A Shortlist of Invasive Ornamental Plants of Concern

By:  Linda Dietrick, Master Gardener

You have probably heard of kudzu and purple loosestrife, two invasive plants that were originally introduced to North America with the best of intentions. Brought into new environments where there were no natural checks on their spread, they became bullies, outcompeting the regionally native plants, with consequences for whole ecosystems. We now realize that these and many other invasive plants can do serious harm to local economies, human health, and the environment. Kudzu, planted in the US as a soil stabilizer in the 1930s, has reached Ontario. Purple loosestrife (Lythrum salicaria), introduced in the 1800s and until recently sold as a hardy ornamental, now infests waterways throughout North America. As these examples show, it can take decades for people to notice invasive species and control them before they become a serious problem. Surely we can do better by paying closer attention.

Recently, the Manitoba Master Gardener Association sponsored a presentation called “Invasive Ornamental Plants – What Home Gardeners and Professionals Need to Know.” The speakers were Cathy Kavassalis, founder of the Ontario-based Coalition for Invasive Plant Regulation, Andrea Eastham, a Calgary agrologist who advises governments and non-profits on invasive plants, and Kristin Pingatore, Weed Control Supervisor for Selkirk. In case you missed the event, you can view slides and video recordings from it at this link: https://www.mgmanitoba.com/invasive-plants-weeds/.

You can also download the handout from the event, “What Not to Plant in Manitoba: Invasive Ornamental Plants in Our Region.” Most of the listed species are probably ones you would never think of buying, selling or planting. But here’s a shortlist of ten plants whose invasive character and potential you may not be aware of. These are species that some nurseries are still selling and that are invasive here or in places with similar growing conditions. Remembering the importance of paying attention, let’s resolve not to sell or plant these in our province. To learn why, continue reading below.

Please don’t buy or sell these plants in Manitoba:

Woody plants
Russian olive Elaeagnus angustifolia
Barberry Berberis vulgaris and B. thunbergii
Common caragana Caragana arborescens
Winged burning bush Euonymus alatus
Non-native bush honeysuckles Lonicera morrowii, L. tartarica, L. xylosteum
Ural false spirea Sorbaria sorbifolia

Perennials

Bishop’s goutweed Aegopodium podagraria
Dame’s rocket Hesperis matronalis
Chinese silvergrass Miscanthus sinensis, M. sacchariflorus
Periwinkle/vinca minor

Russian olive (Elaeagnus angustifolia) has become seriously invasive in southern Manitoba, as tree experts like Michael Allen have long recognized. Its abundant seeds are spread by birds, and it is outcompeting native vegetation in many unmanaged areas and wild spaces. This may worsen as the climate warms. Both Montana and Wisconsin have prohibited it. There are plenty of other hardy, attractive smaller trees available in the marketplace.
https://homes.winnipegfreepress.com/winnipeg-real-estate-articles/renovation-design/Russian-olive-continues-march-in-southern-Manitoba/id-4957

https://dnr.wisconsin.gov/topic/Invasives/fact/RussianOlive.html

https://fieldguide.mt.gov/speciesDetail.aspx?elcode=PDELG01010

Common barberry (Berberis vulgaris) has long been on Canada’s banned weed list because it’s host to black stem rust, a major threat to cereal crops. But we now know that even interspecific hybrids of common barberry can produce seed offspring that are rust susceptible. Moreover, in Minnesota and Ontario, seedlings of Japanese barberry (B. thunbergii) and its ornamental hybrids are overtaking woodlands and harbouring the ticks that carry Lyme disease. Plant a colourful ninebark instead.
https://inspection.canada.ca/plant-health/invasive-species/directives/pest-risk-management/rmd-21-02/notice-to-industry/eng/1653420323729/1653420324010
https://www.dnr.state.mn.us/invasives/terrestrialplants/woody/japanesebarberry.html

Common caragana (Caragana arborescens), originally from Siberia, became popular as a hedge and shelterbelt shrub on the northern Prairies because of its extreme hardiness. But in Alberta and Minnesota, it’s now being removed as an invasive weed that threatens woods and grasslands. Exception: Green Spires caragana ‘Jefarb’, which is seedless. Other caragana species are fine, too.
https://www.calgary.ca/parks/pests/caragana.html
https://www.dnr.state.mn.us/invasives/terrestrialplants/woody/siberianpeashrub.html

