My Zucchini Is Rotting on the Vine

Ready to pick 40 to 50 days after sown, zucchini (Cucurbita pepo) can bear heavy harvests, but poor weather and calcium deficiency sometimes cause rotting. An annual plant, zucchini grows in clumps of large, hairy leaves patterned with light greenish-gray splotches and streaks. Zucchini fruits are picked young when they're about 6 to 8 inches long and 2 to 3 inches in diameter, depending on the variety. Larger fruits are tougher and less sweet.

Zucchini fruits rot when lying on wet ground. Zucchini plants begin to produce fruit in early summer in cold climates and early winter and early summer in warm areas, when the weather is often wet and rainy conditions make the ground damp for long periods. Zucchini fruits lying directly on wet ground develop fruit rot, which starts at the fruit tips. In order to help control fruit rot, put a 2- to 3-inch-thick layer of clean, fresh straw beneath zucchini plants but not touching their stalks, and ensure new fruits are supported by the straw as they develop.

Female zucchini fruits that aren't pollinated rot and die. Each female zucchini flower has a small, immature zucchini fruit connecting to its stem. If a female flower doesn't receive any pollen from a male flower, then the small fruit begins to develop but suddenly stops growing, shrivels and dies. Bees usually pollinate zucchini, but they're less active than normal during cold, rainy weather. As temperatures rise and dry, sunny periods increase, more zucchini fruits usually develop. In warm climates, excessive heat can reduce bee activity. In order to hand-pollinate zucchinis, pinch off a male flower when it's dusty with pollen, and rub it gently into the central structure of fully open female flowers.

Erratic watering causes blossom end rot in zucchini fruits. Blossom end rot begins as a small watery bruise at the tip of a zucchini fruit and often spreads and covers the whole end, causing a black or brown sunken depression that can turn moldy. The flower petals of an affected zucchini plant can remain attached, and usually the rest of the fruit is unaffected. A symptom of calcium deficiency, blossom end rot is caused by drought, fluctuating moisture levels in the soil, damaged roots and other factors that prevent zucchinis from absorbing calcium from the soil. Fresh, non-rotted manure and excessive nitrogen fertilizer applications also can cause blossom end rot. Water zucchini plants' soil regularly so that the soil is constantly moist but never sodden. Also, spread a 2- to 3-inch-thick layer of organic mulch, such as garden compost or leaf mold, on top of the soil but not touching the plant stalks; mulch helps retain soil moisture.

Zucchini plants growing in the most favorable conditions produce the healthiest fruits. Grow zucchinis in full-sun sites and well-drained soil rich in organic matter. If the soil drains poorly, then grow zucchinis on small hills 6 to 9 inches high. Space plants 24 inches apart, and fertilize the soil with a ready-to-use, slow-release, 14-14-14 granular product applied at a rate of 1/4 pound per 10 square feet when transplanting plants or once while seedlings grow strongly. Sprinkle the granules evenly on the soil around the plants, avoiding the plant stems, and work the granules gently into the surface of the soil, or apply the product according to its manufacturer's instructions. Pick zucchini fruits regularly to encourage the plants to continue to flower and fruit.