How to Grow Veggies in a Hot, Humid Climate

The Eastern Seaboard of South Africa is classified as sub-tropical because of its hot, humid climate, especially in summer. Temperatures in the upper thirties or low forties are common during January and February, and these temperatures combined with a blistering humidity level of 90% makes life for humans very uncomfortable without air-conditioning…and it’s just as bad for plants! Here’s how hydroponics can help you cultivate fruit and veggies, year round…

Keeping plants cool

Did you know that nothing grows over temperatures of 30° Celsius, and pollination becomes increasingly difficult the higher the humidity rises over 65%?

Many people fall for the salesman’s ruse that plants must have protection from the weather and that the best way of protecting them is by covering them with a sheet of plastic. The net result of this action – increased heat and humidity! In fact, the sun can heat up a plastic-covered tunnel to 55°C or more – completely unconducive to growing, something that many an aspirant farmer has learned the hard way, as the many such tunnels littering the countryside demonstrate.

Attempting to cool down hydroponic tunnels with water is fruitless, and the reason why is shown in the example in the following table:

Temperature Humidity (rH) Efficacy
36° 90% 10%


To measure the efficacy of water cooling, take the percentage humidity and subtract it from 100%. The resultant figure gives you the effective potential for cooling – in this case it’s 10%. Then, subtract this 10% from the ambient temperature (in this example) of 36°C. The net result is 32.4°C. As previously indicated, plants don’t grow above 30°C, so water cooling in tunnels via misters and other water cooling apparatus is merely a waste of water, electricity and money!

Shade cloth for plants

Now that we’ve dispelled the myth that plastic sheet covering is the best way to protect plants, what method would we recommend that would provide for their protection, and would create a cool environment for them in which to grow?

The answer is both simple and cost-effective – shade cloth! Plant nurseries have been growing seedlings hydroponically for decades. They do this very successfully under shade cloth, held erect by gum poles and wire, and this is the proven method of growing small sensitive plants. The more shade you require for your plants, the higher percentage of shade cloth will be necessary. For tender plants and those that prefer full shade, such as lettuce, apply 60%-70% shade cloth depending whether you live in a hot or very hot area. Each vegetable has its own preference – peppers, for example, would require 50%-60% shade cloth, as they are most prone to sunburn; vanilla would need 90%, as they prefer nearly full shade.

Seven ways to keep plants comfortable in the heat

Although they’re easy to grow in winter, the intense heat experienced on South Africa’s east coast from January to March makes growing plants like lettuces and salad greens difficult. Temperature affects price, too – in winter, the crop supply is plentiful, driving prices down; in summer, supply is down, which drives prices up. If you’re able to provide a cool environment for them in the hot months, then, you’ll not only be able to grow these veggies successfully, but command a good price for them!

There are many other essential things you can do to provide a cool environment for your plants in sub-tropical or tropical zones. Bear in mind that the whole objective is to keep both the roots and the leaf surface temperature as cool as possible. You can:

    1. Bury the nutrient water tanks in the ground, leaving 15cm protruding so that flood waters do not enter the tanks and contaminate the nutrient water. This will insulate the tanks and keep them cool. Paint the tops of the tanks silver to reflect any sunlight that might reach them.

    2. Cover the tank area with a high percentage shade cloth (90% if possible) as this will help to keep them cool. A shade cloth box is recommended.

    3. Cool the water down with a water chiller – this is essential to keep the roots cool. If the ambient temperature is around 30°C or more, you can cool the water to about 20°C or even lower. With a thermometer, take the temperature of the roots and try and get them to as near as 24°C as possible. You may have to cool the water down quite considerably as it will tend to heat up on the way to the plants.

    4. Bury as much exposed tubing as possible, and wrap exposed tubing with Alucushion® or silver paper. Alucushion® is like silver bubble wrap so it does a great job.

    5. Make sure that your floors are covered with dual extruded plastic sheeting – white-side up for light reflection and black-side down for weed inhibition.

    6. Install a couple of high speed fans at the prevailing wind end of the tunnel, and a couple in the middle. This will help to keep the humidity down.

    7. If you grow with Verti-Gro polystyrene pots, not only will you fit six times the number of plants in the same space, but the pots insulate the roots by 4 or 5 degrees, making them cooler in summer and warmer in winter.