COVER STORY : Beating the Heat : With planning, your garden will thrive during the Valley's searing summer. Try branching trees, succulents and sun-loving herbs.

SPECIAL TO THE TIMES; Susan Heeger writes regularly about gardening for The Times.

Summer in the San Fernando Valley might be ideal for pool hopping, mall crawling or frying an egg on the sidewalk, but it's murder on the average fuchsia. As the mercury ticks past 90 and the Santa Anas start ripping, local gardens can literally dry up and blow away if nobody's watching.

Hot-weather garden vigilance often means more watering and more mulching, as well as introducing shade where it's needed to cool things down. But the most critical action, says Agoura landscape architect Gary Krause, involves planning--during the summer and year-round--to create a garden that can take the heat.

"The key is to start by choosing plants that require less moisture--not just California natives but exotics compatible with our climate," advises Krause, a professor of landscape architecture at Pierce College. "If people want things that are lusher, they can have them, but they should reduce the size of these plantings."

Frank Perrino, a Woodland Hills landscape designer, agrees, calling Valley temperatures and dry winds "a double whammy" for the gardener. He points out that in these conditions, a plant's heat tolerance is just as crucial as its drought tolerance, and both factors should be considered during the selection process.

Among plants that do well on both counts, Perrino recommends succulents, such as flower-like echeverias and blue-gray Senecio mandraliscae, and sun-loving herbs: lavenders, society garlic and assorted sages. Rather than grouping these with their own kind, he advocates mixing them with other blooming greens to make colorful borders that not only hold up in heat waves but present an interesting blend of textures.

Other wise summer choices include bougainvillea (especially good, says Perrino, against a scorching wall), reblooming day lilies and dwarf pampas grass. Pampas grass, which needs a lot of growing room, can work well on a large hillside and looks striking, Perrino says, in combination with something low and spreading such as Mexican bush sage.

While these plants drink far less during the summer than thirsty alternatives such as lawns, even they need some watering, either by drip or spray irrigation. Karen Petersen, assistant manager of Chatsworth Nursery, advises gardeners to make sure sprinklers are working and check plants constantly for signs of wilting. "Don't wait till something droops before you notice it," she counsels.

Large trees, in addition to regular watering, need a slow, deep soak every week or two. This is particularly important, Petersen says, because trees and other tall plants create welcome shade for a garden's lower-growing varieties. And during the hottest months, even tough greens need relief from the relentless bake.

"Open, branching trees that let in filtered light are great for this," she reports. "Our whole nursery is located under old California pepper trees."

She also recommends jacarandas, mimosas, locusts, African sumacs and albizias, all of which can be planted now if they are well watered.

If space is tight, large shrubs can be used for shading, too. Petersen's personal favorites for the job include Pittosporum tobira, raphiolepis, heavenly bamboo and a wide variety of viburnums. Beneath the patchy cool these provide, she says, any number of sunny greens would thrive: fortnight lily, agapanthus, geraniums, day lilies and statice, to name a few.

Of course, humans as well as plants need protection from the sun if they are to get out and garden during the summer. But as Krause points out, deciduous trees, in addition to shading the outdoors, bring the added benefit of cooling the indoors too, thus helping control a property's overall climate. By the same token, when the trees lose their leaves during the winter, the sun can penetrate and heat the house, something that won't happen to the same degree if trees are evergreen.

Deciduous vines, when trained to cover a back-yard lattice or pergola structure, serve a similar function, providing summer shade and letting in winter sunlight. According to Perrino, solanum vines (evergreen in mild winters, deciduous in extreme cold) are a good choice, because their delicate leaves let in the dappled light that patio plants need.

For those whose gardens lack overhead shade structures, Krause recommends a trip to a home-and-garden center, where kits are available for building simple pergolas, trellises and even gazebos. "All you need," he says, "is a basic frame, which you can enhance with plant material. You can drape it, too, with shade cloth--available at any nursery--which comes in different grades, according to how much light it lets through."

He and other experts also sing the praises of ornamental water in summer gardens. "It's amazing how small an amount you need to cool down an area and get that pleasing, relaxing sound," says Krause. A basic fountain, he adds, can be made from a large clay pot and a simple pump from a local garden center.

"It's so rewarding," says Petersen, "to live outdoors and enjoy the summer. We can do that here, from early morning into the evening--work, sit, use our gardens to the max."

Summer Survival Tips

* Water early in the day. Evening watering can cause fungus and mildew problems and leaves plants thirsty during the day's heat.

* Water potted and newly installed plants more often than established greens--once a day if needed. In extreme heat, spray plants lightly with the hose in the afternoon. This cools them and helps prevent the spread of insects and diseases such as rust.

* Make sure lawn areas and flower borders are on separate irrigation systems.

* Don't over-water.

* To help retain moisture, mulch planting beds with composted organic matter such as shredded bark. Avoid fungus problems by keeping the mulch away from root crowns and trunks.

* Cut back on fertilizer and use a low-nitrogen formula to reduce plant growth--and water consumption--in the hot season.

* Avoid heavy pruning to reduce the risk of damaging plants.

* Plant early in the morning or late in the day.

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