IN THE GARDEN TURN ORANGE BUSH BACK INTO AN ORANGE TREE.

Byline: JOSHUA SISKIN

Q: We have an orange tree that looks more like a bush. How do you prune the bottom branches without damaging the tree?

- Leon Callahan, Northridge

A: Pruning bottom branches on fruit trees will not damage them; it will simply mean that more fruit will be produced on higher branches.

Fruit production on any tree is determined, in large part, by the amount of carbohydrate or sugar manufactured during photosynthesis, which occurs in leaves. On any fruit tree, it is helpful to think of leaves as ``sources'' and fruits as ``sinks'' of manufactured sugar. Sugar manufactured in leaves is transported via branches and stems to developing fruits.

It so happens that on citrus (and many other fruit trees) bottom branches produce lots of fruit because, being lowest on the tree, they have the most sugar-producing leaves above them. For this reason, there is often a reluctance to prune these bottom branches.

The best way to turn your bushy plant into more of a tree is simply to let it grow. In general, tropical fruit trees such as citrus and avocado are pruned lightly, if at all. Pruning is done either to shorten the tree so that more fruit is produced at a manageable height or to remove dead wood. Commercial pruning of citrus trees is far from delicate or aesthetic. A long, mechanized hedge trimmer moves along the tree row, lopping off several feet of top growth as it goes.

Q: I have a number of perennial herbs: rosemary, marjoram, oregano, sage. Do I leave them alone or trim them back for the winter? How much should I trim back?

- Gene Rubin, North Hollywood

A: A good rule of thumb is to never reduce the size of a plant by more than one-third - two-thirds of the plant should remain - when pruning. If your rosemary is well-established, you can prune it at any time of the year. Your other herbs which, although perennial, are not woody like rosemary, should not be pruned until late February, just prior to the onset of new spring growth.

TIP OF THE WEEK: If you have a young tree that is failing, check the base of the tree to make sure it has not been damaged by the gardener's string trimmer (weed whacker). Beth Uyehara of Reseda sent an e-mail about a jacaranda tree planted last October that is ``as tall as our mature apricot tree but looks like a pole with short leafy branchlets coming out all around it.'' Trees damaged by string trimmers may take on such a look.

``We still have it staked,'' she continues, ``and for the first months we had it, every time we took the stake off, the tree collapsed on the ground.''

A tree that needs stake support one year after planting was either too skinny when planted, has yet to develop a strong root system, or both. When selecting trees, the most important criterion of overall strength is caliper size (trunk diameter).

It is better to acquire a small tree with a thick trunk than a tall tree with a skinny trunk. Also, I would check the condition of the soil and make sure it is not overly moist. The roots of a young tree will not grow into and anchor themselves in wet ground.