Amanda McNulty: Hedging your bets for privacy screen

March 17, 2011 

My grandparents filled their hot-weather home in Saluda, N.C,, with grandchildren and their youthful companions. Every summer we had an epidemic of green death — resulting in numerous boys and girls leaning over the open porch railings since two bathrooms couldn’t handle a dozen upchucky children. Four or six of us staying in each bedroom, even with multiple beds, created a scenario designed to spread disease.

We’ve created the same situation with plantings of Leyland cypress. Twenty years ago the fact sheets on this fast growing, handsome conifer listed it as practically disease free. The revised pamphlets state that overuse and improper spacing have created a pest house where diseases run rampant.

Touted as a quick-growing and inexpensive screen, Leyland cypress was planted in straight lines along property borders or wherever privacy was desired. Easy to propagate, small plants sold for a song, so why not plant one every three feet; all the better to block off that neighbor who insists on weird yard art or won’t paint the house. If you fell prey to this idea, consider removing every other one if they’re four feet on center since this plant easily spreads 10 to 12 feet, and tops out over 40.

Sadly, this hybrid plant with its parentage in the damp British Isles is shallowly rooted and intolerant of either too dry or too wet soils. When stressed by inadequate water, it becomes more susceptible to insects, particularly bagworms, and several devastating canker-causing fungi. The most common, Seiridium unicorne, usually attacks lateral branches which in death become bright orange-red. Another, Botrosphaeria dothidea, more often causes death of the central leader.

The oozing cankers contain the innoculum and the disease spreads as rain or irrigation water splashes on the wounds. You can cut the infected limbs several inches below the point of infection and destroy the diseased material, being careful to sterilize your tools after each incision with a 10 percent bleach disinfectant — and wear old clothes. Some plants recover from infections of S. unicorne if they are kept well-irrigated in times of water stress.

Please read about Leyland cypress diseases at Clemson’s Home and Garden Information Center, www.clemson.edu/extension/hgic/.

A practice that will benefit your Leyland cypress and all landscape material is to water one inch, all at one time, every seven days in the growing season if we don’t get that much rainfall. (You do have a rain gauge, don’t you?) Equally important is to mulch beyond the drip line to conserve that soil moisture. If you water early in the day, your plants can dry before evening.

Last month I saw a new disease, Passalora (or Cercospora) needle blight, which causes the needles in the lowest limbs and nearest the trunk to die. Often it begins on trees, planted too close together, that receive overhead irrigation. Unlike Seiridium and Bot infection, Passalora can be controlled with a fungicide if you improve air circulation and water without wetting the needles.

So what is a shy homeowner, craving privacy, supposed to do? Thuja “Green Giant” was the recommended replacement, but as it too has been crammed together and overplanted, it’s also falling prey to these diseases.

A mixed hedge, with varying sections of similarly textured plants, can be more attractive than a solid green wall, and if one species develops trouble, you’ve still got the bones of your hedge intact to hide behind if fire ants get in your pants.

Amanda McNulty is an associate extension agent for the Clemson University Cooperative Extension Service and is a co-host of “Making It Grow” broadcast weekly on ETV television stations.

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