Uprooting wisteria, and other nonnative plants

Blooming wisteria at Kanapaha Botanical Gardens in Gainesville, Blooming wisteria at Kanapaha Botanical Gardens in Gainesville, Fla. (March 19, 2009) Photo Credit: AP

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Jessica Damiano Jessica Damiano, Newsday columnist

Jessica Damiano is a master gardener and journalist with more than 20 years experience in radio, television, print ...

DEAR JESSICA: Years ago, I cut down a wisteria. Now, even through zoysia grass, I am still getting about a dozen shoots between lawn cuttings. One is about 15 feet from the rest! Do I just dig down until I hit something, or is there another, easier way to get rid of it? --Len Sposato, Huntington Station

DEAR LEN: Your experience makes a good case for planting native plants. There are several varieties of wisteria, and most of them originated in China or Japan. Those are the ones that become invasive and difficult to eliminate and control, because, however well-behaved they might have been in their native land, they somehow lose their manners in our climate. That's also a good reason you should never sneak seeds or plants out of another country after falling in love with a foreigner. I'll write more about this and suggest alternatives for some favorite non-natives next week, but for now let's focus on your problem.

Ordinarily, the first course of action I would recommend would be to cut down or mow the shoots continually until the plant weakens and eventually dies. This process requires tenacity and can take several years, but since you've already tried this without success, it's time for plan B.

All those baby sprouts you're seeing are originating from the underground roots of the mother plant, even though you cut it down. The best way to control its offshoots is to kill it at the source. Return to the stump and, using a 1/8-inch bit, drill inch-deep holes about a quarter-inch apart all over the surface of the cut trunk and carefully pour a liquid weed killer, such as Roundup, into each hole. The roots will transport the herbicide to the offshoots. If any of the offshoots are larger than an inch in diameter, they should be treated in the same manner. Extreme caution should be taken when applying the herbicide because it has the potential to kill every plant it contacts, including grass, shrubs, perennials and trees.

DEAR JESSICA: I have six beautiful and healthy golden arborvitae trees across the front of my 100-foot property line. I would like to have them trimmed. They are about 12 feet high and at least 63 years old.

A reader from Hampton Bays wrote to you recently and mentioned he had his trimmed on top two years ago. I have been advised that it will ruin the trees' cylinder-shaped top and make them unattractive. Will the trees eventually regrow their pointed tops? If we decide to go through with the trimming, how much should be trimmed? I would like to have at least 2 feet removed. --Mary Shorthouse, Westbury

DEAR MARY: The advice you were given is correct. Although you certainly can remove 2 feet from the top of your arborvitaes without harming the trees, new growth won't replace the removed upward-growing tips, and the tree tops will remain flat.

DEAR JESSICA: Recently a landscaper tied a rope around my dogwood and tied the other end to his truck so that his workmen could take down a large oak tree in my neighbor's yard in an effort to direct where it would fall.

That was accomplished all right, but the rope, which was very thick, removed a nine-inch section of bark from the tree. I had heard that a tree will die if the bark is damaged. Should this bark damage be covered with pruning sealer? --Joan, Wading River

DEAR JOAN: Tree bark is composed of several layers, each serving a different purpose. The outermost layers provide trees with protection against insects, moisture, dehydration and temperature extremes, while the innermost layers are responsible for distributing sugars and water throughout the tree. The harm caused to a tree when its bark is removed is directly related to the depth of the damage as well as the surface size of the injury. Damage to less than 25 percent of the circumference of the trunk should heal with proper care. If the affected area is larger, the tree may eventually decline over the course of several years.

There is one vertical section of bark depicted in your photo that appears to be quite deep, but the rest, assuming the other three sides of the tree are not affected, doesn't seem insurmountable. To prevent the tree from further stress and facilitate healing, it's important to keep it watered and give it a boost with a slow-release fertilizer applied under the drip line of the tree (take care to avoid allowing the product to come into contact with the trunk). You should also clean up the jagged edges of the wound using a sharp knife to score a shallow area outside the perimeter of the damage. This will help the tree heal itself more rapidly.

Pruning sealer could actually encourage decay, so it's best to avoid using it.

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