Growing tomatoes in summer heat

Hot weather has arrived which can be a troublesome time for tomato growers. High temperatures, especially if accompanied by low humidity, do hinder fruit set. Viable pollen fails to form and fertilization doesn't occur. Temperatures above 90 degrees during the day and above 70 degrees at night usually result in poor flowering and reduced fruit set. Research indicates that night temperature likely is more critical than day temperature, with the optimal range for the former being 59 to 68 degrees. Those thresholds were exceeded this past week.And there will be a delay in ripening time. Typically, it takes six to seven weeks following flower fertilization for the tomato to mature fully. Full-sized green tomatoes ripen most rapidly at temperatures between 68 and 77 degrees. The greater the deviation from that temperature range, the slower the ripening process will be. Very high temps can virtually halt the process entirely.The red pigment lycopene and yellow pigment carotene are the two pigments that give a tomato fruit its color. Temperatures above 85 degrees tend to slow or even halt the production of these two pigments. No wonder tomatoes growing in excessively hot conditions produce poorly-colored fruit."Yellow shoulder" is a disorder appearing at the top (shoulder) of the fruit which remains yellow as the fruit ripens and turns red elsewhere. These yellow areas never ripen properly and the tissue below them is tough and poorly flavored. This is due partly to the lycopene deficiency described earlier which occurs most strongly where the direct, hot rays of the sun hit the fruit. "Yellow shoulder" tomatoes are usually deficient in potassium. You can help prevent this by choosing varieties with a uniform ripening trait, making sure the ripening fruit have good foliage cover, and supplying ample amounts of potassium fertilizer to the tomato plant. Under stressful conditions like extreme heat, tomatoes can develop a tough, white core in their center. Older varieties with five distinct cavities are especially prone to this disorder. Newer, meaty "beef steak" types have multiple cavities and tend to show white core less often. Again, insufficient potassium levels play a part also. Choosing meatier varieties, maintaining a fertility program with enough nitrogen to encourage good foliage cover and supplying ample amounts of potassium are best practices for preventing the disorder. (Note: too much nitrogen will overpower the phosphorus and the plant will produce nothing but lovely green leaves.)Blossom-end rot is a tomato disorder caused by a lack of calcium in the blossom end of the fruit and a lack of regularly scheduled water. This disorder results in fruit with brown or tan areas on their blossom end. Since hot weather increases water loss (transpiration) from tomato plants, the incidence of blossom-end rot usually is greatest when temperatures are hot. Maintaining the proper soil pH, supplying tomato plants with adequate amounts of calcium and irrigating on a timely basis can prevent this tomato problem entirely.And keep the gardener as cool as possible too.Trish Grenfell is a Placer County master gardener. Contact Placer County gardeners at 530-889-7388 or pcmg.ucanr.org/Got_Questions.