Biodiversity Invasive Species Weeds

2023 February Weeds

The formal definition of a weed is any plant growing where you don’t want it. For gardeners struggling to eradicate weeds, it can seem like they have super powers. Well, maybe they do…

Whether from nature or a child’s puff, dandelion seeds are sensitive to wind direction, which helps them to disperse widely. Jon Feingersh Photography Inc./The Image Bank/Getty

Why dandelion seeds are so good at spreading widely

On any given dandelion, some seeds are destined to go north, while others are fated to fly east, south or west, and every direction in between. In effect, each dandelion seed is programmed to release for a wind coming from one direction and resists winds from other directions, according to research presented at the American Physical Society’s Division of Fluid Dynamics. Dandelion seeds are susceptible to different wind directions depending on where they are on the seed head, says Jena Shields, a biophysicist at Cornell University. The feathery seeds on the side facing a breeze will let go most easily; the others hold on tens to hundreds of times tighter — until the wind shifts. Shields measured the force it takes to pluck dandelion seeds by supergluing a fine wire to the tufted ends and pulling them from the seed heads at various angles. This seed-by-seed study mimicked what happens when wind, or a child’s breath, pushes them over. Because each seed is most susceptible to winds from distinct directions, it helps prevent seeds from all going the same way, Shields says, and may explain why the plants are so successful at spreading. Once blown off a dandelion, the umbrella-like tuft on a seed carries it on the breeze that pulled it away. In fact, it turns out the movement of air around dandelion seeds is teaching us a new way of moving through air, which also helps the seeds stay aloft much longer than would otherwise be the case!

Why we should all learn to love stinging nettles

Stinging nettles are food for peacock butterfly caterpillars. Keith Hider/Shutterstock

I still remember the itch and burn of stinging nettles (Urtica dioica) encountered while exploring wasteland areas as a child. We always used to rub the stinged skin with dock leaves (Rumex obtusifolius) to relieve the pain. Although there is no scientific evidence this actually works, it always made me feel better. Aside from their sting, which is their defense against being eaten, nettles offer a host of benefits. Permaculturalists love them because their fibrous roots, which make them such a pain to remove as weeds, help draw nutrients up from the subsoil and make them available to other plants. They are amazing at colonizing disturbed areas and bare soil – a useful trait at a time when human activity has disturbed so much nature. In addition to those persistent roots, Charles Darwin was right when he theorized that nettle seeds could withstand a long soak in salty water. This ability may have allowed the humble nettle to become a world traveller, colonizing areas across oceans. Those same seeds can lie dormant in soil for up to five years. Nettles are good for wildlife. They are larval host plants for Question Mark, Milbert’s Tortoiseshell, Red Admiral butterflies

Ouch! The silica hairs (trichomes) act as needles, injecting irritants into skin. Floki/Shutterstock

In the UK, nettles spreading to gardens and cultivated areas have allowed several species of butterflies to expand their range. Ladybugs often lay their eggs on nettles, and when those eggs hatch, the ladybug larvae are voracious eaters of aphids – one of the more persistent garden pests. Nettles are good food for humans, too. They are highly nutritious, full of vitamin A and C along with calcium and iron. Nettles have a long history in folk medicine and there is scientific evidence that extracts from nettle leaves, roots and stems can treat high blood pressure and diabetes.

Nettle fibers were used in Europe when the two world wars caused shortages. Indeed, there is evidence that people in cool climates have used nettles since the Bronze Age to create textile fibres until sheep breeding gave rise to more effective wool-producing sheep. So this is another “weed” with potential super powers.

Dog-strangling vine in Kanata (Photo credits to Green Ottawa)

Study explores control options for black swallowwort

Otherwise known as dog-strangling vine (Vincetoxicum nigrum), this nasty invasive plant has super powers that include rapid spread through wind-dispersed seeds and the ability to outcompete and strangle native species, including small trees. It also fools monarch butterflies into laying their eggs on it, but the monarch larvae can’t eat this nasty weed, so they starve.

I haven’t seen any information about potential beneficial properties and to date, no scientific studies have been conducted to determine how the weed responds to common controls, such as mowing and broad-spectrum herbicides. However, an article featured in Invasive Plant Science and Management, begins to fill this information gap. Over three years, the team explored how black swallowwort responded to two glyphosate products and one triclopyr product. The weeds were either sprayed annually with a two percent solution of one the herbicides at flowering in early July—or were mowed in early July and then treated in late August. The study showed the two glyphosate formulations were effective in reducing aboveground black swallowwort biomass, but they were less effective in reducing cover and stem densities. Researchers also determined that mowing failed to enhance the efficacy of the glyphosate applications on a consistent basis. With or without moving, Triclopyr was generally ineffective.

Citizen Science Humour

Can Plants “See”?

Image from Googly Eyes Gardener – Saturday Night Live.

