IN THE GARDEN MAKE SURE YOUR SHRUBS GET THEIR PROPER DOSE OF IRON.

Byline: JOSHUA SISKIN

Q: Can you tell us how to prune hibiscus and similar plants to make them grow bushy and beautiful? Is this the best time of year to do this?

- Jay Grey, Northridge

A: Hibiscus is a tropical plant. Therefore, it should not be pruned during the winter.

In order to prune any plant properly, you need to know where its flowers are produced. Hibiscus produces its flowers on shoot terminals (stem ends). This means that hedging your hibiscus, as gardeners have a tendency to do, will severely curtail its blooming since flowers will not be given a chance to form.

To keep hibiscus blooming nearly nonstop, remove shoots as soon as they stop flowering. Cut spent flower stems far back into the body of the plant. This will keep the plant thinned out and allow new shoots to emerge. For increased bushiness, prune out around one third of the older stems in March.

As a general rule, continuous flowering and bushy growth of shrubs demand a steady feed of fertilizer and constant attention to pruning. Sometimes, regular fertilizer is inadequate. I have found that slow-release and iron-sulphate fertilizers are essential to keep shrubs such as Japanese boxwood (which may otherwise turn orange), photinia, princess flower, hibiscus, gardenia and escallonia lush throughout the year.

The high pH of our soil means that any iron that is present in the ground is locked up by alkaline minerals. Iron sulfate or other iron-rich fertilizers are needed as insurance against this natural alkalinity. In winter, it is all the more important to add iron sulfate and other fast-acting fertilizers since the biological and chemical reactions that make mineral elements available to plants slow down in cold weather.

To keep a lawn green, it is necessary to apply urea, ammonium nitrate, or ammonium sulfate as a nitrogen source, together with a high-iron product. Most plants, and definitely all tropicals, prefer a slightly acid pH. To lower soil pH, apply a thin layer of gypsum, which is probably the least expensive soil amendment, over the entire landscape twice a year.

TIP OF THE WEEK: Be aware that oleander shrubs are dying all over the Valley. They are being killed by a bacteria that lives in the mouth of leafhopper insects. There is some evidence that aggressive pruning will slow establishment of this disease, known as oleander leaf scorch. Any shoots with leaves showing the slightest tip burn should be removed. Unfortunately, any sort of stress in a plant, whether from too much or too little water, over-application of fertilizer or any other chemical stress, may result in leaf tip burn. A tiny wasp that parasitizes leafhopper eggs has been located in Mexico and Texas and is being bred and released in hopes that it will soon control leafhopper proliferation in Southern Calfiornia.