Controlling Invasive Plants

Herbs become invasive in one of two ways. Some freely scatter their seeds, that seem to sprout with ease even in the most inhospitable conditions. Others send out rhizomes (root-like stems) that meander along just under the soil's surface; sprouting new plants as they go. Both are highly efficient ways to overrun your garden and choke out existing plants. Here are some ways to keep these invasive herbal companions in check.

Effective Methods of Control

• Plant them in pots. Individual herbs can be controlled easily by planting them in containers. The containers can then either be left above ground, or you can dig a hole in the garden and sink them into the ground, leaving an inch or two of the pot's rim above the soil line, so the stems won't creep over the edge and root in. A six inch pot with drainage holes is a good size to start with. Herbs in pots sunk into the ground should be dug up and repotted every 2 to 3 years to prevent the plants from becoming root bound.

• Smother them. Lay down a thick layer of mulch, newspaper, plastic, or landscaping fabric under mature herb plants to discourage seeds from sprouting and to smother young plants.
• Edge them out. Install plastic edging underground to stop herbs that spread via roots or runners. The edging must be planted fairly deep in the soil (at least 10 inches) or rhizomes will simply burrow underground and pop up a few feet away.
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• Pinch them back. Stay one step ahead of invasive growth by pinching herbs back vigorously. This will prevent them from producing flowers and casting seed all over the garden.
• Deadhead them. Remove spent flowers as soon as they wilt to stop invasive plants from reseeding.

Dealing with Escape Artists

If you find that one of the invasive herbs in your garden has managed to escape, dig up the unwanted seedlings, making sure to remove the entire root system. Then sift through the soil with a digging fork to make sure you get any leftover pieces. Keep an eye on the area for several months, and either dig out any plants that reappear or clip them off at soil level and smother them for several weeks with a heavy layer of newspaper or a large, flat stone.

A Short List of Invasive Herbs

The following list of invasive herbs is not meant to be complete. In fact, in your region, the list may be much longer:

• artemisia (Artemisia spp.)
• bee balm (Mondara spp.)
• chives, garlic (Allium tuberosum)
• comfrey (Symphytum officianle)
• costmary (Chrysanthemum balsamita)
• dandelion (Taraxacum officinale)
• fennel (Foeniculum vulgare)
• German chamomile (Matricaria recutita)
• Herb-Robert (Geranium robertianum)
• hops (Humulus lupulus)
• horseradish (Armoracia rusticana)
• kudzu (Pueraria montana var. lobata)
• mints (Mentha spp.) (All members of the family, including pennyroyal, peppermint and spearmint)
• St. John's wort (Hypericum perforatum)
• tansy (Tanacetum vulgare)
• violet (Viola ordorata)
• yarrow (Achillea spp.)

Catalogue Clues: When shopping for herbs in garden catalogs, invasive herbs should come with a warning (mint in particular), but they usually don't, so you'll need to do your homework as well as learn to read between the lines. Plant descriptions such as 'carefree", "vigorous", "establishes quickly", or 'grows anywhere" may be code for invasive tendencies. When acquiring herbs locally, consult other gardeners, local nurseries, and your local extension agent about herbs that are invasive in your area.