From the Ground Up: Be cautious of space invaders in the garden

They are among us.... No, not outer space aliens, but dozens of alien plant species that have become commonplace and accepted by us without notice. They are truly alien invaders, or more accurately invasive species. Invasive species, also called introduced species, alien species, or exotic pest species, are the bane of natural landscapers, a nuisance to gardeners, and some can lurk in the inventory of your local greenhouse.

What is an invasive species? Or a non-native species? An aggressive species? A nativar? There are many terms for the issue of plants that are growing where they do not naturally belong. Isn't that just a weed? Not necessarily. A weed can be a plant that is native to the region but just not wanted in a particular place. It also can be a non-native introduced species. It's confusing.

An invasive plant has the ability to thrive and spread aggressively outside its native range and outside of cultivation. A perfect example is Kudzu.

A naturally aggressive plant may be especially invasive when it is introduced to a new habitat or let loose to grow wherever it pleases like Lamb's Ear or mint.

A non-native species is one that has been purposefully introduced from another region or zone, usually as a specimen plant, that has become widely dispersed like crape myrtle and bush honeysuckle.

Nativar is a native species that has been artificially propagated to preserve desired traits. They are no longer part of the natural reproductive patterns that maintain diversity. An example of a nativar is 'Tomato Soup' coneflower v. purple coneflower.

So what's the trouble with aliens? They can crowd out native species in an area, take over environments and disperse quickly throughout a region. They alter to suit the conditions that natives don't have, like developing a long tap root, ability to withstand wildfire, or a variety of seed dispersal systems. They can change the microclimate. They can make it harder for native plants to retain topsoil or absorb water in marshlands. In the case of new colors of coneflowers, they may not produce as much nectar for pollinators.

What's a gardener to do? Choose your plants wisely. Keep aggressive plants in containers and don't let them go to seed. Most naturalized plants aren't a severe threat to the environment if contained, but the range of their effects can be wide. An aggressive mint plant can be easily pulled out of the garden but honeysuckle bushes are crowding out our native woodland plants. Also take note that placement of the plant is important. A healthy mix of native and exotic can be maintained with the aid of a little research. Watch out for phrases like 'spreads quickly' or 'fast groundcover.'

For gardening questions, call the Linn County Extension Master Gardener Hortline at (319) 447-0647.