Growing cool-season leafy crops under tropical conditions is a challenge faced by many commercial producers as well as smaller-scale growers. Heat and humidity can wreak havoc on sensitive plants, not only from increased pathogen pressure, but with physiological responses ranging from bolting to wilting and overall low productivity. Tropical climates that limit outdoor production of high-quality fancy salad greens, however, can have lucrative local markets for hydroponic produce as well as a growing interest in home food production. The path to success with cool-season crops under tropical conditions involves some understanding of plant physiology and a degree of environmental manipulation that hydroponic methods can provide.
Growing Under Warm Temperatures
Tropical climates are characterized by warm to hot day and night temperatures year-round, with very little variation from one month to the next and can experience significant levels of rainfall with constantly high humidity. Light levels are generally high but can drop below light saturation for many leafy crops during periods of heavy rainfall and thunderstorms.
Many cool-season greens such as fancy lettuce have a temperature optimum in the range of 61-72°F (16-22°C) with conditions over 82°F (28°C) increasing the occurrence of germination failures, bolting, tip burn, wilting, lack of color in red varieties, and low plant weight. High humidity combined with warm conditions also increases disease risk, particularly if foliage becomes wet. When temperatures increase, the rate of plant respiration rises rapidly and the plant may burn up more assimilate that is produced via photosynthesis.
The oxygen requirement in the root zone also increases with temperature, putting pressure on the design and operation of the hydroponic system to provide maximum oxygenation. Root health becomes vital so that sufficient water is taken up and transported out to the foliage to match the rate of transpiration which assists with the cooling the plant surface. A feature of tropical climates is often periods of high humidity, further stressing hydroponic plants as it slows the rate of transpiration, thus reducing the transport of water and minerals, such as calcium, through the plant and out to the leaf tips. Physiological conditions such as tip burn and oedema can become common where transpiration is limited for long periods of time.
While cropping cool-season leafy greens under tropical conditions may seem to have a few difficulties, there are a number of options to make this viable and highly successful. These include careful selection of species, varieties, and manipulation of both the aerial and root zone environment to allow maximum growth.
Greenhouse growers of leafy greens in tropical climates often use a simple rain cover with insect mesh sides to modify the production environment. This is made more effective by having large top vents that allow warm air to rise and escape from the top of the structure, thus drawing in fresh air from around the sides. Further environmental modification and cooling includes misting systems (when humidity is low enough to allow for evaporative cooling), and air movement fans to help drive transpiration. For salad crops, a shade cloth screen can be set over the roof to reduce incoming solar radiation and lower the stress on the crop.
Many growers in tropical areas experience difficulty germinating seed of cool-season crops such as fancy lettuce or spinach which may go into secondary dormancy when temperatures are too high. This can be managed by having separate germination rooms or use of shipping containers under controlled temperatures where seed is germinated for the first 48 to 72 hours before being transferred out into greenhouse nursery areas.
Ask a Grower: What is the ideal humidity level for leafy greens?
For extreme tropical climates, indoor production is often a more viable option, particularly for small, high-value salad greens. This allows more precise control over the environment through use of dehumidification, air conditioning, lighting, and CO2 enrichment. Small-framed salad greens grown in vertical systems can also maximize space within an indoor production facility and be used on a large commercial scale, or for domestic home units.
Root Zone Control
Whether cool season greens are grown in a greenhouse structure or within an indoor system, control over the nutrient solution temperature is one aspect that can significantly improve yields, quality, and productivity under tropical conditions. Root zone temperature strongly affects shoot growth. In fact, root zone temperatures play just as much of a role in growth and development as that of the air surrounding the plant. This is because the root tissue sends numerous non-hydraulic messages to the shoot which influence the way the shoot responds to the environment. There is a proven technique that can be used to fool the physiology of cool-season plants into handling higher-than-optimal air temperatures.
Read also: Keeping Things Cool In Your Growroom
Root zone chilling of the hydroponic nutrient solution is a technique being used commercially by many growers in warm or tropical climates, most often with cool season crops such as butter head lettuce, salad greens, and other vegetables. In Singapore, nutrient film technique (NFT), aeroponic, and deep flow culture systems are utilized with extensive nutrient chilling to grow butter head and Romaine lettuce, crops that otherwise do not grow or yield well at ambient air temperatures. Chilling the nutrient solution down to as low as 61-64°F (16-18°C), allows cool-season vegetables to crop well at ambient air temperatures that are often well above optimal for these crops 82-97°F (28–36°C). Without nutrient chilling, the root zone usually warms to the level of the air and this creates numerous growth problems including slow growth, lack of heart formation, bolting, tip burn, and low marketable yields.
Other researchers have reported that nutrient chilling of lettuce also reduces the occurrence of the fungal root disease Pythium aphanidermatum. However, trials have shown that while root zone chilling via nutrient cooling can have these effects, the root zone cooling must be applied soon after early crop establishment and maintained for the life of the crop for maximum effect. Just a few minutes a day of root zone temperatures higher than 86°F (30°C) have been shown to retard the growth of some heat sensitive crops such as lettuce.
Tropical Hydroponic Systems
The systems most suited to leafy veg production in the tropics are often those that allow some degree of cooling of the root system, or at least prevent excessive heat build up in the nutrient solution. Studies have shown that deep water culture systems (pond/raft or float systems) give the best results for crops such as lettuce and other salad greens where nutrient chilling is being used. Aeroponics and NFT are other hydroponic systems that are used with nutrient chilling, however, these seem less effective than deep flow systems, possibly because more of the root system is continually submerged in the chilled nutrient in these types of system.
Under tropical conditions, the electrical conductivity (EC) in the nutrient solution is run at a lower level than it would be with the same crops under temperate climate production. This helps facilitate the uptake of water and calcium under warmer conditions. Under high light and temperatures, nutrient solutions tend to differ slightly than those run-in systems under cool, winter environments. This includes a higher level of iron and changes in the N:K ratio.
Species and Variety Selection
Within the leafy veg category there is a wide range of different plant species and within each species several varieties to select from. Commonly grown hydroponic crops such as fancy lettuce, while all still being cool season plants, do have some scope with regards to heat tolerance and some increased resistance to production problems such as bolting and tip burn. Commercial seed suppliers often have variety selections for warmer-season cropping, and while these won’t overcome all issues with high temperatures, they are worth evaluating under tropical conditions. Some species of leafy veg are more suited to tropical conditions. These include many of the Asian greens such as mizuna, mibuna, many different types of mustards (that come in a range of leaf shapes, sizes, and colors), Tatsoi, Pak Choy, and Kailaan.
As with all leafy greens, if growing under tropical conditions, it is still best to select cultivars suited to the local growing conditions. One tropical leafy vegetable plant that performs extremely well under tropical conditions, and particularly with solution culture hydroponic systems, is Kangkong, also known as Chinese water spinach (Ipomoea Aquatica). This semi-aquatic plant is a popular leafy green in many tropical areas and has an extremely high rate of production and yields provided temperatures are kept well above 73°F (23°C).
Growing leafy greens under tropical conditions has its challenges, particularly with the cooler-season hydroponic crops such as fancy lettuce. However, with some modification of the system, environment, and careful genetic selection, it is possible to grow productive crops on both a small and large scale.
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