Soft Secrets: Keep it Cool!

Heat and water stress and pests can slow your grow show to a crawl in hot weather. Every year even the best growers forget about the long hot days of summer. Often symptoms of heat stress are subtle and less experienced growers will not notice the damage caused until it is too late. Understanding what happens inside plants will help you solve many heat-related problems before they slow garden growth. Couple this information with an knowledge of microclimates and you will be able to keep your garden, both indoors and outdoors, a few degrees cooler. In many cases, just a few degrees will increase harvests substantially.

Hot weather changes the processes inside plants. When the temperature outside is 20 degrees C., the temperature inside leaves is about 5 degree warmer. The temperature can climb inside leaf surfaces to more than 10 degrees warmer than leaf surfaces. When temperatures raise plants use a lot more water to stay cool. The increase in water consumption puts stress on all plant systems. When this stress is left unchecked, growth slows substantially, causing plants to become susceptible to disease and pest attacks.

Cannabis plants draw water up through roots into the foliage. The water serves to cool plant tissue and move substances inside the plant. Tiny pores on leaf undersides called stomata open and close to regulate the flow of moisture that is released in the form of water vapor. This transpiration of water keeps plants cool, similar to the way humans cool themselves when sweating. Plants use the same amount of fertilizer as they did before hot weather, but they use more water. Often water consumption can double when growing in containers outdoors or in a hot indoor room. This change in water consumption can cause severe growth problems both indoors and outdoors.

This grower put a bamboo fence around his garden. The bamboo wall blocked wind and the neighbor’s view. The bamboo fence lets just enough air through to provide air circulation and at the same time blocked the majority of desiccating wind.

A plastic roof blocked some of the midday sun to help keep plants cooler on his terrace. The grower left air space below the roof to allow for air circulation, which helped keep garden cool too.

Add a little bit of wind and water consumption via transpiration increases rapidly. Rooftop balcony and patio gardens are the most susceptible to moisture loss caused by wind. Often the indoor growers increase air circulation and ventilation to help cool plants. The added ventilation and air circulation help plants stay cool on hot days. It also causes them to use more water.

Air also holds more moisture when the temperature climbs, which causes relative humidity to decrease. Lower relative humidity also causes plants to transpire more moisture and use more water.

Containers and growing mediums also tend to stay warmer during hot weather. Chemical processes occur faster in warm growing mediums and more water is used. Once soil temperature climbs above 25-30 degrees, growth slows rapidly. Outdoors or indoors under HID lights, container temperatures can easily climb beyond 30 degrees. A container on a hot patio can easily climb to 40 degrees or more in less than an hour of full sun. Temperatures this hot actually cook roots. Once cooked, roots cannot absorb water and nutrients, even though water is available in the growing medium. Dead roots start to rot immediately and invite pests and diseases. Once cooked, roots take several weeks to grow back providing conditions are perfect.

When water consumption increases, fertilizer consumption stays about the same. Lowering the fertilizer concentration is very important now to keep nutrient salts from building up in the soil. When nutrient salts build up in the soil, the ability of the roots to draw in water via osmosis decreases. In fact, the ability of roots to extract water from the soil decreases proportionately to the buildup of toxic salts. The more over-fertilized the soil or hydroponic medium, the less water plants can draw in.

Planting in the shade will keep plants cool in hot weather. Although this plant receives less light and grew smaller, it produced some nice smoke.

After plants get big enough foliage shades containers from hot sunlight, which would cook roots and stunt growth.

Ensure against toxic nutrient buildup in soil by diluting fertilizer concentration according to manufactures specifications. Also allow at least 20 percent water to flow out the bottom of the container for each watering. Twenty percent is a lot of water, but applying enough to allow adequate drainage from containers will really save you a lot of problems later. Take a look at the list of problems above and you can see that under-watering can be caused by several things at once. When all of them are compounded, they have a synergistic effect on retarding plant growth.

What are the symptoms of heat stress? If you said plants wilt, good! But, by the time plants wilt, the damage is well underway. Subtle differences will appear first. The first symptom – limited fluid movement from water stress – will occur within the plant and you will not see it. The first outward sign of dehydration in plant tissue manifests by leaf edges turning slightly upwards. FOTO ??? Leaf edges turn up slightly because they dissipate moisture there the fastest. When leaves do not have enough moisture to transpire, the edges turn up trying to shed as much water as possible. However, an excess of potassium (K) can also cause leaf edges to turn upward. Potassium toxcisity is difficult to diagnose because it is usually mixed with the deficiency symptoms of other nutrients. Too much potassium impairs and slows the absorption of magnesium, manganese and sometimes zinc and iron. Look for signs of toxic potassium buildup when symptoms of magnesium, manganese, zinc and iron deficiencies appear. These symptoms normally appear after plants have suffered water stress.

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