Formula for plants with heat stress - Ahwatukee Foothills News: Smart Gardening

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Formula for plants with heat stress

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Gary and Sharon Petterson

Posted: Tuesday, August 14, 2012 3:49 pm

Everyone wants their yards to look their best, have lush and beautiful plants year-round. Summertime, though, seems to make us all wilt, plants and people alike. But, even though it’s hot and humid this month, your plants can thrive and stay healthy by understanding a few basics: what is plant heat stress, how do you know your plant is stressed, and what do you do when your plants have been affected?

Plants, like people, need a certain amount of water to keep their ‘system’ operating. During times of intense heat a plant’s main defense is a process called transpiration. Transpiration is the process where oxygen and moisture are released from the stomata (pores) on leaves, stems, flowers and roots which open and close to take in CO2. This has a cooling effect similar to evaporation which is why you feel cool sitting under a tree.

Dry conditions affect a plant’s ability to perform transpiration by forcing the stoma to close. Wind and high light intensity also stimulate stomatal closure. And, when nighttime temperatures remain high the plant has no chance to open its pores and recover or if the soil is too dry there is no moisture for the roots to uptake, we see the cumulative effects of cell breakdown in symptoms like wilt, leaves that look brownish and crisp around the edges, plants begin dropping leaves to shut down transpiration, branches begin to die back, blooming stops and plants pre-maturely drop their blossoms and eventually looks scorched.

The best treatment is to keep your plants healthy; provide an ideal environment by deep watering, maintaining peak soil conditions through soil acidification, mulching, and when needed use a stress relief amendment called “Great Big Plants.” The object is to keep the plant healthy, watered and the root zone as cool as possible. Of course, all of this is predicated on choosing a plant that is well suited to our temperatures and planted in the right micro-climate location for its needs.

Deep watering and loose soil that water can penetrate is key to a plant’s survival during extreme temperatures. Soil should be loose enough for the water to penetrate and soak the root zone of your plants. Test soil compaction using a long screwdriver. If the screwdriver goes easily into the soil there is good water penetration, if it doesn’t, acidification is necessary. Acidify the soil with First Step Soil Acidifier or similar granular water-soluble sulfur to improve water penetration. First Step Soil Acidifier is a double bonus for plant health as some burning on leaves is caused by stress from high soil pH and salts and the solution is to apply soil acidifier.

Establish the right water duration with a bit of experimentation. Start by setting your drip system three times per week for an hour and a half each time. This should keep the soil profile moist enough for most plants. Yellowing of the leaves and water puddles remaining an hour after you’ve stopped watering is a sign of over-watering, in which case, reduce the watering duration by 15 minutes each time. If, however, the plants show signs of stress before the next watering time or the outer edges of the leaf start turning brown and crisp it’s a sign of under-watering and high alkalinity in the soil causing salt burn on the leaf margins, then increase the watering duration.

Another treatment is mulching. Organic mulch laid on the soil surface helps to insulate and lower the soil temperature around the root zone as well as slow water evaporation. An inch-and-a-half-layer of mulch should be sufficient. For new plantings in rock areas, move granite away from the small plants to lower the temperature around the plant stem and increase its chances of survival.

A soil amendment that has been extremely effective in treating heat stressed plants is a product called Great Big Plants, which is a liquid culture of beneficial microbes, hormones and nutrients for plants. This product increases the microbial action in the soil around the roots allowing the plant to take up nutrients and moisture at a faster rate. It’s even great for houseplants. It also helps rejuvenate the cell structure of heat stressed plants like shrubs and flowers.

The cost of treatment is small in comparison to plant removal and replacement, particularly on larger trees. Fortunately, summer will end and temperatures will drop. Until then, keep an eye on your plants and look forward a cool fall.

Gary and Sharon Petterson own Gardener’s World and Gardener’s Eden Landscaping in Phoenix, 3401 E. Baseline Road. Reach them at (602) 437-0700. For the nursery, call (602) 437-2233 or visit, and for landscaping, visit

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