Summer is the season that kills more greenhouse crops than winter does, and it does it faster. A sealed house in full sun can gain 40 degrees Fahrenheit over outside temperature in an hour or two, and most vegetable crops start failing well before that: tomato pollen is denatured above about 95 degrees, lettuce bolts, and seedlings in shallow trays cook.
The mistake is reaching for a fan first. Ventilation is the second stage, not the first, and it has a hard limit that shade does not: a fan can bring the inside of the house down toward outside air temperature and no further. If it is 95 degrees outside, an exhaust fan gets you to something above 95 inside. Shade reduces the heat arriving in the first place, which is a different and more powerful lever.
Stage one: shade
A greenhouse in the growing season has more light than the crop can use. Tomatoes want a daily light integral of 22 to 30 and a mid-summer greenhouse delivers well above that, so cutting light by half costs nothing horticulturally and removes a large fraction of the heat load.
Fifty percent shade cloth over the roof typically drops peak house temperature by 10 to 15 degrees Fahrenheit. That is a bigger effect than most hobby exhaust fans achieve, for a fraction of the money and no electricity. A BELLEBILL 50 Percent Shade Cloth, 10 x 12 ft ($15.90) over a small house, or SUNNY GUARD HDPE Shade Cloth, 10 x 20 ft ($30.99) over a larger one, is the correct first purchase for summer.
Three details make a large difference. Fit it on the outside of the glazing, because shade fitted inside has already let the radiation into the house and only stops it reaching the plants. Leave an air gap between cloth and glazing so the heated cloth is cooled by outside air rather than radiating into the structure. And match the percentage to the crop rather than the weather: fruiting vegetables want 30 to 50 percent, leafy greens and seedlings 50 to 60, and shade crops such as orchids and ferns 70 to 80. The shade cloth calculator works it out from crop, season, latitude and glazing.
Stage two: ventilation
Once the incoming heat has been reduced, the next job is to get rid of what still arrives. This is what vent area and fans do, and the sizing is covered fully in greenhouse ventilation basics.
The important summer-specific point is the target. Fan sizing assumes a temperature rise of about 4 degrees Fahrenheit between the intake end of the house and the fan end. Accepting a larger rise reduces the airflow needed considerably: at 8 degrees the requirement halves. In a mild climate that is a reasonable trade. In a hot one it is not, because the crop is already close to its ceiling.
Also note what venting does to humidity. Pulling large volumes of outside air through the house in dry summer conditions drops indoor humidity sharply, which raises the vapour pressure deficit and puts the crop under water stress even as the temperature comes down. This is why stage three exists.
Stage three: damping down
Wetting the floor on a hot morning is the most underrated cooling action available in a hobby greenhouse, and it is free.
Evaporating water absorbs roughly 970 BTU per pound as it changes state, so a few gallons spread across a gravel or soil floor removes a genuinely useful quantity of heat from the air as it dries through the day. It simultaneously raises humidity at the driest, hottest part of the day, which reduces the vapour pressure deficit and takes water stress off the crop.
Do it in the morning, and do it on a floor that can absorb and release water, which is another argument for gravel or soil over a sealed slab, covered in greenhouse flooring and paths. Do not damp down in the evening: a floor still wet after dark holds humidity up all night, which is exactly the condition botrytis and downy mildew need.
Stage four: evaporative cooling
This is the only stage that can bring a greenhouse below outside air temperature, and it works by the same physics as damping down applied deliberately and continuously.
A misting system such as the MistKing Starter Misting System, 5th Generation ($199.99), or an evaporative cooler such as the Hessaire DC18 Mobile Evaporative Cooler ($159.00), evaporates water into the incoming air stream. The air gives up sensible heat to supply the latent heat of evaporation and arrives cooler. In dry air the effect is dramatic, commonly 15 to 20 degrees Fahrenheit. In humid air it is close to useless, because the air cannot take up more moisture.
The rule of thumb is that evaporative cooling is worth installing where summer afternoon relative humidity is routinely below about 50 percent, and is not worth installing where it is routinely above 70. Between those, it helps on the driest days and does little on the rest. The full trade-off is worked through in evaporative cooling versus shade and venting.
| Stage | Typical effect | Cost | Limitation |
|---|---|---|---|
| 1. Shade cloth at 50 percent | 10 to 15 degF below unshaded peak | Low | Removes light, so it must come off for winter and match the crop |
| 2. Ventilation, passive or fan | Brings the house toward outside temperature | Low to moderate | Cannot go below outside air temperature at all |
| 3. Damping down the floor | Several degrees, plus humidity relief | Free | Wears off through the day and must not be done in the evening |
| 4. Evaporative cooling | 15 to 20 degF below outside in dry air | Moderate to high | Almost nothing in humid air, and it needs a water supply and maintenance |
What to do on the worst days
On the handful of days each year that exceed everything the system can handle, a few actions are worth more than the equipment.
- Water early and thoroughly. A plant with a full root zone tolerates heat far better than one that is already marginal, and midday watering on a hot day is largely wasted to evaporation.
- Open everything at first light to flush the night heat out before the sun starts adding to it. A house that starts the day cool has more headroom than one that never cleared.
- Add temporary shade. A second layer of cloth, or an old sheet over the sunniest roof plane, costs nothing and buys several degrees on the specific afternoon it is needed.
- Move what can be moved. Seed trays and small pots can go outside into dappled shade for the day. They are the most vulnerable things in the house because their root volume is smallest.
- Damp down twice, in the early morning and again at midday, accepting that the second one will still be dry by evening.
- Do not close the house to keep heat out. This is the instinct and it is exactly wrong. A closed greenhouse gains heat regardless, and closing it removes the only mechanism taking heat away.
Summer cooling equipment
Summer heat and summer light are the same problem seen twice. Set the shade percentage from the crop with the shade cloth calculator rather than from how hot it feels, because over-shading costs yield in a way that is much harder to notice than over-heating.