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Lettuce: salad enthusiasts' crop is a highlight in any garden

In the continuing commentary, much of it uncomplimentary, on the nation's eating habits, there is one bright spot: Americans are eating, and apparently enjoying, fresh salads to an ever greater degree.

Fresh-produce merchants, in particular, appreciate the trend. So do the manufacturers of salad dressings, not to mention garden centers that cater to the grow-your-own crowd. Not every gardener grows a full range of food crops, but virtually every one of them has a salad garden. Among the reasons: Salad crops are relatively easy to grow and the yield is high for the space involved.

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Tomatoes, peppers, cucumbers, and spring onions are all leading contenders, but lettuce is unquestionably king when it comes to the summer salad.

Lactuca savita, as the Latin has it, sprang more than 2,000 years ago from a wild ancestor, Lactuca scariola, that still grows untamed in the temperate regions of Asia. Just how it grows is worth observing, for despite the two-millennium separation, similarities in growth habit remain.

Seeds that have lain on the surface of the soil all winter long sprout in the cool spring weather to produce a rosette of somewhat bitter-tasting leaves. By early summer the intense sunlight and often-searing heat make the leaves almost unpalatably bitter. At the same time a seed stalk springs up and flowers, beginning the process all over again.

Our tamed garden relative behaves in much the same way. It grows sweet and tender in the spring (and again in the fall), but, stressed by heat and thirst, it will turn bitter and bolt (send up a seed stalk) at the drop of a hat.

Keeping the summer lettuce harvest coming, then, presents a problem over much of the United States. But, short of installing expensive evaporative-cooling systems, there are ways to enjoy a harvest over a much longer period. Knowing how the lettuce ancestor performs in its wild state, we can take steps to modify conditions in the garden which turn sweet into bitter and begin the flowering process.

Experienced lettuce growers make these suggestions:

* Choose the right varieties. Tight-heading iceberg types do well in cool weather but won't perform at all when it gets hot. So select loose-heading, butter-head types or the loose-leaf varieties.

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* Keep the seedlings coming. Every other week sow enough seeds to cover your family's needs. Once the weather heats up, keeping the lightly covered seed from drying out before germination often presents a problem.

Jim Wilson, an herb grower in South Carolina, suggests mixing the seeds with some moist peat moss in a plastic bag that is placed in a refrigerator for a week. Next, place the bag indoors in a light part of the room, but out of direct sunlight. The moment you see any sign of sprouting, the seed-peat mix should be carefully scattered into small furrows in flats and allowed to root before going outside.

Outdoors, I place the flats on the east side of the house, partly under bushes, so they get direct early-morning sun but for the rest of the day are in part or full shade.

* Keep the lettuce bed as cool as possible by mulching the soil and providing some shade from the high-riding sun. Garden centers sell shade netting, or you can mount snow fencing on a support to form a miniature lath house.

* Keep the soil constantly moist and the lettuce regularly fed with a weak fertilizer solution, but don't overfeed. ''Little and often'' should be the motto. Remember, too, that lettuce is less tolerant of acid soils than most other vegetables. Lettuce grows best between 6.5 and 7.5 on the acid-alkali scale, where 7 is neutral. Many US soils range from mildly acid to very acid, a factor that is not improved by the presence of acid rains. So be prepared to add some ground limestone or, for a quick fix, a little wood ash.

Finally, for those very torrid weeks, grow a substitute. New Zealand spinach and Swiss chard are two options. After all, the tomatoes, cukes, and peppers will be going great guns.

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