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.

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