The Effects of Temperature and Relative Humidity on Tomato fertilization

It feels like just a few weeks ago we were fighting freezing temperatures in the greenhouse. Most of our tomato plants were moved from our living room to their permanent spot in the greenhouse by May 20th. Night-time temperatures at that time were still dipping below freezing consistently, and did so until about the first week of June. The last three weeks of June and the first week of July (despite some rain) have been great! We’ve had beautiful sunny temperatures in the high teens and low twenties. Our garden is loving it.

Midday temperatures reaching 17-21° Celcius translates into very hot greenhouse temperatures. Paying close attention to temperatures and relative humidity is important to get the most out of your tomato plants. Temperature and relative humidity directly affect a flower’s ability to pollinate and set fruit. And since relative humidity is affected by temperature, both are equally important. Before we go too far, we must point out that the word relative is important here. In theory, the warmer the air temperature, the more moisture the air can hold. However, if moisture isn’t ‘added’ to the warming air, humidity ‘drops’. For example, in Figure 1. a parcel of air at 10° Celsius can hold a certain weight of water vapour. Increasing the temperature of that same parcel of air to 20° C increases its ability to hold more water. If water isn’t added (i.e water vapour level stays the same) then it will still hold the same amount of water and humidity ‘technically goes down’. So how does this effect tomato flowers and fruit production?

Water Vapor and Relative Humidity in reference to tomato temperatures.
Figure 1. Changes in relative humidity with an increase in temperature of a parcel of air.

We aim to keep our greenhouse between 20 and 25° Celsius during the day, and ideally no lower than 12-15° at night. This isn’t always easy to do without a commercially-automated system that you see in bigger operations. Other than a cooling fan that is triggered to turn on at 24.5° and turn off at 23.5°, Kyle and I are the system. If we see a cold front or cold night approaching, we close the windows in anticipation, and vice versa.

Before installing that greenhouse fan, our greenhouse was reaching daytime temperatures beyond 30°. At those temperatures, we noticed that relative humidity would plummet, causing blossom drop (stems of the flowers turn yellow before the blooms fall off). Every flower that falls off your plant is a potential tomato that is now gone. High heat (and low humidity) can also cause pollen to stick to the stamen of the flower (male reproductive parts) making it unable to fertilize the pistil (female reproductive parts) and thus no tomato production.

Blossom Drop on tomato in Yellowknife Market Garden due to bad temperatures and humidity
Figure 2. Example of blossom drop

High humidity will cause pollen to clump, and again unable to fertilize the female flower. Ideally, we try and keep our relative humidity between 40 and 70%. As you see from the graph in Figure 3, this is sometimes challenging. There will be a natural fluctuation between day and night, but our focus is to try and reduce the large peaks and valleys where we encounter pollination issues and blossom drop. Consecutive cool nights below 12° also causes blossom drop. Consistency is key.

Humidity of Yellowknife Tomato Greenhouse
Figure 3. Relative humidity measurements from June 29th to July 2nd, 2020.

If we anticipate a very hot day, we can also anticipate low humidity. In an effort to mitigate those dips in humidity, we have started lightly misting the inside of the greenhouse.

Temperature of Yellowknife Greenhouse for tomato
Figure 4. Greenhouse temperature from June 29th to July 2nd, 2020.

From the two graphs, you can see a clear link between temperature and relative humidity. When temperature spikes, the relative humidity goes down (unless there is intervention). June 29th and 30th were very hot days and relative humidity dipped to 20%. July 1st and 2nd were slightly cooler (and rainy) and relative humidity was consistently above 50%.

We are noticing less blossom drop since we started misting in hot temperatures and we’re hoping this translates into a larger harvest in a few weeks. Time will tell.

If you have any tomato related questions, let us know.

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