Reducing Heat Stress in a Tropical Greenhouse

Tropical Climates can be especially challenging for hydroponic greenhouse growers.

The tropical climate raises the following challenges:

1. High air temperatures inside the greenhouse raises the temperature of nutrient solutions thereby reducing the dissolved oxygen content and putting stress on the root zone of the crop.
2. High levels of humidity reduces transpiration rates and slows calcium uptake leading to tip burn issues on lettuces or blossom end rot on tomatoes. In contrast, high temperatures and low humidity creates excessive transpiration rates which also reduces calcium uptake. A steady rate of transpiration is required.
3. If the high temperatures coincide with high humidity in a tropical climate, evaporative cooling techniques cannot be used as effectively compared to regions that experience high heat but lower levels of humidity.
4. Many tropical islands rely on diesel-powered generators for producing electricity which results in expensive electricity costs for the grower. The grower then tries to limit electrical appliance usage as much as possible to reduce running costs. This limits the type of equipment or methods the grower can use for cooling and the crop suffers.
5. The smaller markets in tropical regions can reduce the income stream for growers, thereby limiting the amount of capital investment. More expensive greenhouse designs and equipment are therefore not affordable for many growers.

Below is a list of the best steps a grower can take to reduce heat stress on the crop in a tropical environment.

Proper Greenhouse Design – Roof Vents, Roof Height and Vented Sidewalls

A proper greenhouse design is by far the most effective way to reduce heat stress.

It is better to install a correctly designed greenhouse from the outset. That way you can immediately start growing your best crops and focus on recovering your investment rather than having to spend a lot of time and additional money on heat reduction techniques that never quite create the optimal environment for your crop and result in substandard yields.

A tall gutter height is preferred to allow the heat to accumulate further away from the crop canopy thereby reducing stress. It also allows any mist from an overhead misting system to fully evaporate before reaching the crop.

Roof vents in combination with vented greenhouse sidewalls (i.e those covered with insect mesh) is the main way to allow heat to escape from the growing area.

Cooler, less humid air is able to pass through the vented sidewalls. As this air heats up in the greenhouse it rises and leaves the greenhouse via the roof vents. This natural convection current allows warm, humid air to be constantly exchanged with cooler, less humid air from outside. This cools the greenhouse and allows a steady rate of transpiration to occur within the crop.

The vents and sidewalls should be open fully at all times unless strong winds risk damaging the vents or risk driving transpiration rates too high.

The sidewall mesh should be fairly porous (i.e 5mmx5mm mesh) this keeps birds and larger insects out and ensures optimal air movement. As soon as a finer mesh is used (i.e aphid or thrip netting) the air movement can be reduced by up to 50%. This results in heating of the greenhouse and pockets of humidity that contributes to calcium transportation-related disorders (tip burn, blossom end rot).

A greenhouse with Twin roof vents (i.e a gull wing style) is the optimal for hot conditions as they give a bigger surface area for venting compared to a single roof vent. The twin roof vents add to the greenhouse cost. The vents should ideally be motorised and controlled with a greenhouse vent controller which utilises a weather station. The weather station monitors external wind speed, wind direction the incidence of rain as well as the temperature and humidity levels inside the greenhouse.

If the wind is too strong from one direction, the vents and sidewalls on that leeward side are closed and the vents on the opposite windward side remain open to prevent damage and ensure ventilation is occurring at all times.

With roof vents it is not the length of the vent arm that is important but the distance that you can open that vent e.g. you can have a long vent arm but might not be able to open it very wide because of the risk of wind damaging it.

It is better to have the vent opening nearer the ridge of the greenhouse roof. This is where most of the hot air accumulates and it creates a chimney effect. If the vent opens lower down near the roof gutter it is unable to release as much of the trapped heat.

Having 33% of the floor area covered by a roof vent is optimal.

In rough figures, one roof vent adds around 17% to the cost of an unvented greenhouse.

To upgrade from a single roof vent to a double roof ventis an additional 15% on top of a single vent (i.e. 32% extra on top of the cost of an unvented house)

Note: These additional costs will vary according to the length of the building as the motor drives and vent bracing becomes proportionally less as the length of the vent increases.

If the base cost of the greenhouse is higher due to extra equipment being added such as roll up side walls, extra doors, and crop support the cost of the roof vent gradually becomes a smaller proportion of the total cost.

