march towards extinction can extend across centuries. But the real challenge – after determining that there are no above-ground signs of life – is determining that no viable seeds remain in the soil (i.e. seed bank). Depending on the species, seeds can remain viable for dozens (even hundreds) of years, so when conditions are right, a species thought to be extinct can emerge once again. (Consider the story of the Kankakee mallow.)

On the other hand, there is plenty of evidence that invasive plant species have had significant impacts on certain native plant populations and have placed such species on, what Downey and Richardson call, an extinction trajectory. It is this trajectory that deserves our attention if our goal is to save native plant species from extinction. As described in the paper, the extinction trajectory has six steps – or thresholds – which are defined in the infographic below:

6-threshold-extinction-trajectory

Downey and Richardson spend a portion of the paper summarizing research that demonstrates how invasive plants have driven native plants into thresholds 1-3, thereby placing them on an extinction trajectory. In New Zealand, Lantana camara (introduced from the American tropics) creates dense thickets, outcompeting native plants. Researchers found that species richness of native plants declined once L. camara achieved 75% cover in the test sites. In the U.S., researchers found reduced seed set in three native perennial herbs as a result of sharing space with Lonicera mackii (introduced from Asia), suggesting that the alien species is likely to have a negative impact on the long-term survivability of these native plants. Citing such research, Downy and Richardson conclude that “it is the direction of change that is fundamentally important – the extinction trajectory and the thresholds that have been breached – not whether a native plant species has actually been documented as going extinct due to an alien plant species based on a snapshot view.”

Introduced to New Zealand from the American tropics, largeleaf lantana (Lantana camara) forms dense thickets that can outcompete native plant species. (photo credit: wikimedia commons)

Introduced to New Zealand from the American tropics, largeleaf lantana (Lantana camara) forms dense thickets that can outcompete native plant species. (photo credit: wikimedia commons)

In support of their argument, they also address problems with the way some research is done (“in many instances appropriate data are not collected over sufficiently long periods,” etc.), and they highlight the dearth of data and research (“impacts associated with most invasive alien plants have not been studied or are poorly understood or documented”). With those things in mind, they make recommendations for improving research and they encourage long-term studies and collaboration in order to address the current “lack of meta analyses or global datasets.” A similar recommendation was made in American Journal of Botany in June 2015.

The language in this report makes it clear that the authors are responding to a certain group of people that have questioned whether or not the threat of invasive plants has been overstated and if the measures we are taking to control invasive plants are justified. The following cartoon that appeared along with a summary of the article way oversimplifies the debate:

04_figure2

Boy: There are no studies that show weeds cause native plants to go extinct, thus we should not control them. Plant: If we wait until then, we’ll all be gone!!! Girl: Just because no one has demonstrated it does not mean that extinctions do not occur. The problem is not overstated!

It seems to me that a big part of why we have not linked an invasive plant species to a native plant species extinction (apart from the difficulty of determining with certainty that a plant has gone extinct) is that extinctions are often the result of a number of factors. The authors do eventually say that: “it is rare that one threatening process in isolation leads to the extinction of a species.” So, as much as it is important to fully understand the impacts that invasive plant species are having, it is also important to look at the larger picture. What else is going on that may be contributing to population declines?

Observing invaded plant populations over a long period seems like our best bet in determining the real effects that invasive species are having. In some cases, as Downey and Richardson admit, “decreased effects over time” have been documented, and so “the effects [of invasive species] are dynamic, not static.” And speaking of things that are dynamic, extinction is a dynamic process and one that we generally consider to be wholly negative. But why? What if that isn’t always the case? Extinctions have been a part of life on earth as long as life has been around. Is there anything “good” that can come out of them?

Confidential Carnivore

This is a guest post. Words and images by Jeremiah Sandler

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If you live in North America or Europe, chances are you have seen Dipsacus fullonum, commonly called teasel.  Its tall (up to 2 meters), spiky flower stalks with large purple flowers are easy to spot in low-lands, ditches, or along highways.  Since this prolific seeder’s introduction to North America from Europe, it has steadily increased its habitat to occupy nearly each region of the United States. Of course, like all plants, teasel has its preferences and is more frequent in some areas than in others.

dipsacus fullonum_jeremiah sandler

Teasel is an unassuming, herbaceous biennial.  It takes two years to complete its life cycle: First-year growth is spent as a basal rosette, and second-year growth is devoted to flowering.  Standard biennial, right?  As of 2011, an experiment was conducted on this plant that changed the way we see teasel, and possibly all other similar plants.

“Here we report on evidence for reproductive benefits from carnivory in a plant showing none of the ecological or life history traits of standard carnivorous species.” -Excerpt from the report titled Carnivory in the Teasel Dipsacus fullonum — The Effect of Experimental Feeding on Growth and Seed Set by Peter J.A. Shaw and Kyle Shackleton.

We all have favorite carnivorous plants, Venus flytraps, pitcher plants, sundews, etc.. Their showy traps and various means of attracting insects are all marvels of evolution in the plant kingdom.  These insectivorous plants evolved these means of nutrient acquisition in an answer to the lack of nutrients in their environment’s soil.  In some of these plants, there is a direct relationship between number of insects consumed and the size of the entire plant. In others, there is no such relationship.

The unassuming, biennial teasel can now join the ranks of carnivore, or protocarnivore.  It didn’t evolve in bogs or swamps where soil nutrients are depleted.  It has no relationship to the standard carnivorous species. It doesn’t have any flashy traps. In fact, it has no obvious traits which suggest it can gain nutrients from insects. Teasel’s carnivorous habits can be likened somewhat to the carnivorous habits of bromeliads; water gathered in their leaves traps insects.

In Shaw and Shackleton’s experiment (done in two field populations), maggots were placed in water gathered in the center of some first-year rosettes of teasel.  Other rosettes in the same population were left alone as controls.  Not surprisingly, the teasels which were ‘fed’ larvae did not change in overall size.  The size of the overwintering rosette did not offer any predictability towards the size of flower shoots for the coming year. However, something strange did happen:

“…addition of dead dipteran larvae to leaf bases caused a 30% increase in seed set and the seed mass:biomass ratio.”…“These results provide the first empirical evidence for Dipsacus displaying one of the principal criteria for carnivory”

Teasel has some physiology to absorb nutrients from other macroorganisms despite teasel evolving in an entirely different setting than typical carnivorous plants.  Teasel’s already proficient reproductive capacity is enhanced by using insects as a form of nutrients in a controlled setting.  

Many exciting questions have been raised by this experiment. How has this absorption mechanism come about, without the obvious use of lures or other structures to attract insects? And how does teasel maximize upon its own morphology in the wild, if at all?  What would the results be if these experiments were recreated on other similar species?

There are studies being conducted all the time that further the boundaries of what we know about these stationary organisms. There are new discoveries waiting just around the corner. Carnivory in plants is amazing because it transcends common notions about plants; especially in the case of the unassuming teasel.

Selected Resources:

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Jeremiah Sandler lives in southeast Michigan where he works in the plant health care industry. He is currently pursuing a degree in horticulture and an arborist license. He is interested in all things plant related and plans to own a horticulture business where he can share his passion with others. Follow Jeremiah on Instagram: @j4.sandler

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