Dr. Bob Gilbert – Columnist
I moved here from Atlanta where thousands of Leyland cypresses were planted. It looked like there had been a ticker tape parade with Leyland cypress dropped out of the sky. For 10-plus years they created a plant bonanza. Nurseries could not keep them in stock. They sold like hot cakes. We had a border of them at Smith Gilbert Gardens and we were rooting cuttings as fast as we could give them away. Then Dr. Michael Dirr famed horticulturist from UGA commented that there was something wrong with them mostly caused by monoculture. This is a man-made condition caused by over planting and repeatedly planting just one (mono) species. Long ago farmers learned to rotate crops partly to avoid spread of diseases. Too many of the same species over time allows the proliferation of even rare and uncommon pests and diseases. Plus, Leyland cypress used to create hedge rows on property boundaries. They were planted close together. So, if one got sick they all got it. It was like crawling in bed with someone who has measles.
I have gotten ahead of myself. Leyland cypress’s have a most interesting history. It began as a chance seedling inEngland. An Alaskan cypress or Nootka cypress Chamaecyparis nootkalensis and Cupress macrocarpa or Momterey Cypress were planted very close together on an English estate owned by Christopher Leyland. Because these plants were not related, cross pollination was not expected. Their natural range is about 400 miles apart. In 1888, a seedling appeared that was a natural or accidental cross between a female cone of the Alaskan and a male cone of the Monterey. It is not clear to me how they could tell which plant supplied the male and which the female parts, but the same cross has since occurred naturally at least 20 times over the years. Because this is a cross between two plants of different genus Leyland cypress is written in the scientific literature as x Cupressocyparys leyandii (ku-res-o-paris). The X always indicates an intergeneric cross. Like most intergeneric crosses the resultant plants are sterile. The thousands of existing trees planted all over the world all came from those few change seedlings in England propagated from cuttings. Over 4o named natural variations-cultivars have since occurred and are on the market. These are variations found on a branch. A cutting is rooted and if the change in structure or foliage color is maintained and does not revert back is becomes a cultivar and is named.
Leyland cypress needs lots of light, can stand high levels of pollution, just about any soil type is ok and it grows fast. The tallest one is listed at 130 feet. When used as a hedge or as boundary fence it can block sunlight on the adjacent property. There have even been law suits involving neighbors around this shading issue.
Monoculture of Leyland cypress has allowed a dramatic increase of an uncommon but naturally occurring canker disease. For once we cannot blame a Japanese plant introduction for this disease. We are to blame, because there were so many hosts the canker has proliferated. A canker is defined as a symptom of injury resulting in an open wound that has become infected by bacteria or fungi. It can weaken a plant. Browning of the foliage at the tip of a branch is the first indication that the canker disease is present. Left unchecked the disease progresses down the branch all the way to trunk. There is no treatment for this problem except cutting off the infected branches back to healthy wood. Sterilizing pruners before and after is extremely important as the canker organisms can be transferred on pruners and saws. Once the disease reaches the trunk the entire tree is infected and it needs to be totally removed. Infected parts that are trimmed off need to be burned or put in the trash.
A minor problem on Leyland’s is bagworms. These can be controlled with sprays and if accessible can be pick off before they hatch.
It is curious that it takes up to ten years before the canker on Leyland’s shows up. When I first moved here about nine years ago I found little evidence of it. Now I am finding it often. The question is do you wait for evidence of the canker or be proactive and replant? I would guess that if you have a Leyland Cypress 10 years old or older you have a 90 percent chance you will get the canker disease. This estimate is not based on scientific facts but is simply my opinion. The Leyland’s at Smith Gilbert were removed years ago before they got infected. The replacement plant materials are now mature so there are no gaps in the landscape.
Leyland’s have a local twist. J.C. Raulston, founder of the Raulston Arboretum in Raleigh, commercially introduced this plant after testing it for hardiness. He has been quoted as saying that the one and only thing he regretted having done in his life was introducing this plant. He traveled all over the world collecting plants and introduced other new and wonderful plant materials for us to enjoy so he can be forgiven.
Fortunately, there are some beautiful and mostly disease resistant substitutes that can make great hedges. These three hould be easy to find:
1. Thuja plicata – Western Red Cedar or Giant or Western Arborvitae
2. Thuja standishii x plicata ‘Green Giant’
3. Cryptomeria japonica ‘Yoshino’. If you need ideas for a hedge google “tapestry hedge- images.” The photos suggest ways to create a hedge and avoid monoculture with some uniquely beautiful examples.
Dr. Bob Gilbert, now living in Franklin is cofounder of Smith Gilbert Gardens in Kennesaw Ga. Karen Lawrence is a prefessional photographer of botanical subjects and wildlife is from Franklin, N.C.