How to improve the natural tolerance to Heat Stress in Plants

It’s around the time of year again where ambient temperatures can effect the natural growth of your plants, and provide massive headaches in trying to achieve optimum temperatures inside your grow room. At temperatures above 30 degrees your plants begin to suffer, and although it’s not always possible to prevent heat stress hitting your plants it is possible to increase the tolerance before the heat wave arrives.

Heat Stress in Plants is not Necessarily Bad!

Stress to your plants is not necessarily a bad thing, exposing plants to moderate stress can result in better colour, vitamin content and aromas to the fruit. As example tomato growers often increase the EC of their nutrient solution during fruiting and flowering. The increased salt content in the root zone prevents as much water uptake, resulting in sugars and organic acids condensing in the fruit. Through EC manipulation it is possible to double the lycopene content (red colouring), and increase vitamin C content by as much as 50%. A direct proportional relationship between EC and sugar content also exists; the higher the EC, the sweeter the tomatoes. Additionally, by increasing the EC, the plants accumulate more sugars and dissolve solutes in their roots, making them more tolerable to any future salt stress.
So moderate stress can have a positive impact, however too much stress and can be terminal. Exposed to too much heat or salt stress and the plant can’t take up enough water to meet its basic needs. The edges of leaves will start to curl up and turn brown and plants can show signs of calcium deficiency such as tip burn in lettuce or blossom end rot in tomatoes. High heat and humidity can make the problem considerably worse. So as temperature levels increase above the optimum levels, it is important to begin to dilute the EC levels of the nutrients solution to aid in uptake of water and nutrients. It is also important to ensure there is sufficient air movement in the garden so the plants scan continue to transpire and naturally cool themselves.

Plants Produce their Own Plant Protection Agents

Plants have an uncanny ability to produce their own plant protection agents and cope with a wide array of environmental stressors. Sometimes however, plants need a little assistance. In nature, plant growing micro-organisms in the root zone make compounds that improve the plants tolerance to diseases and stimulates the plant to become more tolerant to environmental stress.
Through studies of the einteractions between plants and their microbial guests, plant scientists are beginning to learn how to harness the power of natural biostimulants to provide new and improved plant protection agents. Among the most promising of these are seaweed extracts, humic acids and l-amino acids.

Benefits of Humic Acid and Seaweed Extracts on Plant Stress

Virginia Tech carried out a 10 year study on biostimulants and found that the use of humic acid and seaweed extracts had a profound effect on the stress tolerance of a plant. Seaweed extracts are rich in natural plant hormones called cytokinins. Cytokinins stimulate cell division. So when applied to the root zone, seaweed extracts stimulate increased lateral root growth and root mass, providing a healthy root mat for the uptake of water and nutrients. If a gardener can help plants develop greater root mass before the plant begins to experience stress, the plant will be much better prepared to efficiently take up water when the summer heat arrives.
When the seaweed extract was combined with humic acid the scientists discovered even greater benefits. The best ratio that was found was 5 part humic acid to 2 part seaweed extract (5:2). This combination catalysed the production of 50% more superoxidant dimutase (SOD) – a powerful plant protection agent. SOD neutralises harmful free radicals before they can accumulate in plant cells and damage cellular membranes. Under normal conditions plants produce enough SOD, however during times of excessive stress plants can’t cope with production. So the addition of a humic acid/seaweed extract stimulates higher levels of SOD, protecting against harmful free radicals and help the plants keep green during the summer hear.
Humic and fulvic acids are additionally rich in beneficial trace elements, which are particularly good at helping the plant take up iron. During times of stress, plants require iron and other trace metals to activate important enzymes. For example, SOD is activated by either a zinc/copper complex or an iron/manganese complex. If the trace elements are unavailable, the enzymes are turned “off” and they will be unable to protect the plant. However if the enzymes are turned “on”, a single molecule of SOD can perform over 1000 chemical reactions per second in the cell! That’s a lot of plant protection from a very small amount of humic acid.
Seaweed extracts/humic acids also contain vitamins and amino acids that can help strengthen the plant and provide an additional level of plant protection. Amino acids are intermediate chelators. In other words, amino acids help keep trace metals soluble and available to the plant. In addition, some amino acids dramatically improve the uptake of calcium. Certain amino acids, such as glutamic acid and glycine, stimulate root cells to open up calcium ion channels, allowing calcium to be taken up many times faster than osmosis. Calcium forms the glue that glues cell walls together, strengthening the plant tissues against temperature stress. So when temperatures and humidity start to rise, the enhanced calcium availability provided by amino acids will continue to have a protective effect on the plant.

A few practical tips to help prepare plants to beat the heat:

    • Supplement plants with a 5:2 ratio of humic acid and kelp extracts. The combination works 50% better than either product alone.
    • Provide good drainage. Stagnant water can cause root rot which can ultimately reduce the uptake of water and nutrients.
    • Maintain nutrient temperatures between 68-75 degrees. As water temperature increases, dissolved oxygen in the water decreases. Plants need plenty of oxygen for healthy roots.
    • Avoid large swings in temperature and humidity. Try to allow no more than a 10 degree difference between day and night temperatures.
    • Provide adequate air movement. All of the leaves in the garden should be moving gently to keep the transpiration stream flowing.
    • Don’t let pH drift too high. As pH becomes more alkaline (above 6.5), iron and other trace metals start to become unavailable to the plant.


  • Provide carbon dioxide supplementation. Higher CO2 levels allow plants to grow at higher temperatures (up to 5 degrees higher) without negative side effects.
  • Don’t apply too much nitrate-nitrogen. Excessive nitrates promote lush top growth, but restrict root growth. Plants become more susceptible to heat stress.
  • Use silica supplements. Silica strengthens plants and helps alleviate heat stress.

Remember, healthy plants are naturally resistant to stress and disease. There are limits to the amount of heat stress that even healthy plants can tolerate, so we should do everything that we can to provide the best environment possible. But if a plant has a strong root system, thick cell walls, abundant trace minerals, and a reserve of antioxidants and plant protection agents, it will be better prepared to beat the heat. Don’t wait until it’s too late! Give your plants a boost with kelp and humic acid before the summer comes on strong.
Copyright© 2013
Harley N. Smith


Easy Grow Ltd | Hydroponic Wholesale
Easy Grow Ltd | Hydroponic Wholesale