GARDENING : EARLY SUMMER TURNS ATTENTION TO TREE TRUNKS.

Byline: Joshua Siskin

With spring in Los Angeles arriving midwinter, it only makes sense that summer should come in early spring. A new kind of sun has arrived - one that turns the sky pink and mauve and purple in its wake, the same colors seen in flowers that have bloomed in recent weeks.

Most prominent among the pink and purple bloomers is the purple orchid tree (Bauhinia variegata), the flowers of which are actually more pink than purple. This tree is as famous for its problems as for its beauty. Its greatest difficulty in life is standing up straight. It tends to bend and flop all over the place unless staked, restaked and staked again. Then, not too many years after you finally straighten out its trunk, it dies, not being possessed of genes for arboreal longevity.

In general, staking of any tree should be avoided, if at all possible, when planting. Except in windy areas, tree staking should not be necessary - as long as the tree has been properly trained in the nursery. Most trees are acquired in 15-gallon containers at a height of 6 to 8 feet. Unfortunately, many of these trees, when put in the ground, are incapable of standing on their own.

It makes more sense to plant a strong 5-foot tree than a weak 8-foot tree. A 5-foot specimen that can stand on its own is preferable to an 8-footer that needs staking. Once a tree is staked, two kinds of problems may develop: The tree becomes dependent on the stake, and can never stand on its own; and wire used to stake the tree - even if looped through a short piece of old hose - eventually restricts the flow of water and carbohydrate up and down the trunk, weakening and perhaps killing the tree.

When you purchase a nursery-grown tree, there is usually a 2-by-2-inch stake attached to the trunk. This stake has been used to make the trunk grow straight and, it would appear, to protect the tree from damage during handling. Throw this stake away; instead, take two 8-foot lodge poles and sink them into the planting hole on either side of the root ball prior to backfilling the hole. Use elastic rubber ties, and never wire, when staking. Loop the ties around the trunk - and secure them to the stakes - at the minimum height necessary for the tree to stand erect.

One of the most important criteria for tree selection is the caliper size (diameter) of the trunk. Trees require staking because they are top heavy, which means that the trunk is not thick enough to support the tree canopy on its own. It would appear that trees are forced to grow rapidly to a saleable height at the expense of proper trunk development. The person planting the tree is compelled to pay the price - in staking - for a poorly proportioned, unbalanced specimen. After the tree has grown for awhile - and its caliper has thickened - thin out the canopy before removing its stakes. This will ensure that the tree won't fall over when it loses its lodge-pole support.

If you don't wish to be bothered with the staking dilemma, try to find multitrunked trees which, being shorter and more balanced than the standard or single trunked specimens, should not require artificial support when planted.

As if the orchid tree didn't have enough problems standing on its own, its leaves are frequently chlorotic (yellow with green veins) and suffer from tip burn, the result of sensitivity to alkaline soil. Put peat moss in the planting hole and mulch heavily to minimize the symptoms of chlorosis.

One of the orchid tree's (Bauhinia sp.) common names is camel's foot, so called because of the resemblance of its two-lobed leaf to that of the camel's hoof. If you ever wondered how plants get their botanical names, such as Bauhinia, consider this explanation from ``What Tree Is That?'' by Stirling Macoboy: ``In the 18th century, when a new genus of tree was discovered that bore uniquely twin-lobed leaves, a suitably paired name was soon forthcoming. A diligent search revealed the names of two 16th-century botanists, twins perhaps, brothers certainly. And so the brothers Bauhin became immortalized in the curious foliage of these lovely trees.''

Another famous tree that has just now finished its period of pink-purple flowering is the redbud. The western redbud (Cercis occidentalis) is native to California, and the eastern redbud (Cercis canadensis) is native to the eastern United States. The most outstanding exhibition of these trees is at Descanso Gardens in La Canada Flintridge. The many specimens on display there are mature and multitrunked. The ``Forest Pansy'' variety of the eastern redbud, which is available at many nurseries now, has purple leaves and requires some sun protection to grow in the Valley.

For a matching ground cover under your orchid or redbud trees, consider the Mexican evening primrose (Oenothera Berlandieri). This remarkable plant disappears into oblivion during the winter, only to emerge as a fresh carpet of pink in the first warm days of spring. Although it is often labeled invasive, this is the kind of plant you don't mind being invaded by - on account of its superficial roots. Let it grow around any kind of tree or in the rose garden; it hardly needs watering.

Tip: For long-lasting cascades of pink and purple flowers, select perennial Verbena. Keep this plant in full sun, since it is likely to develop mildew anywhere else.

MEMO: Joshua Siskin's column appears every Saturday. He welcomes questions from readers. Write to him in care of the Daily News Features Department, P.O. Box 4200, Woodland Hills, Calif. 91365-4200. You can also reach him through this on-line mailbox: JoshSiskinaol.com.

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Photo: Unless high winds pose a problem, a healthy tree sho uld be able to stand up without staking.

Gene Blevins/Special to the Daily News