Native, exotic, invasive: what it means and why it matters

Regular followers of this blog will have noticed that when I introduce a plant, I like to mention where it is native, and that I'm working on a list of plants that are native to Toronto.

When I first heard people talking about the importance of preserving native plants, as opposed to plants that evolved elsewhere, I didn't see why it was important. Sure, I have a sentimental attachment to our native trilliums, hepaticas, etc., and would be sad to see them go, but then I also have a sentimental attachment to a lot of the non-native weeds, like ox-eye daisy (Leucanthemum vulgare). I wondered if the concerns about the non-native plants "invading" Canada were a bit xenophobic, which seems rather hypocritical since most people criticizing invasive plants are also non-Native.

I know better now.

First, some definitions:

A native plant is one that has evolved in an area for many, many years. In practice, for North America people generally consider that the plants that were here before the Europeans came over to be native. Note that for this purpose "area" has nothing to do with political boundaries, but rather with ecoregions. Our native flora is quite similar to that of the neighbouring American states, but a plant that is native to British Columbia may not be native here. (I've seen national Canadian gardening magazines describing plants as "native" without indicating which part of Canada they're native to.)

An exotic, alien, or non-native plant is one that is not native to an area. This includes many garden plants, which have been brought over from Europe, Asia, Africa, South America, Australia, and even other areas of North America. It also includes many of our familiar wildflowers, which were brought over either deliberately or accidentally and now live without human assistance, for example Queen Anne's lace (Daucus carota, native to Europe and southwest Asia).

Why does it matter?

Why does it matter where a plant came from originally? Most of Canada's human population comes from all over the world, why shouldn't our plants reflect that? It's understandable that people would want to bring over plants from the old country. (I will admit that when I find out a plant is native to Australia, I am tempted to get it in honour of my late father, who grew up in Melbourne.)

There are two potential problems with non-native plants:

Some non-native plants have difficulty living here

A plant from a wildly different climate will have a lot of trouble coping with Canadian conditions. For example, an Australian plant would be pretty much guaranteed to die in winter. Some exotic plants require a lot of coddling to cope with our cold winters, hot summers, hail storms, snow, ice, etc. This not only means more work for the gardener, but in the case of plants that won't get through the summer without people watering them, it also means a greater drain on our natural resources. These delicate plants may also lead gardeners to use more fertlizers and pesticides than they otherwise would have done.

Some non-native plants find it too easy living here

A plant that flourishes without human attention, is "pest"-free, and tolerates a wide variety of conditions, is understandably going to appeal to a lot of gardeners. Unfortunately, these plants often don't stay in our gardens. Plants that produce tasty berries are the hardest to contain; the birds that eat the berries may poop them out kilometres away, possibly in a vulnerable ecosystem. Other plants produce seeds that travel on the wind; deadheading can prevent this, but how do you deadhead a Norway maple (Acer platanoides)?

Once these easy-going plants start growing outside of cultivation, being "pest free" gives them an unfair advantage over native plants. Most plants contain toxins in their foliage as a defence against herbivores; insects that have co-evolved with these plants have developed ways of coping with various toxins. Most herbivorous insects are specialists; these insects can only eat certain species of plants that they can detoxify. Non-native plants feed fewer or no native insects, allowing them to out-compete the native plants that our native insects eat. These species can become invasive.

An invasive plant is an exotic plant that lives in the wild without human assistance, and disrupts ecosystems. (The Society for Ecological Restoration has a useful list of invasives in southern Ontario (PDF).) Invasive plants spread prolifically, replacing native plant communities. Since most native insects can't eat these invasive plants, the population of insects declines.

So what? Even if you don't like insects (I find many of them beautiful and fascinating), insects are an important food for more "charismatic" animals. Most songbirds feed their young caterpillars; if the native plants those caterpillars eat are gone, the baby songbirds starve. The loss of native plants has repercussions throughout the foodweb. Not only the plants, but the animals who eat them, and the animals who eat those animals, and so on, go into decline as invasive plants take over.

Can we predict which exotic plants may become invasive?

Many popular garden plants, particularly food crops, are non-native. It would be nice to know which ones pose a risk to our ecosystems, so we could avoid them. Unfortunately, ecosystems are so complex and ever-changing that it's difficult to predict which plants will become invasive. Still, there are a couple of things to watch out for:

• Plants that self-sow or "naturalize readily". Yes, most of us love getting free plants, but when exotics start reproducing, will you be able to control their spread?
• Plants from a similar climate. It's no coincidence that our invasive plants come from temperate Eurasia. Here they can find the climate they're adapted to, without the insects and diseases that kept their populations in check back home.

When I first learned about the dangers of invasive plants, I assumed that "heirloom" varieties would be safe. Surely if people have been growing a plant in Toronto for a century without a problem, it's not going to invade now, I thought. Unfortunately, I was wrong. In Bringing Nature Home (Portland, OR: Timber Press, 2007), Douglas W. Tallamy documents many seriously invasive plants which were well-behaved garden plants for decades before becoming a problem.

You would think that an exotic plant that is very similar to a native plant could fit into our ecosystems without causing problems, but we can't assume that either. Tallamy reports that even closely related plants support far fewer insects than their native counterparts. Here in Ontario, European frogbit (Hydrocharis morsus-ranae is one of the most serious invasive plants according to the Canadian Wildlife Service. It forms dense mats on the waters surface which prevent native plants from growing underneath. Yet European frogbit looks virtually the same as the native American frogbit (Limnobium spongia); the only visible difference is that the native frogbit has a grooved stem.

Implications for our gardens

Although most of us will continue to grow some non-natives, especially foods and herbs, it's clearly better for the environment to

• choose natives rather than non-natives whenever possible (see the Ontario section of the North American Native Plant Society Sources List)
• don't buy plants known to be invasive (check the list! (PDF))
• prevent non-natives in the garden from spreading elsewhere if possible (deadhead, plant in containers rather than directly in the ground, etc.)
• remove invasive plants from the garden, especially those whose spread is difficult to control (if it's a tree, check out Toronto's Private Tree By-law before proceeding—you may need a permit)

Increasing our use of native plants can let us enjoy beautiful gardens while preserving the beauty of our wilderness.