IN THE GARDEN MYSTERIES OF A NORFOLK PINE.

Byline: JOSHUA SISKIN

Q: I live in Van Nuys in a house with several old walnut trees. One of the trees in front is dying and I want to replace it. I saw a beautiful tree at Orcutt Ranch Park (east of the rose garden) and was told it was a Norfolk pine. The only Norfolk pine I can find anything about is the Norfolk Island pine, but the way it's described doesn't sound like it would do well in the San Fernando Valley. However, the tree at Orcutt Ranch looks great. Could you tell me anything about the Norfolk Island pine? Would it do OK in my area?

- Rhonda Haendiges, Van Nuys

A: The Norfolk Island pine (Araucaria heterophylla), which is not a true pine, may be the most distinctive tree you will see growing in our area. It has an undeniably exotic look with elegant needle leaves that hang down tightly along the stem, reminding you more of a spruce than a pine.

You are correct in wondering whether this tree could grow in the Valley. It is a subtropical species that is sensitive to cold; its habitat of Norfolk Island is located 900 miles east of Australia in the South Pacific. Yet, in protected locations such as where it is growing in Orcutt Ranch (on Roscoe Boulevard east of Valley Circle Boulevard), the Norfolk Island pine will thrive. By the way, it is as sensitive to Valley heat as it is to winter cold. Plant it where it is protected from the elements by taller trees such as elms, oaks, Shamel ashes or pines.

While you are at it, you may wish to examine two relatives of the Norfolk Island pine. One is the monkey puzzle tree (Araucaria imbricana); the other is the bunya-bunya (Araucaria bidwillii). Both have highly distinctive foliage and are more tolerant of cold than the Norfolk Island pine. All three of these trees do well as floor plants indoors.

You mention that you will be planting where a walnut tree once grew. Some people may wonder about this, since many plants will not grow under walnut trees, which have allopathic roots; such roots exude a chemical that inhibits root growth of surrounding species. Roses, for instance, simply will not grow in the vicinity of walnut trees. However, only living walnut trees present a problem to surrounding plants. Once the tree is removed (even if roots remain), there is no longer a threat to other plants.

Q: I have three hibiscus bushes in my back patio yard, and with the warm weather and now the rains, they have grown almost into trees. One is over 6 feet high, and the other two are 5 feet tall. I know they need to be pruned, but they are blooming so beautifully that I hate to cut them down. My question is should I trim them now, or would it make any difference if I waited another month.

- Mitzi Carver

A: Hibiscus, by nature, are arboreal plants and will grow to a height of 20 feet when left unpruned. They bloom on and off throughout the year - but only on shoot tips. If you prune hibiscus when they are flowering, you will have to wait another month or two until newly developed shoot tips receive enough sun to form flower buds.

Whatever you do, allow your hibiscus to grow naturally - and keep away the hedge shears. Hibiscus that are pruned into hedges never flower, since the tip growth that turns into flowers is never allowed to develop.