Hot, dry climates with clear skies and temperatures reaching up to 120* F don't have to be stressful for marijuana. Most plants in hot climates die because they don't get enough water or have shallow roots which are fried in the hot upper part of the soil.
If the marijuana plants are watered satisfactorily enough, the water will penetrate straight down into the medium and the roots will follow it down. Deeper soil layers won't exhibit the same radical temperature changes found in upper layers.
During the summer, it's particularly important to never let the soil dry out. The roots draw water up that is then transpired by the leaves. If the water source dries up, the leaves will start to lose the water from their cells. They will also lose their turgidity and begin to wilt. As the drying progresses, more and more cells start to die. Even a minor wilt due to water stress damages the plants. If they are not watered at the first sign of stress, they can die in a matter of hours. Download my free marijuana grow bible for more watering tips.
Sometimes the plant's leaves will droop slightly during the hottest part of the day, even when it's had enough water. This is a normal reaction that might just be a protective measure used by the plant. When the sun gets too intense for the leaf, it shifts its angle by wilting so that it receives less sunlight.
Marijuana plants grown under partial shade with these same conditions perform well. The soil is unlikely to go through any radical changes in temperature, and the plants get enough light to grow and produce. In some areas, indirect light might provide enough intensity (if there is no cloud cover) to produce decent yields.
Marijuna growers might be interested in a few commercial products to help with those sunny, hot, and dry climates. Anti-transpirant sprays will mitigate water loss during particularly stressful periods. Directions on these sprays indicate that they're useful for transplanting and for enduring stressful climatic conditions like hot, dry winds. You can find these products at many nurseries.
Another option is water-holders which are made from starch and polymers. They resemble corn flakes and are relatively lightweight when dry. You mix them into the soil and, when they contact the water, they sort of balloon to hundreds of times their weight in water. A single tablespoon of flakes can hold 6 to 8 ounces of water. When the soil dries, the particles start releasing their moisture. The soil maintains its dampness longer and less water is lost overall.
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