Tropical climates are more susceptible to cyclones and hurricanes. A properly selected greenhouse is one that can provide optimal ventilation and has a steel frame that is properly engineered to withstand the maximum wind speeds at that location.

All too often tropical growers fall into the trap of installing tunnel houses that have no roof or sidewall vents and the roof height is too low. The heat quickly accumulates under the ridge of the roof and puts stress on the crop.

Foreign aid money that is used to promote horticulture in poorer tropical countries often stipulates that a certain brand of greenhouse from the donor country should be used. This creates additional business in the donor country through manufacturing greenhouses, but the design is not always optimal for the country the greenhouses are being sent to.

The tunnel houses are usually low cost, with lightweight frames that are not designed for cyclone conditions. The roof height is also lower which may be suitable for soil grown crops but not hydroponic crops which are usually raised off the ground to a comfortable working height. As a result the crop canopy grows closer to where the heat is accumulating and the steel greenhouse frame becomes damaged in the first major storm.

It is far easier and faster to replace damaged plastic film covers than it is to replace damaged sections of steel frames.

A well-engineered greenhouse is always the last one standing after a strong cyclone

The growers who use a well-designed greenhouse can capitalise on the after-effects of a heavy storm. All they need to do is replace the plastic film covers and get growing again. Other growers who used cheaper greenhouses remain crippled for a few months while they await steel frames to be manufactured and shipped from overseas – as well as the arrival of the specialised enginerring staff that are needed to install them. This delays the planting of their replacement crop or results in continued losses amongst the new crop (i.e due to rain damage) until the greenhouse damage is fully repaired. In those instances the benefits of a strong, purpose-built greenhouse is quickly realised and pays for itself.

Horizontal Air Movement Fans

Horizontal air movement fans are designed to move air within the greenhouse but are not designed as extraction fans. The purpose of the fans is to promote a gentle movement of air throughout the greenhouse to create a more uniform environment. This helps to prevent pockets of humidity or hot air and promotes a steady rate of transpiration. If the air is humid and not moving you are at risk of fungal disease or calcium transportation issues. A slight movement of the leaves is required at all times to allow the plant to steadily transpire and cool itself and to move the pockets of humidity.

Shadepaint

Shadepaint can be applied to the greenhouse film reduce light intensity coming into the greenhouse. This reduces heat.

Some types of shadepaint can be applied once and then gradually wash off in the rain.

Other types need a special shadepaint remover to ensure a more durable coating.

Shadecloth

Shade cloth can be suspended outside the greenhouse above the film or on horizontal wires inside the greenhouse.

A white 40% shade cloth is best as it reflects the infra-red rays and can stay in place year round. If you use a heavier grade of shade cloth it will need to be retracted during cloudy or low light periods or else the plants can stretch (grow elongated).

Custom shade cloth panels can be made which have hemmed edges and a draw string inside. Then all you need to do is pull a high tensile wire through the hem and use shackles to clip the shadecloth to a wire rope running parallel to the High tensile wire.

Or just use butterfly clips and standard shadecloth to attach the cloth to a support wire on the edges.

More modern shadecloth systems can be retrofitted with motorised winders so the shadecloth panels can be pulled over and back again in response to an enviro sensor depending on the level of light intensity.

Misting Systems

If the hottest times of the day regularly coincides with humidity levels under 75% then a grower can look at installing a misting system. This involves pulsing a fine mist into the air and this evaporates and draws the heat out of the air. A lower humidity level is required for it to work well.

If you have high temperatures e.g. 35 Degrees C but the humidity is high as well e.g. 80% then the misting system won’t work as well because the moisture won’t evaporate as readily and the mist will make the environment even more humid. This can put additional stress on the crop.

75% humidity or lower is o.k as the evaporating mist won’t typically push it beyond 85% for a long period.

Growers are advised to visit Wunderground.com where they may have a weather station logging data near your area with historical hourly temperature and humidity data for each month. A grower can then gauge if their local climate is too humid to make misting a viable option.

The idea is to do a 1 – 3 second pulse of mist, the mist then evaporates and the fans and convection current then removes the humidity then it mists again. The misting systems are quite water efficient but you need to check on humidity levels before considering one. If your humidity levels are constantly above 75-80% from 11am – 3pm then a misting system would not be effective.