Plant science is endlessly fascinating and I often think we can learn as much from the controversies as for the so-called settled science. Below are two articles that explore a very controversial topic – can plants “see”. While biomimicry in plants is well known and well documented, the controversy arises on the question of whether plants have organs that actually allow them to “see” as our eyes allow us to see. The pro argument is based on recent work initiated by a citizen scientist in Utah and supported by several European researchers. Linda Chalker Scott presents the con argument in The Garden Professors Blog.

Can Plants See? In the Wake of a Controversial Study, the Answer’s Still Unclear: A tiny pilot study found that so-called chameleon vines mimicked plastic leaves, but experts say poor study design and conflicts of interest undermine the report. The vine Boquila trifoliolata is a shapeshifter. As it winds its way through the Chilean rainforest, its leaves change to resemble those of the plants it uses for support or, sometimes, neighbors it isn’t in contact with. It does such a good job of pretending to be other plants that although the vine was first described in the 1800s, its talent for impersonation remained secret until only about a decade ago. In the early 2010s, Ernesto Gianoli, a plant ecologist with the University of La Serena in Chile, realized that what appeared to be a strange-looking stem from a tree was in fact a B. trifoliata vine, the leaves of which perfectly blended in with the tree’s actual leaves. Once he saw that, he spotted the vine mimicking all sorts of plants—more than 20 species so far—by tweaking the size, shape, and color of its leaves. Gianoli reported his findings in a 2014 Current Biology paper, but to this day, no one is certain how B. trifoliolata pulls off its impressive masquerades. Most recently, he discovered microbiome similarities between the mimicking vines and their models, hinting that bacteria could be involved. But in a paper published online in Plant Signaling and Behavior, citizen scientist Jacob White and University of Bonn graduate student Felipe Yamashita claim to have found evidence for a different hypothesis: that the vines can “see” other plants’ leaves, at least well enough to copy their looks. Some found these results thrilling, while others were deeply critical. White, a homemaker in Utah with a passion for science and plants but with no formal scientific training, says he got the idea for the study after reading about the eye-spots of Chlamydomonas algae and the lens-like cells of certain cyanobacteria. he came across a 2016 mini-review by University of Bonn plant physiologist František Baluška and University of Florence plant neurobiologist Stefano Mancuso suggesting that plants have eye-like structures that afford them a form of vision. He read about botanist Gottlieb Haberlandt’s 1905 hypothesis that the upper epidermal cells of leaves may function as simple eyes (ocelli)—and about B. trifoliolata’s touchless mimicry. To eliminate the hypotheses about B. trifoliolata’s mimicry hinging on the transfer of biological compounds, he paired a B. trifoliolata plant with a fake for it to grow on. Lo and behold, he says he observed what appeared to be an attempt by the vine to mimic the fake leaves as it grew up the artificial plant. In correspondence, Baluška suggested he try a fake plant that looks more like something that could be found in Chile. White did, and the vine seemed to mimic it as well. The balance of this longish article details White’s further experiments, and critiques from peer reviewers.

The plants have eyes! Another foray into B(ad) S(cience): The article found here reports on the ability of leaves to mimic other leaves. While the concept of leaf mimicry is not new and has been seen in agricultural weeds for decades, this article goes a step further in claiming that plants can actually see the leaves they are to meant to mimic. But let’s back up a bit to explore leaf mimicry, which is a thing. Leaf mimicry serves to protect plants against herbivory and other types of removal (like weeding). This phenomenon was reported decades ago where agricultural weeds were shown to change their morphology to more closely resemble the desired crop. The benefit is obvious: if a weed looks like a crop plant, it is unlikely to be removed through hand weeding. Likewise, if a weed resembles a poisonous plant, herbivores that are visual learners will avoid these weeds. When some plants of a species are disproportionately allowed to survive (i.e., not eaten or removed), they reproduce better. Higher reproductive capacity means more offspring: this is the process of natural selection. We can even see this in dandelions in our lawns and gardens. Dr. Scott notes a number of clues that we’re dealing with bad science. There are a lot of problems with this paper; it would take me a separate blog post to critique the Materials and Methods section alone. But the biggest red flag for me was the following paragraph:

Each plant was assigned a number and placed on a growing rack. Two artificial vines were placed above the plants on a wooden trellis. During the winter, the plants grew quickly through the leaves showed poor mimicry of the artificial plants leaves. The original plant that we had did not show good evidence of mimicry until the spring and summer. We decided to continue the experiment and see if there were better results in the warmer months.

This reflects significant author bias: the experiment didn’t work in the winter, so they did it in the spring and summer to see if they got results they liked better. And apparently they did. Other potential red flags that careful readers might note include: a lead author with no apparent connection to an academic institution; a journal (Plant Signaling and Behavior) that focuses on the questionable field of “plant neuroscience”; an experiment performed under vague and uncontrolled conditions; and typos, grammatical errors, and awkward writing throughout.

As a counterpoint to her criticism, Dr. Scott includes this link to a wonderful video featuring the late, great Christopher Walken.