Tomato Won't Set Fruit

Low light levels, extreme temperatures, dry or wet soil and high-nitrogen fertilizers can prevent a tomato plant (Lycopersicon esculentum) from setting fruit, or there may be other problems. Sometimes a tomato plant is green and healthy but doesn't flower, or the plant may produce flowers but no fruit. A perennial in U.S. Department of Agriculture plant hardiness zones 10 through 11, this plant is usually grown as an annual. Depending on the variety, a tomato plant can grow 3 to 6 feet tall and 2 to 3 feet wide.

A tomato plant needs high light levels and warm, even temperatures to set fruit. When a tomato plant receives less than eight to 10 hours of direct sunlight every day, it grows spindly and develops healthy leaves but little or no fruit.

This plant also needs nighttime temperatures between 55 and 75 degrees Fahrenheit for good fruit set. Most fruit sets when temperatures are 60 to 70 degrees Fahrenheit at night.

Growing a tomato plant too early or too late in the year can mean temperatures are too low or high for fruit to set. Continue to care for a plant that was planted at the wrong time, and it will probably fruit when the weather warms up or cools down.

Irregular watering and applying an unsuitable fertilizer can prevent flowering and fruiting in a tomato plant. The soil around the plant should stay evenly moist but not soggy. When the soil surface is dry, water a tomato plant until the water puddles, then stop watering. Spread a 2-inch layer of garden compost, leaf mold or another organic mulch around the plant to conserve soil moisture.

Apply a balanced fertilizer, such as an 18-18-21 product. Dilute an 18-18-21 liquid fertilizer at a rate of 1 tablespoon per 1 gallon of water, and apply the solution every one to two weeks, or apply the fertilizer according to the manufacturer's instructions. You can apply fertilizer solution in place of water if the plant needs watering.

Sometimes a tomato plant produces flowers but no fruit, and this is often due to poor pollination. Tomato flowers shed most of their pollen between 10 a.m. and 4 p.m., and wind moves the pollen to the female parts of the flowers. In greenhouses and very still conditions pollination may not occur. Low and high humidity also prevent good pollination.

At midday, gently shake the tomato plant to help disperse the pollen. You can also hand-pollinate the plant by brushing each flower with a small, soft paintbrush, or use a blossom set spray, which contains chemicals to encourage fruit to set. Spray each flower and the leaves surrounding it with blossom set solution until the leaves are wet. Apply the spray every two weeks until fruits appear.

Aphids and thrips can attack a tomato plant, and a severe infestation can prevent the plant from setting fruit. Aphids are small insects that cluster on soft stems and the undersides of leaves. Thrips are difficult to see, but infested tomato flowers are flecked, spotted, deformed and may not open. Thrips also attack tomato leaves, causing a silvery appearance, withering and curling, and black specks on the undersides of leaves.

Pick off heavily infested stems, leaves and flowers, and spray the plant with a ready-to-use insecticidal soap on a dull day. Spray all the plant parts every week, if necessary.