Q. My tomato plants flower but don’t set fruit. Why not, and what can I do about it? — Madeline W., Nipomo
A. There are several reasons why tomato plants blossom but don’t set fruit. Assuming the plant is healthy, free from insect or disease damage and is getting at least six hours of direct sunlight each day, some likely suspects are temperature, soil moisture, lack of pollination or faulty nutrition.
Here on the Central Coast, low nighttime temperature is a common cause of poor fruit set. Tomatoes are a self-fertilizing “warm season” crop native to Central and South America. When nighttime temperatures fall to 55 degrees F, the ovum becomes unreceptive to pollen. Fertilization will not occur and the unfertilized flowers will abort. The result is healthy plants with no fruit.
A hormone spray can help set fruit, but should be used carefully and in strict accordance with the label instructions for application. Tomato varieties vary in their sensitivity to cold. A helpful guide to varieties suitable for our local climate is the UC Davis Home Vegetable Gardening brochure “Tomatoes,” available online.
Too-hot days can also cause poor fruit set. Daytime temperatures greater than 90 degrees F for extended periods can affect fruit set in varieties susceptible to high heat. Choose a variety suitable to our climate, and maintain uniform soil moisture through proper irrigation and a layer of mulch around the base of the plant. Apply enough water to wet the root zone thoroughly. Use a drip system or soaker hose to avoid wetting the foliage.
Although tomatoes are self-pollinating, they do need some type of movement to transfer the pollen inside the flower. This normally occurs by wind, mechanically or by insects. This is sometimes the problem in small yards that are protected from wind or inside a greenhouse.
Faulty feeding can also affect fruit set. Fertilizers high in nitrogen stimulate lush leaf growth but inhibit flowering and fruit set. Wait until fruit is set before side dressing the plant with fertilizer. Repeat side dressing every 4 to 6 weeks, and water well after each application.
So choose a variety suitable for our climate. Plant it where it gets full sun. Feed it and water it properly. Mulch to maintain soil moisture and suppress weeds. You will be rewarded for your efforts. An excellent resource for tomato gardening information is found online at http://ucanr.org/sites/gardenweb/Vegetables.