Space invaders

By Ashley Womble
This Old House

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(This Old House) -- Beloved by gardeners for its ability to camouflage an unsightly chain-link fence or cover ground where the sun barely shines, evergreen English ivy has long been a top-seller at nurseries. But this vine's vigorous growth habit has also earned it a more dubious distinction as an invasive plant species that will flagrantly cross into neighbors' yards and, when conditions are friendly, adjoining fields and forests, too.

Dating to the 1800s when it was imported from Europe to brighten American gardens, English ivy is most certainly a grande dame in the invasives kingdom, but a new crop of dangerous debutantes now nip at her heels. "Hundreds of invasive species have been established in the last few years, and they're crowding out our native plants," says ecologist James Ackerson of the National Forest Service. And though the problem is national, just as climate and soil conditions vary from region to region, so do problem-plant lists. Some species that are rampant in Southern California aren't a threat in New England states that experience a hard freeze at least not so far. Many experts cite global warming as a cause for the rise in invasives.

So before you plant your beds and borders, check with your local cooperative extension service and visit Weeds Gone Wild to see what species pose threats in your area. If you have your heart set on a potential renegade, you need to impose strict controls. "When a plant is aggressive, it means that it will spread until you do something to stop it," says This Old House landscape contractor Roger Cook. So plant rampantly spreading groundcovers behind steel edging sunk deep to contain their roots. Prune invasive vines often to keep them from wandering. And divide and deadhead prolific perennials mercilessly before they set seed and multiply.

Better yet, stick with species native to your region. It might take them a little longer to cover new ground, but your conscience will be clear and your neighbors will thank you.

Buyer Beware

When browsing for plants at the local nursery, watch out for tags bearing potential euphemisms for "invasive species":

• Aggressive
• Self-Naturalizing
• Quick-spreading
• Exotic
• Fast-growing
• Vigorous
• Prolific
• Adaptable
• Pest-Free

Problem Plants

Here are some plants still commonlly found at nurseries and garden centers that have made the invasive list in many regions of the country.

Vines:
English ivy (Hedera helix)
Fiveleaf akebia (Akebia quinata)
Porcelainberry (Ampelopsis brevipedunculata)
Oriental bitttersweet (Celastrus orbiculatus)
Wintercreeper (Eunymous fortunei)
Japanese honeysuckle (Lonicera japonica)

Herbs and Perennials:
Yarrow (Achillea millefolium)
Wormwood (Artemisia absinthium)
Oxeye daisy (Leucanthemum vulgare)
Purple loosestrife (Lythrum salicaria, L. virgatum)
Peppermint and spearmint (Mentha varieties)
Common mullein (Varbascum thapsus)

Shrubs and Woody Vines:
Japanese barberry (Berberis thunbergii)
Exotic bush honeysuckles (Lonicera species)
Multiflora rose (Rose multiflora)
Japanese spiraea (Spiraea japonica)
Exotic wisterias (Wisteria floribunda and sinensis)

Groundcovers and Grasses:
Fountain grass (Pennisetum setaceum)
Goutweed (Aegopodium podagraria)
Running bamboos (incl. Bambusa, Phyllostachys, and Pseduosasa)
Vinca, or periwinkle (Vinca minor)

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