What To Do When An Invasive Herb Takes Over Your Garden

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Question: What can you do about herbs that grow too well?

Answer: Most beginning gardeners experience the occasional problem of enthusiastically planting something new, only to discover by the end of the season that it seems to want to eat up every inch of available garden space before it goes on to conquer the world. And sometimes a plant that is well-behaved in your climate might be Attila the Hun in another region of the United States. Oftentimes, you don’t know its growth habits until you grow it.

Many examples spring to mind. Take, for example, the dandelion: The leaves are nourishing greens for salads and the root has a number of traditional medicinal uses, but it is widely regarded as a meace. Its puffballs toss so much seed to the winds that one plant can become a yard full if you let it. Because it has a deep taproot, it can be tough to pull without breaking off the top.

Other plants may become invasive because they spread in all directions via runners or suckers (some of the mints forget their manners in this way). Sometimes any small bit of root or rhizome left in the ground lives on, perhaps sprouting two plants in place of the one you thought you pulled. Tilling the soil can aggravate the problem as roots are chopped up into many pieces and plants sprout off of each piece.

Some plants may become troublesome just because they grow much taller or wider than you expected, or because once established they are difficult to eradicate.

Remember that vigor and fast, strong growth are assets in a plant–up to a point. That point can depend not only on the plant and the climate, but also on the amount of space you have in which to garden and your level of tolerance.

What can you do about the problem of invasiveness? Sometimes the best approach is to know about a plant’s tendencies before you decide to put it in your garden. Your best allies may be your neighbors and experienced gardening friends, your local garden center, the Cooperative Extension Office in your area, and the library.

If you want to grow a plant no matter how much trouble it is, here are some tips:

• Plant it and other equally tough plants in the same area of the garden and let them duke it out. Keep your eye on them to be sure that one doesn’t overwhelm the others.
• Put an aggressive plant in a pot or a whiskey barrel on the patio instead of in the ground.
• Look in garden centers and mail-order catalogs for a cultivated variety of the same plant that doesn’t grow as tall or as fast. Sometimes, for example, a variegated plant (one whose leaves are splashed with white or yellow) is not as rambunctious as its all-green counterpart.
• Sink a plastic barrier into the ground to contain the roots (although many plants, given time, will hop over such a barrier with ease or burrow beneath it).
• If you have the room, plant it in an out-of-the-way spot where it can roam as it pleases.
• Plant it in a location where its roots are confined. Have you got a narrow strip of ground along a back alley or alongside a driveway or sidewalk? You may be surprised at the niches you can find.
• Don’t provide the conditions it needs to thrive. Have you got a shady spot under a large tree where nothing else seems to grow? Try it there.
• Don’t unwittingly spread the unruly plant via your compost pile, where weed seeds will survive if the pile doesn’t heat up enough to kill them.

Watch the plant for a season to decide whether it’s worth the fight. Cultivate around it regularly, pulling out unwanted seedlings or taking a shovel to the edges of a spreading clump to keep it in its place. Remember that while you may admire and enjoy a vigorously spreading plant, your grumpy next-door neighbor might not.

Here are some plants that you may want to think twice about if you have a small garden (or at least keep your eye on them): fennel, wormwood and some other artemisias, evening primrose, some mints, comfrey, sorrel, common yarrow, horsetail, clary sage, and bamboo.

Kathleen Halloran, former editor of The Herb Companion, is a freelance writer and editor living in Las Vegas.

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