We’ve had mild temperatures during this unusually long spring. Hot spells are normal in May in the central San Joaquin Valley with temperatures regularly exceeding 90 degrees. High temperatures cause tomato flowers to drop prematurely and are a major factor in failure for tomatoes to set fruit. This year tomato plants are still loaded with flowers and are already setting immature fruit at the end of May.
Here are a few suggestions for keeping your tomatoes in good health when high summer temperatures return.
Consistent soil moisture prevents two common tomato problems. Cracking of tomato skins occurs during hot spells when the soil is allowed to dry out. The lack of water followed by irrigation causes the meat of the tomato to expand faster than the skin can. Blossom end rot is caused by a calcium deficiency as well as soil moisture fluctuations which lead to water stress just as fruit are developing.
This year, tomato roots will have had more time to become established and will be better able to draw up water and nutrients, especially if you’ve amended the soil with compost or humus. Irrigate tomatoes when the top 2 to 3 inches of soil is dry. That may mean checking soil moisture levels daily during hot weather depending on the water retention capabilities of your garden soil.
Your tomato transplants should have been fed with a low-number starter fertilizer when planted. Tomatoes are very sensitive to excess nitrogen which causes them to produce more green leaves than flowers and fruit. Fertilize your tomatoes at first fruit set with a low-nitrogen food (around 5% to 6% N) and then every four to six weeks until early July when really high temperatures cause all plants to enter a state of semi-dormancy. Cut back or stop fertilization during the hottest months. If you didn’t add extra calcium to the planting holes in the form of broken eggshells, bone meal, bulb food or a starter fertilizer with at least 3% calcium, make sure that your tomato food contains extra calcium to help prevent blossom end rot.
Bush-type or determinate tomatoes (Roma, Celebrity) set most of their fruit at the same time and then their productive season is over. A heavy crop can break branches. Tomato cages that can provide support for individual branches work well for bush-types. Indeterminate or vine-type tomatoes (San Marzano, Early Girl Hybrid) produce flowers and fruit throughout a long growing season. Pinching back growing tips on indeterminate tomatoes can keep vines in check and keep the plant bushier. Vine-type tomatoes may need taller supports.
Most of the tomato varieties sold as transplants in local nurseries are better-adapted to the high summer temperatures and scorching sun rays in our planting zones, zone 7 in the foothills and zones 8 and 9 in the Valley. If you’ve experimented with varieties normally grown in cooler zones, try to provide some afternoon shade in June, July and August with market umbrellas or shade cloth structures.
The first hot days will stress all the summer vegetables and flowering annuals in your garden. Monitor your tomatoes daily in summer for whiteflies, bagrada bugs and tomato hornworms – the three most common tomato-attacking pest insects.
More on identifying and controlling for tomato pest insects in next week’s column.