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Improper pruning can lead to sunburn damage in trees
By Tony Tomeo
This past winter I spent much of my "free" time restructuring some of the ancient apple trees back at the nursery. Many of the trees are more than 100 years old and have been abandoned since 1974.
Most have been severely damaged by improper pruning. Lower parts of their canopies have been shaded out long ago, and restructuring involved removal of so much dead and damaged material in the upper canopies that some of the remaining limbs were dangerously exposed to the sun.
I do not earn enough to purchase Coppertone by the gallon, so I hung fertilizer bags over these limbs and fastened them in place with duct tape. I tried to leave the bags loose and open on the north and lower sides so insects would not proliferate under them. However, the trees look like they lost their tops to a tornado which previously struck a nearby fertilizer factory. As much as I enjoy the apple trees, I would not want any of them in my garden looking like that.
Some might wonder why sun protection would be so important to trees which normally grow in full sun exposure. It is because pruning and structuring is very unnatural for apple trees, as well as many other species, which rely on their own foliar canopies for shade. Prior to selective breeding, apple trees did not produce such heavy fruit in such abundance, so they could live quite well in the wild without pruning.
Pruning is done so that trees are not strained to overproduce (often inferior) fruit, the weight of which breaks the trees apart. Properly pruned trees maintain a well foliated canopy. Unusually severe pruning is often necessary, however. Both fruit trees and ornamentals, such as flowering cherry trees, are staked and trained on single straight (exposed) trunks, because low shrubby trees which shade their own limbs are not practical for most gardens.
Sensitive species which are grown on tall trunks may require sun protection when young, but not at maturity when sufficient canopy has developed. Because sun protection is a temporary concern, I recommend temporary solutions.
For trees with straight trunks, such as cherries and hollys, a light plank may be tied to the south side of the trunk. A strip of duct tape from the top of the trunk to the bottom is also effective. Newspaper may also be used, but probably looks just as shabby as fertilizer bags. I prefer to plant pole (climbing) bean seeds at the base of the trees every spring. The bean plants grow up the trunk before hot weather arrives.
Although unsightly, shading is only necessary during warm sunny weather from late spring to early autumn. Painting trunks white is effective, but stays with the trees long after it is no longer necessary. (If you choose to paint, I recommend paint which was mixed improperly for a previous client and sold very cheaply. As long as it is nearly white, it will be fine. Trees are not very fashion-conscious.) Shading is usually not necessary for trees shaded by other trees or shrubbery during the hot parts of the day.
Newly planted trees are more susceptible to sunburn, not only because of their small canopies, but also because they are not conditioned to grow in direct sun exposure. In the nursery environment from which they came, their trunks were shaded by other trees growing with them. In the landscape, nearby trees and shrubbery help absorb some of the glare from the sun. However, pavement, walls or any reflective surface will enhance the severity of sunburn.
Cherry and flowering (fruitless) cherry trees are probably the most sensitive to sunburn. Cherry relatives, such as apricot, peach, nectarine, plum and almond, are also very sensitive. Apples and crabapples are moderately sensitive as well as various holly, photinia, avocado, camphor, maple, locust and ash.
There are many more species which are subject to sunburn than I can list. Generally, those which have smooth bark are more sensitive. Trees like avocado and camphor are only sensitive when young, prior to development of thicker bark. Shrubs which are trained onto single straight trunks, standards such as holly and photinia, though sensitive to sun exposure as trees, sufficiently shade their own trunks when grown as shrubs.
Sunburn may be seen on the bark surface facing the sun as the surrounding unaffected bark expands past it, leaving it sunken or flattened and peeling. Eventually, bark peels away, exposing dead wood which is often infested with boring insects. On straight vertical trunks, burn will be seen as a symmetrical vertical strip. In severe cases, affected trees or limbs are not worth salvaging. The time and resources might best be used on replacement and proper maintenance of new trees. Minor cases may recover, especially if the burned areas are protected while healing. Fertilizing will improve a tree's vigor and ability to compartmentalize or heal.
Tree of the Week: Norfolk Island Pine
My old colleague from Cal Poly, San Luis Obispo, Brent Green and I always joked about the Norfolk Island Pine, Auaucaria heterophylla. Its towering conical form and perfect symmetry make it look like something one would build in their garden rather grow. While driving past a large specimen in Santa Barbara, Brent asked me, "Do you think the Parisians will want it back?" Although they can grow to 100 feet tall, they are usually grown as houseplants or specimens in tubs on patios where they are protected from cold. Most of us are familiar with its small, half-inch long, needle-like foliage and soft texture. Few middle-aged trees may be seen around the Monterey Bay Area or San Francisco.
Horticulturist Tony Tomeo may be contacted at 408.358.2574.
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Saratoga, California Since 1955
