Managing Summer Heat in Ontario Commercial Greenhouses

For a lot of the greenhouse operators across the Niagara Peninsula, July is harder than January.

In winter, you know what you’re up against. The heat is on, the setpoints are set, and if something goes wrong, the alarm wakes you up. In summer, the problems sneak up on you. It’s 9am on a Tuesday and already warmer than your target. Your fans are running, but the CO2 you paid to enrich is disappearing out the roof vents. By afternoon, you’re watching your tomato plants show signs of heat stress during their most critical growth period.

Sound familiar? You’re not alone. Here’s how to get ahead of it.

Why Summer Is Actually Harder Than Winter

In winter, the math is simple: your growing target is warmer than outside, so you add heat. You have a big temperature differential to work with, and passive losses mostly work in your favour.

Summer flips that around. When outdoor temperatures hit 28 to 30 degrees and your greenhouse target is 24, outside air isn’t cooling your space — it’s adding to your heat load. The solar radiation coming through your glazing on a clear July day adds even more. And if you’re on the Niagara Peninsula, that lake-effect humidity means outdoor air is already carrying significant moisture, which limits how much work your ventilation can do.

We’ve covered the specific climate challenges of growing in the Niagara region in detail in our guide to commercial greenhouse HVAC in Grimsby and the Niagara Peninsula. If you haven’t read it, it’s a good companion to this article.

Your Four Tools for Keeping It Cool

There’s no single answer to greenhouse cooling. Most successful operations in Ontario are using a combination of these four approaches, matched to their crop, their structure, and their budget.

1. Natural ventilation

Roof vents, side wall panels, and gutter-to-ridge openings let hot air escape and cooler air in. It’s your lowest-cost option and it works well in spring and fall, or on cooler summer mornings. But on a hot July afternoon when outside air is 30 degrees, it’s not going to get you where you need to be on its own.

2. Forced air ventilation

Exhaust fans paired with controlled inlet openings move much higher air volumes than passive venting can manage. A well-designed exhaust system can cycle through the full greenhouse air volume several times an hour. For most Ontario operations, this is the backbone of the active cooling strategy.

The key word is “designed.” Fan placement, inlet sizing, and airflow direction all matter. Fans that are in the wrong location push hot air around instead of removing it.

3. Shade screening

Here’s the one that’s easy to overlook: if you can stop solar radiation from converting to heat inside the greenhouse in the first place, your mechanical systems have a much easier job. Interior or exterior shade cloth at 30 to 50 percent reduction can knock four to eight degrees off your peak internal temperature on a clear summer day.

It’s not an HVAC solution, but it’s often the most cost-effective first step at reducing the cooling load. Worth a conversation if you haven’t already explored it.

4. Evaporative (pad-and-fan) cooling

Pad-and-fan systems pull outdoor air through water-saturated cellulose pads, cooling it through evaporation before it enters the growing space. When conditions are right, you can drop inlet air temperature by eight to twelve degrees. That’s meaningful.

The limitation in Niagara is humidity. Evaporative cooling loses effectiveness as outdoor relative humidity climbs, and on your most humid summer days, the pads can’t do much. But on drier days, it’s a practical alternative or supplement to mechanical cooling, and a lot cheaper to run.

The Emissions Problem Nobody Likes to Talk About

Here’s the tension that trips up a lot of growers in summer: the same ventilation that cools your greenhouse also flushes out your CO2 enrichment.

When your exhaust fans are running at full capacity on a hot afternoon, CO2 you’ve invested in replacing or enriching is leaving through the roof almost as fast as you can add it. The result is that your most productive growing months — when your crop is biggest, your light levels are highest, and photosynthesis could be working hardest — are also the months where CO2 management is most difficult.

The solution is smart ventilation control. A building automation system can manage exhaust fan speed dynamically based on both temperature and CO2 concentration at the same time. When temperature is in range and CO2 is dropping, it throttles back the fans. When temperature climbs above threshold, it prioritizes cooling. Manual control can’t achieve this balance — the response time is too slow and the tradeoffs are too frequent.

