Perhaps you’ve come across the phrase “invasive species” in the newspapers, on a sign at a nature preserve or maybe in a glossy booklet produced by a conservation group. But what do these words really mean – especially to you as a gardener?

Water hyacinth

There aren’t many ponds or lakes that water hyacinth (Eichhornia crassipes) wouldn’t like to take over.

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Japanese knotweed

Japanese knotweed (Fallopia japonica) loves to invade streams.

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Ice plant

Ice plant (Carpobrotus edulis) carpets dunes with growth.

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Purple loosestrife

Avoid purple loosestrife (Lythrum salicaria) – it invades wet areas.

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Cape ivy

Cape ivy (Delairea odorata) has beautiful foliage, but it’s highly invasive along the California coast.

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Not everyone agrees on the term’s exact definition, but basically an invasive species is one that rapidly reproduces and takes over more and more space. For a gardener looking for a nice groundcover, a plant that’s a little invasive can sometimes be a good thing. But plants that spread too rapidly can become a gardener’s worst nightmare because they cause so much work to keep in check. In fact, battles between gardeners and their most detested problem plants can reach epic proportions!

But the invasive species you hear environmentalists talking about are a little different. These are non-native plants introduced by people to natural areas – forests, deserts, rivers and so on. Most introduced plants perish after a year or two, like house pets abandoned in the wild. But a few species – a very few – grow and grow so rapidly, they start to crowd out the native plants. The natural area, previously home to a diversity of plant species, can become overrun by one invader.

Why is this bad? First, the native plants in the area have a tough time competing with the invasive species. And the rich, natural beauty can be eliminated. Worse still, when the native plants start disappearing, the native animals that feed on them start suffering, too.

Further complicating matters is the fact that a plant can be invasive in one country (or in an area of a country), but it might not be considered “invasive” in its homeland. Why? Because in a given plant’s land of origin, that plant is surrounded by animals and diseases that evolved along with it. Even though it might have bad-tasting leaves or poisonous sap, the animals have learned to cope with it. When the plant is brought to a new country (or an area of a country), it can start to grow and grow without the usual animal or disease burdens it fought against in its native land.

Invasive species find their way into the wild from many sources. Yes, some have been introduced by gardeners. But many were brought in accidentally by farmers using impure bags of seed or by scientists and government officials trying to improve the land. The good news for gardeners is that of the countless numbers of plants in horticulture, it’s only a tiny fraction that environmentalists fret about.

So should you worry about introducing an invasive species? If you’re an urban gardener and you compost your garden clippings, you don’t have to worry too much. But if you live near nature preserves, have hungry birds eating berries from your bushes or have a stream flowing through your property, what you grow in your own garden could escape its borders.

What can you do? Try to find out what plants are thought to be “invasive” in your area. If you have any in your garden, look for noninvasive alternatives. This is particularly important if you live anywhere near a natural area or have a stream running along your property – or even if you have storm drains that could wash seeds from your property into a river.

If you discover that some plant in your garden has a tendency to reproduce so much that you’re endlessly weeding out seedlings or sprouts, maybe you should consider growing something else in its place. It’ll save you weeding time in the future – and it might help protect the native biodiversity in your nearby nature preserves!