Digging In: How to Get Rid of Thistles Without Harming Other Plants

By Scott Aker
Special to The Washington Post
Thursday, June 4, 2009

Q We have been establishing a bed of pachysandra over the past seven years, adding more plants each year to fill a larger area. Over the past three years, the bed has been invaded with thistles. We have tried hand-pulling these weeds, but they return six weeks later. How do we get rid of the thistles without harming the pachysandra?

A The long-term solution will be to plant a tree to shade the bed. Pachysandra likes shade; thistles do not.

In the meantime, you can apply any herbicide containing glyphosate from mid-summer to early autumn. The herbicide is non-selective; it will also kill the pachysandra, so be careful when applying it. Wear a sturdy rubber glove that has been thoroughly checked for leaks. Put on a larger cotton glove over the rubber one. Mix the herbicide according to package directions in a small container. Dip your hand in the herbicide to saturate the cotton glove. Make a fist over the bucket or container to squeeze out any excess herbicide. Gently grasp each thistle to apply a dose of herbicide. If you are careful not to let any herbicide drip, you will not injure the pachysandra.

You may need to make several applications before each thistle is eliminated because it has deep rhizomes that are not easily killed.

You can get rid of thistles without chemicals by hoeing them weekly. An entire season of hoeing may be needed to dispatch the very persistent weed. You also should search your neighborhood for possible thistle stands that could be the source of your woes. Thistle seeds are carried by breezes.

My gardenia has been attacked by tiny cottonlike balls. Most of the leaves have turned brown and died, though it has buds growing. Previously, the flower buds turned black and died. How can I fix this?

Your gardenia may be afflicted with mealybugs. You can remove them individually with cotton swabs dipped in rubbing alcohol, but they are very hard to get rid of, especially if they have infested the roots as well. You may want to throw out the plant and start again. If you want to keep the container, make sure all the old soil has been removed and the pot sterilized with a bleach solution.

You would also need to change your winter care regimen. The plant was probably kept in a room that was too warm and lacked enough humidity, which is why the flower buds shriveled and turned brown. Gardenias are most happy if kept just above freezing during the winter with as much humidity as you can give them. If you decide to replace your gardenia, keep it in an unheated sunny room during the winter and be absolutely certain that it is not infested with mealybugs before placing it near other plants.

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

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