Astrid Muschalla, Coordinator Master Gardeners of Rideau 1000 Islands and Certified Organic Land Care Professional gave an overview of invasive plants. This blog is an abridged version of her presentation with links for further information. Key terminology is described, in particular, the idea of understanding the impact of non-native invasive species on a continuum. There is discussion about how non-native invasives enter into our province and their role in our landscapes, ranging from positive soil builders to harmful transformers. Astrid then highlights some of the most harmful species and suggests what we as gardeners can do. The blog ends with some online resources to for further information.

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Key terminology

A  Non-native (alien) is a species outside of its natural geographic range that arrived with human assistance.  

The impact of the introduction of these species into the ecosystem is best viewed on a spectrum as follows:

Exotic: a species that is unable to survive in temperate climate and survive the harsh Canadian winter (i.e. Banana plants – it will die once the frost hits)

Naturalized: where a species, like daffodils is able to establish in a natural environment and maintain a self-sustaining population, but the impact is neutral: it doesn’t harm or benefit the species already existing in that ecosystem. i.e. peppermint (Mentha x piperita) or dandelion. Although these species can be weedy and hard to eradicate, control is not a priority.

Invasive: The extent of invasion is variable as follows:

  • Minimally invasive – species that do not pose an immediate threat to natural areas but do compete with more desirable native species. Once established, many can reproduce aggressively and become difficult to eradicate. Some are similar to native species and are often substituted by nurseries. Control where necessary and limit their spread to other areas. Examples are purple loosestrife, lily of the valley, and periwinkle
  • Moderately invasive – species that are moderately invasive but can become locally dominant given certain conditions e.g. soils, recreational impacts or disturbances. Control where necessary and limit their spread to other areas. Examples are goutweed, orange ditch-lily daylily.
  • Highly invasive – species that are highly invasive but tend to dominate only certain niches or do not spread rapidly from major concentrations. Many spread by vegetative means or seeds that drop close to the parent plant. Most persist in dense populations for long periods. Control where necessary and limit their spread into other areas. Examples are Norway maple, amur honeysuckle, Japanese knotweed.
  • Transformers – species that exclude all other species and dominate sites indefinitely. Plants in this category are a threat to natural areas wherever they occur because they tend to disperse widely (e.g. through transport by birds or water). Examples are buckthorn, dog-strangling vine, garlic mustard, common reed (Phragmites).

How Do Non Native Species Get Here?

There are two pathways – human and natural.

Human pathways include:

  • International travel and tourism
  • Trade, including packing materials like wood pallets which can often harbour potentially invasive insects
  • Importation of aquarium & water garden species, can carry invasive insects, snails, plants and fish
  • Recreational activities such as ATVs and bikes. Trail equipment should be cleaned after use. Eco tourism – walking through an infested site and not cleaning off seeds or plant parts before entering a new area also allows for spread.

Natural pathways include wind, water, and animals spreading seeds and plant material

Horticulture Industry as a Pathway

Each year thousands of new plant species are introduced for use in the horticulture trade. Most are harmless garden additions, however a very small percentage can become problematic.

In Canada of the estimated 245 species for which there is a known pathway for introduction, 73 were introduced as horticultural species and 86 were unintentionally introduced as hitchhikers with plant products.

Invasive species can “escape” from gardens in a variety of ways.

  • Soil associated with nursery stock can carry new insects and diseases, and hitchhiking plant seeds
  • Importation of live plants (fruits and vegetables) can often harbour plant pests
  • The plants themselves can spread uncontrolled (goutweed) or be shared with fellow gardeners (periwinkle).

Invasive Species as soil builders

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Weeds spread best in disturbed sites, unhealthy soil and unhealthy  ecosystems. We should temper our approach to these plants based on their propensity to “invade”. Arden Andersen, US-based soil scientist, notes that vegetation is an indicator of soil conditions and that certain plants grow best in certain conditions (types of soil, nutrient content, air and water availability).  

