Greenhouse Tomato Tips for Beating the Heat: July in the Greenhouse

The Trouble with Greenhouse Tomatoes

Do your greenhouse tomatoes have heat stroke?

If your tomato blooms are bending and falling off, flower and all, they are having a heatstroke. When extreme heat hits, greenhouse tomatoes fail to set fruit even as the leaves keep growing and new blooms appear.

Tomato flower breaking off when it doesnt get pollinated.

The University of Delaware says "Tomato plants can tolerate extreme temperatures for short periods, but several days or nights with temperatures above 90^oF (32^oC) in the daytime or 72^oF (22^oC) in the nighttime will cause the plant to abort flowers and fruit. At these temperatures, the pollen can become sticky and nonviable, preventing pollination from occurring and causing the blossom to dry and drop." Read More Here.

Another bloom breaking off when it doesnt get pollinated in high heat

Strong junction where tomato flower hooks to main stem on my Cherokee Purple heirloom tomato

Most references say temperatures from 90-95^oF (32-35^oC) damage the pollen and when the pollen fails to fertilize the stigma, the flower aborts. 

What can greenhouse growers do in summer heat?

Shade Cloth

Shade cloth is like having a tree overhead but without all the sap and falling leaves!

Install shade cloth on your greenhouse as soon as the temperatures rise or as soon as you can get your hands on the cloth. This year we had extreme heat for a week in May, but then it got cool and rainy again in June. If the shade cloth is installed in cloudy weather, it will slow plant growth.

Based on the long-range forecast, we installed our shade cloth in early May. I use 60% shade cloth because a lower number offers less shade. Anything from 40% to 70% is best for greenhouses. Lower ratings (from  30% to 40% diffusion) is usually sold for shading outside  crops, like lettuce, since it bolts and goes to seed in super bright light and summer heat.

You can hang shade cloth inside the greenhouse, lay it over your greenhouse in the evening when the automatic vents are closed, or stretch shade sails between framing bars of your greenhouse.

Fans

I keep a fan going constantly within my greenhouse. I have the Schaefer Horizontal Airflow Fan that can be purchased through BC Greenhouse Builders.

Exhaust Fan

I'm happy about it since it moves the air around and helps with circulation. My big greenhouse is in full sun so I also installed an exhaust fan system. You can find out more about these here.

Screen Doors

Regardless of the time of day, once it gets hot out my screen doors stay open. This allows the cool air to come in at night passively as the hot air is exhausted.

Watering the Soil and Paths

When the temperatures skyrocket, as they did in Summer 2021 here, I am thankful I work from home. I pop up to the greenhouse a couple of times a day to water the paths and even the sides and tops of my greenhouse. This usually shuts down the exhaust fan as the extra humidity temporarily cools the greenhouse.

Watering plants in the evening and sometimes again mid-morning helps cool the soil and air. This increases the humidity and sometimes this is enough to help hold flowers as they become pollinated and form flowers.

Mulch

Outdoors I use a lot of mulch, but in the greenhouse, I have been slowly adapting this trend. Many insects, like pill bugs, multiply quickly under wood chips and this is my main source of mulch outdoors.

The Farmer’s Hive, an electronic device for tracking soil moisture and temperature, has alerted me to extremes in my soil temperature so I am adding extra compost as mulch this afternoon.

Hardy Tomato Plants

I have noticed some greenhouse tomato plants have dropped their blossoms due to the heat more often than others. During heat spells, I noticed, as a general trend, the larger-fruited varieties like Aussie, Ananas and Tundra drop more flowers than smaller fruited ones like Barry's crazy cherry, Candyland and Isis. Roma tomatoes often produce just a single heavy crop on bush plants. So, the Roma are also in heavy fruit during these warm spells and unaffected by the heat.

Some web pages provide lists of heat-tolerant tomatoes.  There are hundreds of known tomatoes and my favorites are not on these lists.  As such I will continue expanding and developing my own list of new and heirloom plants. One of the ones just loaded with fruit already this season but absent from the lists is Juliet (a small Roma style) and a mid-size heirloom Cherokee Purple which popped onto my list last year after I spoke with epic tomato grower Craig LeHoulier.

I have been picking greenhouse tomatoes for a couple of weeks already and plan to continue as the summer progresses. Salsa, sauce and platters of fresh ripe tomatoes are refreshing and lovely to snack on. Fruit is picked as it changes colour, before it is overripe and cracking. With a greenhouse I am able to control the climate but with a little personal attention, I am able to maximize my harvest and enjoy the benefits of fresh food longer. The best of both worlds.