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How to Kill Invasive Clematis

By Scott Aker
Special to The Washington Post
Thursday, May 14, 2009

Q Our garden has a fall-blooming clematis that just won't die. It tries to smother neighboring plants. I have tried Roundup, boiling water, vinegar and digging at the roots. Unfortunately, the plant is well-established close to the fence, making it hard to excavate the roots. Do you have any suggestions that won't harm a nearby rose bush?

A This lovely vine with vanilla-scented flowers looks innocent enough, but it is one of our worst invasive plants. Begin by cutting it to the ground in late summer, before it has a chance to bloom or set seed. Treat the cut ends of the vines with an herbicide containing triclopyr, such as Ortho's Brush B Gon. Follow up by checking the area frequently for any seedlings so you can dig them out before they become established.

I have been fighting the blight with my tomatoes for several years. I have sprayed fungicide regularly, removed leaves and cleaned up all the old plant foliage. Still, it returns. I've read of three different approaches to fighting the disease: Move the tomato plants elsewhere (unfeasible for me), cover the existing tomato bed with a black tarp in late winter, or have a pro spray a chemical and then lay a tarp. What are your ideas?

Try solarizing your soil to reduce the reservoir of spores that can lead to early blight and a host of other tomato diseases. You still have time to do this for the current growing season. Till the soil and moisten it thoroughly. Place bricks or plastic bottles at intervals on the soil surface to elevate a layer of clear plastic sheeting. The plastic should be clean and unbroken. Dig a shallow trench around the area and bury the edges of the plastic. Three weeks of bright sunshine will create enough heat under the plastic to kill many plant pathogens, insects and weed seeds. You can use a soil thermometer to verify that the soil reaches a temperature of 140 degrees for several hours each sunny day.

After solarization, remove the plastic and the spacers and plant your tomatoes, mulching them to conserve soil moisture. Avoid any overhead irrigation. Stake the tomatoes to keep their foliage from drooping, and allow enough space between plants to encourage good air circulation.

The blight spores are carried by wind, and infections may still appear if neighbors are growing tomatoes or related plants and weather conditions favor the disease. Even then, you can minimize problems by planting tomatoes late, until around Independence Day.

As the vines mature in late summer, the blight may be less of a problem. Late-season tomatoes often have very good flavor because they mature in conditions of cool nights and warm days.

Scott Aker is a horticulturist at the U.S. National Arboretum.

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