Thin-bark trees offer beautiful flowers but care is needed to protect their trunks

15-thinbarkMany people consider redbuds, although more pink than red, to be the most brilliant of all spring flowering trees.

In full bloom, they appear as blinding flashes of magenta-pink against a background of smooth gray branches. Before a single leaf appears, branches are studded with flowers. And if the flowers themselves are not sufficient to keep your interest, a display of heart-shaped leaves, with broad bronze margins, are soon to follow. Eventually, these leaves turn to a pleasant lime green and, in the fall, will change to gold, orange and red.

There are both western redbud (Cercis occidentalis), a California native, and eastern redbud (Cercis canadensis) trees. A popular cultivar known as ‘Forest Pansy’ is noted for its burgundy foliage. All redbuds are extremely cold hardy and grow fine in the Antelope Valley. Their flowers attract hummingbirds and their mature form, when given freedom to grow in every direction, is a 20-foot diameter globe that never requires pruning.

Yet plants, no less than people, are often flawed, regardless of their physical beauty. In the case of redbud trees, the problem lies with their bark, which cracks easily. It has been my observation that western redbuds, whether planted in gardens or parks or just kept in containers, are likely to exhibit cracked bark, particularly on the crown. When speaking of trees, “crown” refers to the lower portion of the trunk, from roughly a foot above the soil line to where it meets the ground. Many redbuds are also grafted trees and bark around a graft union, located in the crown area, is the weakest bark on a tree.

Thin-barked trees, which include redbuds, maples, and nearly all fruit trees — from peaches to citrus to avocados — are susceptible to so-called southwest injury. The hottest exposure on the side of a building or the side of a tree is that facing southwest, and thin-barked trees are most likely to exhibit cracked bark on their southwest side.

But it is not only scorching summer heat that causes thin-barked trees to crack. Bark cracks are due to the sensitivity of the crown area to sudden changes in growing conditions. If there is a prolonged dry spell followed by sudden, heavy rain, growth of interior cells may be more rapid than exterior bark, and cracking may occur. A frost followed by warm weather may have a similar result. Finally, excess fertilization in the fall and a sudden growth spurt, followed by a frost, may also lead to cracked bark.

A similar set of circumstances, incidentally, leads to split citrus and tomatoes. When soil suddenly goes from dry to wet, and there is a sudden influx of water into a tree and its fruit or into a tomato plant, pulp expands faster than peels or skins can grow and so the fruit splits. This is an excellent argument for mulching, which tends to keep soil moisture levels even.

In their habitat, thin-barked trees protect themselves by developing a naturally shrubby growth habit so that their bark is shaded from exposure to the elements. Nearly all fruit trees, left to their own devices, are more shrublike than arboreal. As horticulturists, we train them into trees so that we can fit more of them into an orchard and maximize our harvest.

Many fruit trees have strong suckering tendencies and, knowing what we do about cracked bark, it is easy to understand why. Suckers, which grow up from the base of the trunk, provide protection from heat and cold to the easily cracked crown area.

To prevent cracking, paint your tree’s bark, or at least the bottom portion of the trunk, with white latex paint. Alternatively, cover the crown with a tree wrap, available at some nurseries and through online garden supply vendors.

Orchid trees (Bauhinia spp.) are leguminous kin to redbuds and, like them, flower at this time of the year. You really have to get up close to appreciate orchid tree flowers. From a distance, all you see is their pink to purple colors.

Close inspection reveals their corsage dimensions, best appreciated by floating them in a bowl on your dining room table.

Enjoyment of this tree is mitigated by its amorphous growth habit. It is a challenge growing orchid trees as standard, single-trunked trees, since they sucker profusely. Their pods also can be a nuisance. However, there is a hybrid species known as Hong Kong orchid tree (Bauhinia x blakeana) that does not produce pods.

For years, cinerarias (Pericallis x hybrida) were grown in the spring for their unmatched flower power but discarded soon after as blooming subsided. Now, however, there are repeat blooming cinerarias that are worth a second look. Grown in a properly shaded bed in soft, crumbly earth, the type of conditions in which azaleas and ferns would feel at home, cinerarias may even self-sow so that you will have an expanding crop of them from one year to the next.

Q: I have had a flowering plum tree growing in a north-facing backyard for approximately 17 years. It was a consistent bloomer for 15 years and in the past two years I have noticed 1/3 of the tree does not get new leaves or blooms. My gardener, for the first time, cut back the branches in late January of this year. Could he have cut back the new growth? This year it only partially bloomed and now it has very few leaves. What is the solution for more leaves/blooms?
— Karen Mansky, Oak Park (Conejo Valley)

A: 17 years is old age for a flowering plum tree. Flowering plums have weak immune systems and generally go into decline between 15-20 years of age. That’s the trade off with small, flowering trees. They are easily maintained but they seldom look good for more than 15 years or so.

Tip of the week

If you are reluctant to apply snail and slug bait because of apprehension over its consumption by pets or wildlife, you can relax thanks to a product called Sluggo. Not only is Sluggo non-toxic, but it contains iron phosphate that feeds your plants as it dissolves into the soil. Sluggo is widely available in nurseries and home improvement centers.

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