This is one of the strongest use cases for greenhouse BAS integration. If you want to understand the broader picture of how automation pays off, our article on saving money with building automation upgrades covers the ROI logic in detail.

And if you’re also thinking about what humid summer nights can do to your crop on the disease side, our botrytis prevention guide covers exactly how HVAC and humidity management interact during the most vulnerable periods.

A Word on Your Hydro Bill

Ontario’s time-of-use electricity rates put peak pricing between 11am and 5pm on weekdays — which is precisely when your cooling systems are working hardest. If you’re running mechanical cooling, exhaust fans, and possibly supplemental dehumidification during those hours, your energy costs in summer can climb fast.

Demand management strategies — pre-cooling the greenhouse before peak hours, deploying shade screening to reduce midday load, and using BAS scheduling to shift what can be shifted — can make a real difference. We went deep on the energy efficiency side of things in our article on understanding real HVAC savings versus false promises, which is worth reading if you’re evaluating whether automation investments make sense for your operation.

Before the Heat Arrives: Your Pre-Season Checklist

The worst time to discover a problem with your cooling system is during the first heat wave of the season. Run through this before summer gets serious:

• Exhaust fan motors and belts checked — confirm fans are actually hitting their rated airflow, not just spinning
• Ventilation panel and vent actuators tested through full range of motion
• Pad-and-fan systems serviced — fresh cellulose pads, clean distribution lines, functioning float valves
• BAS temperature sensors calibrated and positioned at plant level, not overhead
• Shade screening cleaned, deployed, and confirmed on the right schedule
• Alert thresholds set for high temperature, and notifications confirmed to reach the right people

If you’re on a preventive maintenance contract, most of this gets handled for you as part of the seasonal transition. If you’re not, this is a good reason to consider it! It’s a lot easier than calling us in July when the fans aren’t moving enough air.

Once summer is behind you, the next thing to plan for is winter. If you want a head start on that, our guide to

Looking for more information? Check our article on winter greenhouse heating reliability and redundancy The section on backup heating is worth thinking through before the shoulder season catches you.

Frequently Asked Questions

What is the ideal temperature for a commercial greenhouse in summer in Ontario?

For most fruiting vegetable crops — tomatoes, peppers, cucumbers — the target range during summer daylight hours is 22 to 26 degrees Celsius. Above 32 degrees, heat stress starts affecting pollination and fruit set. The goal isn’t just hitting an average; it’s preventing spikes above threshold during peak solar hours, which is why monitoring at plant level matters more than a single overhead sensor.

How do I cool a greenhouse without losing all my CO2 enrichment?

The practical answer is staged ventilation control through a building automation system. By managing exhaust fan speed based on both temperature and CO2 concentration simultaneously, you can hold onto CO2 enrichment during moderate heat periods and only shift to full ventilation when temperature is the real priority. It’s a balance that’s genuinely difficult to achieve with manual control — the tradeoffs happen too fast and too frequently.

What is pad-and-fan cooling and does it work in Niagara?

Pad-and-fan cooling draws outdoor air through water-saturated cellulose pads, cooling it through evaporation before it enters the greenhouse. It works well when outdoor relative humidity is below about 70 percent. Niagara’s lake-influenced humidity reduces its effectiveness on the most humid days, but it’s still a useful supplemental tool on drier summer days and is significantly cheaper to run than mechanical refrigeration.

What temperature causes heat stress in tomato and pepper crops?

Tomato pollen becomes non-viable above roughly 32 to 34 degrees Celsius, which shows up as poor fruit set for weeks after the heat event. Pepper crops behave similarly. The problem is that even a few hours above threshold during flowering has lasting effects on yield — which is exactly why real-time temperature monitoring with prompt alerts is so important during Ontario summer heat events, not just a nice-to-have.

How do I manage humidity in a Niagara greenhouse during hot, humid summer days?

On the most humid summer days, ventilation is less effective for both cooling and humidity control because outdoor air is already warm and moisture-laden. Mechanical dehumidification becomes more important in these conditions. Maintaining consistent air movement throughout the canopy and keeping drain pans and condensate lines clear are also critical during sustained humid periods — a clogged condensate line in July can escalate fast.