“it’s a mistake to think that a healthy soil supports crops and weeds equally well. By this logic quinoa, blueberries and alfalfa should all grow well in the same soil. But they don’t.” ….

(Arden Andersen – Soil Scientist)

Some “weeds” are pioneer plants, able to grow in soil unsuited for edible or domesticated plants.

The successive growth and decay of these “weeds” lays down an absorbent mat on the soil which prevents erosion from rain runoff and wind. This mat traps the rain water and causes it to soak into the soil for future needs. The “in-soak” keeps the springs flowing, which feed our rivers, keeping them crystal clear and running at an even rate, instead of flooding after each rain. It also feeds the wells from which many people get their water supply.

Weeds play a role of construction engineer in the soil. In performing this role, the weed directly deposits nutrients and metabolites into the soil or rearranges those nutrients already in the soil. The weeds don’t rob, they only borrow, as eventually it all returns to the soil for future crop use.

(Arden Andersen – Soil Scientist)

Because the growing “weeds” take up and store soluble plant food in their tissues, the wasteful leaching of phosphate, nitrate and other minerals is prevented. This helps prevent the pollution of our water with an excess of these nutrients.

As weeds grow and decay, the soil is improved. Nature gave these plants a means of protection, such as a bitter taste, thorns, or even made them poisonous so they wouldn’t be eaten by animals. This is so they could continue the soil-making and building processes until finally good soil is made. Then the edible plants can move in and take over.  Thus, weeds can be caretakers of the soil and are a vital link the soil fertility and food chain. 

Harmful invasive non-native species/ transformers

Non-native species have the potential to harm the environment, economy or society, and control becomes a priority. Such species can degrade natural areas, interfere with agriculture, be economically burdensome on land managers/stewards, reduce forest productivity and/or be a danger to human health and safety, like giant hogweed.

Characteristics of invasive plants

  • Massive amounts of seed production
  • No enemies/ predators
  • Many don’t need disturbed or bare soil to spread
  • They can change their environment to favour their own spread by: exuding chemicals (allelopathic) to change the soil environment, altering light availability, changing soil temperatures and moisture conditions, and changing vegetative litter constituents thereby disrupting decomposition patterns and soil nutrient cycling.

Some of the worst offenders

Dog strangling vine (DSV) (Cynanchum rossicum) is so invasive because of copious seed production… It has polyembryonic seeds (each seed contain multiple embryos and multiple seedlings germinate from each seed).  It also has good survival as it invests heavily in roots in the first 3 years, it is a good competitor as it promotes its own growth including chemical attacks (allelopathic) on its neighbours and it has no enemies. Its highly toxic chemicals are unique, so much so that the monarch butterfly gets confused because it’s in the same family as the native milkweeds but is unable to support the monarch’s lifecycle – the caterpillars can’t feed on DSV.

You have to dig this voracious perennial up by the rhizomatous root crowns (don’t worry about the roots below). Then dessicate or solarize in black plastic bags for 4 weeks and dispose in garbage (or a dedicated compost). Fresh plant material has to be put in the garbage. Note, that the seed bank in the soil can persist for years so you have to be vigilant.

Purple loosestrife (Lythrum salicaria)

Biological control can be very effective, but it can take many years.  It took 15 years after beetles were released to control loosestrife before heavy damage was observed on a large scale. The goal is not to eradicate an alien plant, but to turn it into a minor member of the plant community.

Common (European) Buckthorn (Rhamnus cathartica)

Buckthorn has female and male plants with fruit on the females. This fruit is dispersed by birds that eat the fruit which gives them diarrhea. It’s easy to see these plants inthe fall after most native plants have dropped their leaves because buckthorn keeps it’s leaves and berries until much later. Buckthorn has a (somewhat soft) point at the end of the older branches. The inner bark is yellow. It is best to dig buckthorn out when it is small. Cutting out is much harder. Cut back, especially the females with fruit, to stop seed dispersal. Branches will  re-sprout with vigour so you have to maintain the cutting back regularly until the plant eventually weakens and dies.  An Extractigator is a very helpful tool for removing woody plants without having to dig them out.

