Farmers, gardeners, and scientists have known for some time that tomato (Solanum lycopersicum) is sensitive to heat stress at flowering and fruiting. Pollination and fruit formation can be disrupted when temperatures >90⁰ F. during the day and >70⁰ F. at night. Other fruit problems like yellow shoulders and white internal tissue are also caused in large part by heat stress, especially when determinate (self-topping) varieties are grown and pruned heavily.
If you feel that high temperatures are reducing tomato flowering and fruiting in your garden you can try moving crops to spots receiving late afternoon shade or you can cover plants with 30% shade cloth (a mesh material that blocks about 30% of sunlight). Another option is to try some of the many heat-tolerant tomato varieties. Heat tolerance is a major focus for tomato breeders around the world.
This year, some HGIC staff tried four determinate, hybrid varieties developed by Southern breeders (the first three are from the University of Florida) that I started from seed at home. They all have excellent disease resistance:
Florida 91 (F1) 72 days (transplant to harvest). 9 to 11 oz. red tomatoes
Heatmaster (F1) 75 days (transplant to harvest). 7 to 8 oz. red tomatoes
Jamestown (F1) 80 days (transplant to harvest). 9 to 10 oz. fruit. Purported to have a deep red crimson gene and high lycopene content
Phoenix (F1) 72 days (transplant to harvest). 8 oz. red tomatoes
HGIC staff observations
We did a taste test in the office of the four varieties in mid-August. Florida 91 was rated the most flavorful with Heatmaster close behind. Phoenix and Jamestown were rated as bland with a slightly mealy texture.
Dan Adler was unimpressed by the two plants he took home and planted. “We had a week or so of heat and they got that leaf curl and didn’t recover. For a while, the tomatoes took forever to turn from green and started to have problems being so big and long on the vine. Newer branches that grew did not have the leaf curl and those tomatoes seemed better.”
Christa Carignan planted Phoenix and Heatmaster. “I planted them a little bit later than the other tomatoes I started from seeds. Both were among the first to get leaf spot symptoms, and I did nothing else but cut back the lower leaves as symptoms progressed up the plant. I found them to be less productive compared to other varieties I grew (all my others were indeterminate varieties). I had both heat-tolerant plants next to Mountain Magic which were more prolific by far (but I realize it’s a different type of tomato – so not an apples-to-apples or tomatoes-to-tomatoes comparison). I really did not notice a difference in flavor between the two heat-tolerant varieties, and they did not impress me compared to the flavor of Black Krim and Big Beef.”
Ria Malloy says that she “grew 4 Heatmaster tomatoes- 2 in the ground and 2 in containers on my deck at home. None of them were prolific producers. The fruit produced was the size of a tennis ball or smaller, symmetrical, and very hard. Even after they turned red and were on the vine for several weeks, they remained a light orange-red color. Their flavor was better than most store-bought tomatoes, but not what I look for in home-grown tomatoes. They did not succumb to any diseases or have insect problems. The plants produced fruit in the summer heat. But overall, they were unimpressive. That said, I could have done a better job of fertilizing them.”
The author planted two plants of each variety. All varieties had strong determinate vines and produced fruit from late July to early October. Average fruit size from a mid-season harvest:
- FL 91- 18.3 oz.
- Jamestown- 13.5 oz.
- Phoenix- 12.0 oz.
- Heatmaster- 9.4 oz.
The flavor and texture of these varieties did not quite match up with Better Boy, Supersonic, Celebrity, and other common home garden varieties.
UME research in Southern MD
Vegetable growers in Southern Maryland have been experiencing issues with “summer slump” when heat stress reduces pollination and fruit set. Alan Leslie, Ph.D., and Ben Beale, Ag Educators for UME in Charles Co. and St. Mary’s Co., respectively, are working with growers to conduct on-farm field trials of 13 tomato varieties. Trials are also taking place at the Central MD Research & Education Center in Upper Marlboro. In 2022, Thunderbird (determinate) and Carole (indeterminate) performed significantly better than the two standard varieties being grown. We’ll keep you updated with the results from this important research project.
Interestingly, a recent Texas A&M heat stress study (greenhouse and field) of 43 tomato varieties showed that Heatmaster and Celebrity, a common determinate home garden variety, were two of the top producers.
Related blog posts:
- High night temperatures can lower yields
- Heat-tolerant vegetable crops and cultivars for the changing climate
- UDEL- heat stress article
- UDEL- Heat Stress Trial With Tomato
- UDEL- Heat Tolerant Vegetable Varieties
- Genetic and Molecular Mechanisms Conferring Heat Stress Tolerance in Tomato Plants
By Jon Traunfeld, Extension Specialist, University of Maryland Extension, Home & Garden Information Center.
Read more posts by Jon.