Nancy Phillips, age 13, of Visalia, California, for her question:

When do carrots, potatoes and other vegetables die?

Few garden vegetables get a chance to die from old age because we consume them before they complete their life cycles. There is no general ruling on when they would die from natural causes because each species follows its own program. This is one reason why a garden plot of your own is such fascinating fun. The road to success reveals the unique life styles, likes and dislikes, of dozens of plant species.

The well planned vegetable plot has a few perennial plants that live through several years, donating their gifts every season. A few species are biennials that complete their life cycles in two years. As a rule, they reach the edible stage during their first year and we consume them before the second year, when they concen¬trate on setting their seeds. Many of our vegetables are annuals that complete their life cycles during a single season. We harvest the leafy species before they run to seed and the bean types when their seeds are ready.

The carrots are biennial herbs. Prepare deep crumbly soil enriched with compost and partly decayed organic material. Sow the fine seeds very early in the spring. They tend to germinate slowly and unevenly. Keep the growing youngsters moist but not soggy. Thin out the plants to avoid crowding    and sample the tiny surplus ones. The rest will add to their feather green tops and dig their golden roots down deeper. Harvest them after 60 to 80 days, depending on the species. If you sow more seeds every few weeks you get relays of fresh carrots through summer until late fall.

The carrot grows its sweet root to store food for the great event next season. If it survives in the ground until spring, it sprouts a tall stalk topped with tiny flowers that become seeds. You can pamper your best carrot through the winter and gather its seeds to sow for another series of crunchy roots.

The potato plant thrives for about three months, growing a leafy greentop and an underground nest of edible tubers. It will sprout from a healthy old potato with a tough skin, especially if you let the sun turn it greenish for a few days. Or you can buy so called potato seeds, which are prepared from tuber sections with several eyes, or sprouting centers. Prepare a deep row of crumbly rich soil and mound the greenery with mulch as it grows.

The small white potato flowers, as a rule, do not set growable seeds. But they tell you that, with tender care, you may sift out a few baby potatoes. When the flowers and leaves die down, the main underground crop is ready for harvesting. These plants have completed their life cycle.

Members of the corn, bean and squash families are annual plants that complete their life cycles in one season. The cabbage and carrot families are two season biennials. Asparagus is a long lived perennial, getting better year by year. Our tomatoes, peppers and eggplants are treated as annuals because they perish with the slightest frost. But in their native tropics they survive many years as perennial shrubs.