Multiflora Rose (Rosa multiflora)

This plant was introduced in late 1800’s. It is a climbing and rambling, very thorny shrub which can grow into a very large thicket, up to 20 feet, smothering everything. It has small 1” white flowers which appear from May through June. It was widely planted for erosion control and as living fences for farms to keep animals in/out. It was also used as root stock for breeding roses and widely promoted until it was too late. Individual plants may produce as many as 500,000 seeds per year. Seeds stay viable in the soil bank for 10 to 20 years depending upon soil conditions (Munger, 2002). It’s easy to spot this plant in the fall when it’s covered in tiny red rosehips. The extractigator tool works great on this vicious plant.

Garlic Mustard (Alliara petiolata)

Garlic mustard is a major threat to native woodland species such as trout lily, trillium, maple. Its populations can double in size every 4 years. It can enter and establish itself within a stable healthy forest site. This plant is allelopathic – it produces chemicals that hinder the growth of other species. Garlic mustard reproduces by seed only in the second year when it’s easy to pull in May and June, before the seeds ripen. However, these seeds can live in the soil for at least 7 years and it can take up to 3 years to fill in with native plants – so have good native seeds available; Virginia waterleaf is a good one for shade.

Wild Parsnip (Pastinaca sativa)

Wild parsnip outcompetes native species and adversely impacts pollinators. It reduces the quality of some agricultural operations. Wild parsnip contains toxic compounds that can cause serious rashes, burns or blisters to skin exposed to the sap and then sunlight. It can grow up to 1.5 metres tall

Others are Japanese knotweed and giant Hogweed.

Impact of Invasive Plant Species in Ontario

Invasive species represent the second most significant cause of species extinction globally, after habitat loss. As reported by Ontario Invasive Plant Council (OIPC) from International Union for Conservation of Nature’s Red List of Threatened Species…. the challenges are, lack of resources and proper training, large infestations seem overwhelming and it is difficult for land owners to know where to begin a management plan, (IUCN, 2014).

Why is biodiversity important

  • More biodiversity means more healthy/resilient systems  – able to withstand change, for example, climate change
  • These systems (ecosystem services) clean the air, water, cycle soil nutrients, neutralize wastes.

We are absolutely dependant for our survival on the living and non-living resources that nature provides from forests to meadows, oceans/lakes, and the earth’s mantle.

What you can do now

  • Learn to identify terrestrial invasive plant species that are a threat in Ontario. Fall is a good time to identify DSV, buckthorn, multiflora rose, amur honeysuckle, but with practice, you’ll learn to identify them all throughout their lifecycle using the bark, leaf shape and arrangement, fruit/flower/seed pods, thorns, etc.
  • Avoid using invasive plants in gardens and landscaping
  • Buy native or non-invasive plants from reputable garden suppliers but note that native plants provide habitat and food sources for our diminishing wildlife.
  • Dispose of invasive plants in the garbage. Do not put them in the compost or discard them in natural areas, unless properly dessicated or solarized.
  • Report all sightings to the Invading Species Hotline 1-800-563-7711 or visit EDDMapS Ontario to report a sighting. EDDMapS is meant to be fast and easy, requiring no knowledge of GIS. “Early detection and rapid response is extremely important as it helps stop or control invasive species before they become an unmanageable problem” (OIPC). Another tool for sharing information is iNaturalist .

Online resources  

The Ontario Invasive Plant Council is a multi-sector non-profit agency founded in 2007.

Its mandate is to provide “leadership, expertise and a forum to engage and empower Ontarians to take action on invasive plant issues”. It has numerous resources which could be of help to you including the following:

Best Management Practices Guides and quick fact sheets to control invasives

Grow Me Instead suggests alternative species to plant including ground covers, wildflowers and grasses, trees and shrubs, vines and aquatics. It is available in English and French. A Northern Ontario guide was completed in Spring 2012 and is available on OIPC, OMNR and many other websites.