Skip to content

Invasive plant releases cyanide into soil

Garlic mustard was introduced in the late 1800s by settlers

When settlers came to North America from Europe, they brought along old-world plants – some by accident, others deliberately.

Lilac bushes were a reminder of home, blooming each spring by the cabin door. And plants like garlic mustard provided important additions to the kitchen and the medicine cabinet for immigrants struggling to build a new life.

Unfortunately, some of those plants have become invasive in a new world setting.

Garlic mustard, Alliaria petiolata, is also known as Sauce-Alone, Penny-hedge, Jack-in-the-Bush and Poor Man’s Mustard. The plant lives for two years, producing a rosette of kidney-shaped, roughly veined leaves in its first year; and a metre tall plant with a flowering stalk of white, four-petalled flowers in the second year.

Introduced in the late 1800s by settlers, it was used as a flavouring for fish and other dishes, and medicinally as a diuretic. With its first-year leaves surviving under the snow, it was a good source of vitamins A and C in winter.

But there’s a dark side to the plant, said Spencer Leava, Stewardship Assistant with the Lake Simcoe Region Conservation Authority. The survival over the winter of first-year rosettes gives garlic mustard a head start in the spring over native species. And while it spreads slowly, it can form dense stands, invading even undisturbed forests and woodlands, and threatening species at risk like American ginseng.

Garlic mustard also carries diseases that include cucumber mosaic virus, cabbage black ringspot virus, and turnip mosaic virus, and it can impart an unpalatable garlic flavour to milk when it is eaten by dairy cows.

But the worst impact is the longest lasting: Garlic mustard releases allelopathic chemicals from its roots (including glucosinates and cyanide) that suppress the growth of other plants, and are especially toxic to soil fungi.

That means that the forest’s mycorrhizal fungi – so important for the survival of tree saplings, and their uptake of water and nutrients – are suppressed.

Leava has made a study of garlic mustard. The allelopathic chemicals “can account for up to three per cent of the plant’s mass,” he said – natural pesticides that not only target the soil fungi, but persist in the soil long after the plants themselves are removed.

A study of the compounds found that the soil was effectively sterilized. In tests, “native plants were still not succeeding,” even when all other traces of garlic mustard were removed, Leava said.

Not only that, “Compounds in garlic mustard leaves may accelerate leaf litter decomposition rates around the plants,” Leava noted – reducing the amount of leaf litter on the forest floor, so important to ground-nesting birds, salamanders and other forest dwellers.

Garlic mustard has yet another strategy that helps it dominate. Leava was at one time a member of the Hit Squad, a group of young professionals and students hired by the Ontario Federation of Anglers and Hunters to help conservation authorities and other groups target invasive species and educate the public.

Hit Squad members found that garlic mustard was fairly easy to remove – yet the patches persisted. Leava had the answer: “It has an outstanding seed production for a plant. In ideal conditions, a single plant can produce 3,500 seeds.”

According to the Ontario Invasive Plant Council, each plant produces up to 150 seed pods, with up to 22 seeds per pod. One study found that a dense stand produced more than 105,000 seeds per square metre. It is also self-pollinating – which means a single plant can produce seeds and launch an infestation.

Even more importantly, the seeds remain dormant and viable in the soil for up to 10 years – which means, Leava said, that any control plan has to be long-term, lasting for at least 10 years. “It should be about depleting the seed bank.”

The Invasive Plant Council calls for the removal of garlic mustard. “Controlling garlic mustard before it becomes locally established will reduce its impacts on biodiversity, the economy and society,” the Council wrote, advising of the need for a long-term strategy to address a “daunting” problem.

Best practices suggest removal of outlying patches, and those along trails and in areas of endangered species, before attacking major stands. The small black seeds are mostly spread by animals – including humans and dogs – which is one reason why it has been showing up along recreational trails.

Control includes pulling individual plants and small patches in April and May, with mowing and clipping of flower heads before the seeds set in May. Chemical control is also recommended, early in the year or in late fall, for larger patches – and here, Leava said, the adaptation that gives garlic mustard such a strong head start in spring can be used against it.

Early in the year, rosettes of garlic mustard may be the only green plants in the forest. Everything else is dormant, making it easier to target just the invader.

So far, Leava said, “we haven’t actually done any chemical control” within the LSRCA’s jurisdiction, but that doesn’t mean garlic mustard isn’t a significant problem. It shows up in 12 per cent of all Ontario EDDmaps (Early Detection and Distribution Mapping System) reports, he said, “and accounts for one-third of all invasive plant occurrences in the areas we monitor.”

Since an “occurrence” refers to a stand, no matter how many plants are within the patch, the extent of garlic mustard “is probably a lot higher.”

Plants that are removed shouldn’t be composted. The recommendation is to bag garlic mustard and any flowering heads in black plastic garbage bags, leave them out in the sun to ‘cook’ for a week, before disposal at landfill or by burning.

Residents should watch for it, report it, stay on the trails in woodlands and keep clothing, gear and pets clean to avoid spreading this invader. “It’s quite a big problem,” said Leava. “It’s going to be a long removal strategy.”

While garlic mustard not a “regulated” species, it is a threat to woodlands. Sightings can be reported to [email protected] or here.


Comments

Verified reader

If you would like to apply to become a verified commenter, please fill out this form.




Miriam King

About the Author: Miriam King

Miriam King is a journalist and photographer with Bradford Today, covering news and events in Bradford West Gwillimbury and Innisfil.
Read more