Summer School: Lessons learned from another hot, humid Ontario summer

Happy first day of fall! It was a dry, sweltering summer across Ontario, and I’ve heard from some growers that it was the most challenging summer they could recall in recent memory based on the heat alone.  Oppressive heat and humidity pack a punch that leaves everyone feeling sluggish – you, your staff and your plants.  While we won’t complain too loudly (winter’s coming!), learning from this year’s challenges will help in summers to come as we experience more extreme weather.

In general, knowing your crop, and walking it more often in times of stress will help to catch problems early.  Look for hot spots where shade curtains or whitewash are not effective and rectify the problem as soon as possible. Check irrigation lines regularly to avoid disaster – a few hours or a day without water can be enough to kill a sensitive crop. Ensure your climate control system is correctly calibrated and all vents and fans are in working order, this will help air circulate and stay cooler.

Keeping up with the heat and avoiding drought stress can be tricky business.  Your climate control software will help – both relative humidity (RH) and Vapour Pressure Deficit (VPD) should be monitored.  VPD can be especially helpful, because it measures the difference between the current moisture held by the air, and the maximum moisture that can be held (100% humidity). RH is a similar measurement, but it depends on the temperature and hotter air can hold more moisture.  Did you notice your plants were looking a little droopy, were late to flower or aborted flowers altogether?

Tips for another year:

  • Try altering the timing of irrigation – plants get the same end volume each day, but it’s spread out in shorter bursts which happen more often. Make sure that adequate water is available at bloom development, especially if you’re growing outside.
  • Keep the VPD higher (i.e. lower the humidity if possible) when plants are in the finishing stage. Not only does this allow them to stay cool (it’s just like human sweat!); the active transpiration helps nutrients to get taken up effectively.
  • Adjust your climate control settings to avoid any problems you had this year. A checkup of your system is a good idea anyways considering the cooler weather to come!

The Relationship between High Humidity and Calcium Deficiency

Here’s a complaint I saw in multiple crops this summer.  Calcium deficiency is usually caused by low calcium availability or drought stress which results in low transpiration rates. BUT it can also be caused by low transpiration rates because of high humidity (low VPD).  Calcium uptake is directly related to the plant’s respiration rate and deficiencies can occur even if there is enough calcium in the feed.  The infographic below shows this concept.  When stomata are open in a greenhouse with lower humidity, the plant is actively transpiring – good for calcium uptake and keeping the plant cool.  However, if the humidity is too high in the greenhouse, more stomata will remain closed, and the transpiration rate will be low – leading to deficiencies and a warm, droopy looking plant.  If you notice signs of calcium deficiency, lower the humidity before adding more calcium.


Calcium is involved in many different plant processes.
Calcium is involved in many different plant processes.

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