Most non-native species we introduce to our gardens pose no threat to natural plant communities. But some invasive species such as purple loosestrife have spread so quickly and widely—often carried by birds or the wind—they've begun to threaten the existence of endangered native species and biological communities. And plants can be invasive in certain parts of the country and not in others. For example, English ivy is a pest in British Columbia but not across most of Ontario.
Gardeners can help control the spread of invasive plants by not introducing new invasive species. Here's how.
• Use native plants. When you choose a plant native to your area, you likely won't be unleashing the next invasive menace. Learn what native species can be used in place of exotics. There's almost always an alternative. For example: use blazing star (Liatris spicata) instead of the now-illegal purple loosestrife.
• Plant hybrids. The majority of showy garden hybrids developed by breeders are safe to grow, as they don't self-seed.
• Use plants native to other North American regions with similar climates and growing conditions.
• If you still decide to choose an exotic, make sure it's a non-invasive species such as peony, petunia or forsythia.
For more information on invasive plants see Invasive Species or Royal Botanical Gardens web sites.
Still, there are steps a gardener can take to control a plant's unacceptable behaviour. Here are some tips for taming garden bullies.
Deadhead: Remove all flower heads as they fade to limit the offspring of many would-be parent plants. Coneflower (Echinacea) and black-eyed Susan (Rudbeckia hirta) can produce thousands of seeds in a season. Pulling out or hoeing seedlings, in combination with deadheading, will usually control the majority of rampant self-seeders—including yarrow, lambs' ears (Stachys byzantina) and spurge.
Cut them down: Mow or shear back hard after flowering to temper shallow-rooted plants such as periwinkle (Vinca minor), spotted deadnettle (Lamium maculatum cvs.), soapwort (Saponaria officinalis) and sweet woodruff (Galium odoratum).