Winged burning bush (Euonymus alatus), a prodigious seeder that has been declared invasive in most of the eastern US, Minnesota, and Ontario, can produce dense thickets threatening native woodland. The equally hardy dwarf Turkestan burning bush (Euonymus nanus ‘Turkestanica’) is a noninvasive alternative.
https://www.invasive.org/browse/subinfo.cfm?sub=3023
http://www.invadingspecies.com/invaders/plants/winged-euonymus-2/

Non-native bush honeysuckles (Lonicera morrowii, L. tartarica, L. xylosteum) are already wreaking havoc in Ontario, Minnesota, and, anecdotally, Manitoba. Birds spread the seeds, and plants can take over large areas. The native bush honeysuckles or Diervilla species are fine alternatives. (http://www.invadingspecies.com/invaders/plants/invasive-honeysuckles-2/; https://www.dnr.state.mn.us/invasives/terrestrialplants/woody/bush-honeysuckles.html)

Ural false spirea (Sorbaria sorbifolia) is another Asian species that has been declared invasive in the US Northeast and Ontario. Manitoba gardeners who have fought its aggressive root suckers will agree that we shouldn’t be using it. Plant a real spirea, elder, or sumac instead.
https://www.invasiveplantatlas.org/subject.html?sub=14050

Bishop’s goutweed (Aegopodium podagraria) is still being sold in nurseries, despite being declared invasive in Ontario and an “alert” species in Alberta. Anyone who has ever tried to remove it from their property has surely cursed the person who planted it there. Alternative low-maintenance, shade-tolerant ground covers include wild ginger (Asarum canadensis), star-flowered Solomon’s seal (Maianthemum stellatum syn. Smilacina stellata), Virginia creeper (Parthenocissus quinquefolia), and barren strawberry (Waldsteinia ternata).
https://www.ontarioinvasiveplants.ca/wp-content/uploads/2022/05/OIPC_BMP_Goutweed_April122022_D3_WEB.pdf
https://abinvasives.ca/wp-content/uploads/2020/09/Identification-Guide-for-Alberta-Invasive-Plants.pdf

Dame’s rocket (Hesperis matronalis), a biennial or short-lived perennial from Europe, is regulated as a noxious weed in both Saskatchewan and Alberta. Though not yet regulated in Manitoba, the Invasive Species Council of Manitoba has flagged it as invasive. It threatens woodlands, for it is both shade tolerant and a prolific seeder. Its seeds are still widely sold and often included in so-called wildflower mixes. Garden phlox, which has similar flowers, is a noninvasive alternative.
http://invasivespeciesmanitoba.com/site/index.php?page=dames-rocket; http://biodiversity.sk.ca/Docs/InvasiveSpeciesCouncilFactSheets/Dames Rocket.pdf

Chinese silvergrass (Miscanthus sinensis, M. sacchariflorus) is an Asian grass now considered invasive in most of the US Northeast and Ontario. There is no reason to plant it in Manitoba, given the many beautiful noninvasive species available, including sterile ‘Karl Foerster’ feather reed grass and native grasses like big and little bluestem, tufted hairgrass, and sideoats grama.
http://www.invadingspecies.com/invaders/plants/miscanthus-2/
https://www.invasiveplantatlas.org/subject.html?sub=3052

Periwinkle/vinca (Vinca minor) is a perennial ground cover that has now been declared invasive in Ontario and the eastern US, where it has taken over woodland understories. Variegated cultivars, often sold as annual basket stuffers in Manitoba, are of lesser concern. For alternative groundcovers, see above under bishop’s goutweed.
https://www.ontarioinvasiveplants.ca/wp-content/uploads/2016/07/NorthernGMI_2014_FINAL.compressed.pdf; https://www.invasive.org/browse/subinfo.cfm?sub=3081

If you’re a Master Gardener, you have a role to play in educating people about the invasive character of these and other plants. You can set an example by not sharing invasive plants with others and by eliminating any that are growing in your own garden. And: you can support your local nursery by purchasing new plants that are not on this list!

If you’re a nursery or landscape professional, consider joining the Canadian Nursery Landscape Association in supporting the National Voluntary Code of Conduct for the Ornamental Horticulture Industry, here: https://canadainvasives.ca/wp-content/uploads/2019/06/2019-01-16_CCIS_Code_of_Conduct_FINAL.pdf.

Linda Dietrick (education@mgmanitoba.com)
April 2023

Published: May 2023