Flowing Water on Greenhouse Roof

Some growers have lengths of PVC pipe or lateral tubing running along the length of the greenhouse roof at its peak. They punch a lot of small holes in the pipes then pump water though the pipes. As the water flows down the greenhouse film outside it cools the film and the air inside the greenhouse. This method is cheap to install but isn’t as water efficient. You also have to check that it won’t wash off your shade paint.

Pump Room Considerations for Recirculating Systems

A larger than normal hydroponic tank for recirculating NFT systems and plenty of cascading fall from the return pipe back into the tank helps to reduce plant stress. E.g. 2 – 2.5 plants per liter of tank solution. So if you have 15,000 plants you should have a 6000 – 7500 litre tank.

A bigger tank takes longer to heat up during the day and enables more dissolved oxygen to be shared around. It prevents impurities such as sodium building up as quickly and ensures a more evenly balanced hydroponic solution can be maintained for longer. Smaller tanks heat up faster, lose dissolved oxygen quickly and promote the build-up of impurities which contribute to nutrient imbalances.

Venturi Oxygen Injectors

Recirculation pumps should also have a venturi on the pump bypass for additional oxygenation and to bring plant waste gases to the surface of the tank solution. The tank should also be well ventilated with an extraction fan to remove any waste gases. Removal of waste gases by using an extraction fan or ventilation can significantly improve growth. Always let your tank and pump room breathe with a good cascading return solution falling through clean air.

Venturi Oxygenation Systems

Nutrient Solution Chillers

Nutrient solution chilling is not really an option in many tropical regions as the electricity is usually expensive on tropical islands and the capital outlay for a chiller is high. If a chiller is used then the pipework leading to and from the growing benches also needs to be insulated or buried and the tank needs to be insulated as well.

Low EC During the Day

During the hottest period of the day, the plants do not feel like feeding vigorously. They are more interested in transpiring and keeping cool. During these times a slightly lower EC level (conductivity) can be used to promote more water uptake and calcium uptake. The EC level can then be boosted in the early evening by manually adding nutrients to the tank or adjusting the setpoint on the dosing controller at the end of the day. The plants will then uptake more nutrients overnight to help make up for the reduced levels of uptake during the day. E.g for lettuces run at 0.9 – 1.0 EC during the day and boost to 1.1 – 1.2 EC at night. The ideal level you increase the EC to can be fine-tuned by seeing how much the plants have reduced the EC by the morning when overnight auto dosing has been disabled. Any more than is necessary is wasteful. E.g if the tank is dosed from 1.0 to 1.2 EC at night and is 1.1 EC in the morning then it may be more efficient dosing to 1.1 EC instead.
This is because the solution needs to be diluted back to the normal growing strength in the morning before the sun gains too much intensity to avoid tip burn.

Correct Selection of Varieties

The correct plant varieties also need to be selected for each particular time of year. E.g. Red and Green Oak lettuces tolerate the heat the best. Sometimes it is best to grow those varieties over the hottest months and switch to other varieties over the cooler months. Crisphead lettuces do not grow so well in warm climates. It is difficult for them to form a tight head and tip burn issues are more common.

Other Variables

Regularly check that the plant roots are nice and white and are in good health. A root disease on the crop can cause wilting and heat stress so you need to be sure there are no other factors at play.

Dumping Cycles in Recirculating Systems

For the recirculating systems like an NFT lettuce system you need to dump the main nutrient solution mixing tank once every 4 weeks to remove any sodium build up and return the solution back to balance. (unless a lab test shows that it should be done sooner or can be extended longer). If the procedures at the farm have worked well previously then it is best not to change them. Quite often if a procedure is changed to try and save costs (e.g. dump less frequently) there can be an unintended consequence to that (e.g. sodium build up = more plant stress, slow growth, poor nutrition). Or alternatively a water treatment procedure may be skipped to save on chemical costs and this leads to root disease due to poor sanitation.

A seasonal lab test on the recirculating nutrient solution is highly recommended so you can monitor the build-up of impurities and set the timing of your dump cycle perfectly. This saves money, optimises nutrient uptake and reduces stress on the crop.

If a grower can address all of these points above then they are sure to see some crop improvements in their crop.