Keep Your Greenhouse Cool During the Summer

While greenhouses are essential for growing plants like tomatoes and cucumbers that thrive in warmer temperatures, during the summer months Greenhouse temperatures can rise to potentially damaging levels for your plants. Even a warmth-loving plant like the tomato can suffer pollination damage and growth problems when temperatures rise above 32°C during the day or 25°C after dark.

Knowing how to regulate the temperature and keep it from rising to levels that could potentially kill your plants is essential if you’re to have a consistent, healthy crop and while in Britain moderating winter temperatures is more difficult, maintaining productive temperatures during the summer can be just as important.

Ventilation

Whether you own a Halls, Elite or Swallow greenhouse, a good flow of fresh air is one of the most effective ways to prevent a greenhouse from overheating. Due to the natural way hot air rises, by putting the roof and side vents to good use you can create a convection current of fresh air to cool down any plants at risk of overheating.

As hot air inside the greenhouse rises and escapes the roof vents, it creates a mild vacuum effect, which draws cooler air in from the side vents.

It is important that the temperature is regularly monitored as although good ventilation can help prevent a greenhouse from overheating, it takes far longer to cool down an already overheated greenhouse. Keep an eye on weather reports and on days forecast to be hot, head out early in the morning to open doors and vents.

As a general rule of thumb, to get a good movement of air inside the greenhouse, you want to have an area roughly equivalent to 1/5th of your floor space open.

Get a thermometer and mark it with minimum and maximum temperature markers, so you can easily see when the temperature is getting too high or low.

There are also automatic venting systems available. More old-fashioned venting systems use wax cylinders that expand and contract as the temperature increases and decreases, opening and shutting vents. There are also electronic automatic venting systems, but they can be prohibitively expensive and are no more-or-less effective than the wax based systems.

Shading a Greenhouse

Shading is another effective way to handle heat, but can be more difficult to handle correctly as plants depend on light levels to survive and grow, especially during the summer months.

Shade paint is an easy and cheap way to filter out some of the heat from the sunlight, and can add more as the months get hotter. The great thing about shade paint is that it is easy to wash away, so that when autumn starts closing in you can simply wash it away to trap as much heat as possible during the colder months.

Not all greenhouses are suitable for shade paint, such as those made from unpainted timber.

Another great shading alternative is to use external or internal blinds. External blinds tend to be better as they filter the sunlight before it passes through the glass and can easily be removed, so your plants can get full sunlight during the mornings and evenings when the heat is not as direct.

One pitfall of using external blinds is that they can block vents and reduce airflow if applied carelessly, so always make sure that the blinds are not interfering with any vents. Mesh and netting can also be effective in reducing heat when the sunlight is not too intense.

Water Cooling

During very hot periods, it might be necessary to reduce temperatures through water cooling. This is the process by which hard surfaces such as flooring and table tops are doused in water.

The benefits are two fold; not only does this increase humidity inside the greenhouse which has been proven to help plants deal with hotter temperatures, and as well as this the very process of evaporation reduces the heat inside the greenhouse, in much the same way as sweat evaporating cools animals.

It also keeps paving slabs and hard surfaces cooler, which in and of itself can help greenhouses stay cool. Materials such as stone absorb and retain a lot of heat, which can add to the temperature inside a greenhouse and keep it hot during the night. While this is great during the cooler months, it can be yet another source of heat during the summer.

On particularly hot days, it’s worth thoroughly drenching any hard surfaces in the morning, late afternoon and at mid-day, if possible.

As an added bonus, the added humidity also makes it harder for pests such as the red-spider mite, that thrive in dry conditions, to develop.

Heat Stress

Plants require significantly more water in hotter weather, not only due to water evaporating from the potting soil, but due to water being used in transpiration.

Plants push moisture through their stomata (small pore-like holes in the leaf surface) which then evaporates, reducing heat. The mechanism behind this process is similar to how we sweat. Therefore, it is important the root level moisture is measured regularly to ensure that plants have enough water to effectively cool themselves.

Remain attentive to common signs of heat stress in plants such as immature foliage dying, plants wilting and scorched leaves.