Many gardeners, worried about an early onset of high temperatures common during a severe drought year, hurried to put in their tomato transplants in early and mid-March. Tomatoes will not set flowers if temperatures are below 55 degrees at night or above 85 degrees during the day. We've lucked out so far. A prolonged, mild spring has proved optimal temperatures for flower set and blossom hold on tomatoes. Whether these flowers will produce a good crop this dry summer depends on gardeners carefully monitoring soil moisture levels and controlling fertilization.
Tomatoes ripen about 10 to 12 weeks after they are transplanted. During the first three to four weeks the soil should be kept consistently moist. Drip emitters are the best means of directing water to tomato root zones during this period.
After the first three or four weeks, tomatoes should be watered deeply when the top two to three inches of soil has dried.
Plan on watering three times a week during the hottest months and adjust timers so that the soil is moist at a depth of at least 12 inches — tomatoes will grow deeper roots than many other plants if water is available and a deep root system will help keep plants alive during dry spells. A 3- to 4-inch layer of compost or humus that can be worked into the soil after the plants are removed in fall also will help conserve water and moderate soil temperature fluctuations.
Tomatoes are highly nitrogen sensitive. When fed high nitrogen fertilizers or when fed too much too often, tomatoes produce rampant green growth instead of flowers. Feed lightly with a low-number (like a 5-10-5 formulation) granular fertilizer every four to six weeks until July.
When temperatures soar in mid-summer and flower production stops completely, stop fertilizing until mid-August when nights are longer and cooler.
We're likely to see two other problems with our tomato plants this summer. Smog or high ozone levels cause lower leaves to turn yellow and drop and cause blossom drop as well. Smog-affected tomatoes do not thrive or produce well.
High temperatures and high intensity light levels cause 'solar yellowing' or scorching during our summers. To give fruit more shade, don't remove secondary or axial leaves.
Note: Check www.fresno.gov/water for information on the Water Wise Plant event today at the Fresno State Horticulture greenhouses from 8 a.m to noon.
This is a give and take on water-wise plants with many participating agencies providing free workshops and information booths.
Master Gardeners will be available to answer your questions.