July in the Greenhouse: The Trouble with Greenhouse Tomatoes

Posted on June 30, 2021 | Categories: Donna Balzer

Do your greenhouse tomatoes have heat stroke?

greenhouse tomatoes

Tomato flower breaking off when it doesn’t get pollinated.

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


The University of Delaware says “Tomato plants can tolerate extreme temperatures for short periods, but several days or nights with temperatures above 90oF (daytime) or 72oF (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: https://sites.udel.edu/weeklycropupdate/?p=7056

greenhouse tomatoes

Another bloom breaking off when it doesn’t get pollinated in high heat

Most references say temperatures from 90-95 degrees F damage the pollen (32-35 C) and when the pollen fails to fertilize the stigma, the flower aborts. What can greenhouse growers do in summer heat? Shade cloth, fans, doors and watering.

growing tomatoes

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

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 bolt or as soon as you can get 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 slows plant growth.


Based on the long-range forecasts, we installed our shade cloth last week. I use 60% shade cloth because a lower number offers less shade. Anything from 40%-60% is best for greenhouses. Lower ratings (from 30%-40% diffusion) is usually sold for shading outside crops like lettuce, because 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






I keep a fan going continuously.  I have this one: https://www.bcgreenhouses.com/product/haf-horizontal-airflow-fan/

I am happy for that because it moves the air and keeps it moving. My big greenhouse is in the full sun so I also installed an exhaust fan system:  https://www.bcgreenhouses.com/accessories/exhaust-fans/18-exhaust-fan-package/


Screen Doors


Day and night I leave my screen doors 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 get way above normal, as they have this week, 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 fruit.




Outdoors I use a lot of mulch but in the greenhouse, I have been slow to adapt 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 Plants


I have noticed some greenhouse tomatoes drop their blossoms due to heat more frequently than others.  During the recent heat spell I see, as a general trend, the larger-fruited varieties like Aussie, Ananas and Tundra drop more flowers than the smaller fruited ones like Barry’s crazy cherry, Candyland and Isis. Roma tomatoes often produce just a single heavy crop on bush plants so they are also in heavy fruit right now, unaffected by the heat.

growing greenhouse tomatoes

A collection of tomatoes harvested early (today) includes Barry’s Crazy Cherry – the yellow one

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.

cherry tomatoes

Candyland cherry tomatoes are thriving in the heat



For more great tips from Donna, visit www.donnabalzer.com.

You can also read Donna’s gardening books: No Guff Vegetable Gardening with Steven Biggs and her just released Gardener’s Gratitude Journal:  Part Diary, Part Personal Growing Guide.

donna balzer