In the Garden:
Lower South
February, 2005
Regional Report

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This Mexican petunia is a great heat-tolerant landscape plant, but it's prone to disagreeing with the gardener on just where it should and should not grow!

Trespassing Plants

"Wanted: A plant that will quickly grow to fill its allotted space and then stop growing, requiring no trimming or dividing."

We gardeners would love such a plant, right? No one wants to wait for a new planting to mature or fill in. Our goal is a full, attractive perennial border, ground cover, lawn, or herb garden. Most plants, however, fall into one of two groups. Some are slow growing and need to be coaxed into accepting our landscape as a suitable home. Others hit the ground running and never look back. They fill in rapidly alright, but then ignore their assigned spaces in our landscape and jump their borders in an assault on their neighbors.

Managing these trespassing plants can be a bit of a challenge. It is often best to avoid them, choosing less invasive options. In some cases, however, they may be good choices. There are things we can do to keep them in bounds; that is, to make them behave amidst their more well-mannered neighbors.

What's the Alternative?
Consider choosing less-invasive varieties of plants you like. Bamboo is a wonderful plant that offers beauty and makes a great screen. Running types are prone to invade, especially in moist soil conditions. Clumping types, on the other hand, stay pretty much in place, making them a better choice for most landscapes.

The standard berrying types of Nandina also tend to spread underground, as that bush begins to dream of becoming a thicket. Most dwarf types are much less prone to spread.

Chinese Privet (Ligustrum sinense) and Japanese Privet (Ligustrum japonicum) are notorious invaders throughout the south. They produce berries that are eaten by wildlife, especially birds, spreading the plant far and wide. There are many other great shrubs you can substitute here in the south to avoid problems.

Curbing Wanderlust
When a plant with invasive tendencies is considered an important choice for your particular landscape, there are often steps you can take to keep it in bounds. This is especially true of those plants that spread by runners or rhizomes.

Vertical barriers are an effective way to block rhizomes. The basic idea is to install a sheet of galvanized metal or thick plastic to form an underground wall that extends from just above the soil surface to a depth below which the rhizomes won't grow. If you lean the barrier slightly outward at the top, so the below-ground portion is closer in toward the invasive plant, the rhizomes will be less likely to turn downward and attempt to grow under the barrier.

The key to success with this technique is to make sure the barrier is plenty deep to prevent the plant from growing underneath it. Mint, for example, can be fairly well contained by a sheet metal barrier placed to a depth of about 10 inches.

Another way to confine such a plant is to plant it in a black nursery pot and sink the pot into the ground so the edge of the pot extends a couple of inches above the soil surface. You will need to trim the runners periodically so they won't simply jump the fence and take off running again! I prefer to grow such plants in containers on the porch or patio where I can keep an eye on them so I don't have to be so diligent about pruning runners!

A few of my favorite plants are prone to seeding themselves about the garden. The common, tall type of Mexican petunia (Ruellia brittoniana) is a prime example. I have learned that to keep it from taking over, I am going to have to maintain a pretty thick mulch and be willing to hoe or pull some of the volunteers each year. If removed soon after they pop up in unsuitable locations, they won't have time to develop rhizomes and start the underground spread.

Sometimes a little spreading is okay. After all, cottage gardens are basically an excuse to allow plants to decide for themselves where they want to grow. We just move in to provide a modicum of order. Unless you are dealing with a truly brutal invader, the plants will tend to settle into a nice mix, leaving you with a little hoeing and mulching to prevent some overzealous species from taking more than their fair share of space.


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