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Tuesday, July 17, 2018

RX for Plants with Heat Stress

Everyone wants their yard to look its best all the time and have plants that stay lush and beautiful year round. Summertime in the Valley of the Sun, 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 questions:  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 are just like people when it comes to heat stress–they need a certain amount of water to keep their system operating. During times of intense heat, transpiration is accelerated. According to Wikipedia, “Transpiration is a process similar to evaporation … it is the loss of water vapor from parts of plants(similar to sweating), especially in leaves but also in stemsflowers and roots. Leaf surfaces are dotted with openings … called stomata … which are bordered by guard cells … that open and close the pore. Leaf transpiration occurs through stomata, and can be thought of as a necessary “cost” associated with the opening of the stomata to allow the diffusion of carbon dioxide gas from the air for photosynthesis. Transpiration … cools plants … and enables mass flow of mineral nutrients and water from roots to shoots.” More simply put, plants release oxygen and some moisture when taking in the CO2 , which is why you feel cool when sitting under a tree or walking in a field of alfalfa (as I like to do). Transpiration is the primary mechanism plants have to cope with heat. Dry conditions affect a plant’s ability to perform the transpiration process by inducing stomatal closure. 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 and we see the cumulative effects of cell breakdown in symptoms like wilt. Those symptoms include leaves that look brownish and crisp around the edges, plants that begin dropping leaves to shut down transpiration, branches that begin to die back, blooming stops and plants pre-maturely drop their blossoms and eventually look scorched.


Stress Relief Amendments

The best treatment is to keep your plants healthy and help them do their job well. 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 maintaining soil that water can penetrate is key to a plant’s survival during extreme temperatures. Make sure that the soil is loose enough for the water to penetrate and soak the root zone of your plants. Test your soil compaction using a long screwdriver to penetrate the soil. If the screwdriver goes in easy you have good water penetration, if it doesn’t press in easily acidification is necessary and probably a longer duration of watering. Acidify the soil with a soil acidifier to improve water penetration. 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.


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 evaporation of water from the root zone. 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 useful in treating heat stressed plants is a product mentioned above called Great Big Plants, 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 is extremely effective and is even great for houseplants. Locally, it is used extensively for ball fields after heavy use because of its ability to rejuvenate the cellular structure of the grass after heavy pounding by the players. It will also rejuvenate the cell structure of heat-stressed plants like shrubs and flowers.



Watering Schedule

At the present time we find that operating your drip system three times per week for an hour and a half each time has kept the soil profile moist enough for the survival of most plants. Plants that start turning yellow is a sign of over watering and then you should back the duration of the watering down to perhaps an hour and a quarter three times per week. If, on the other hand, 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. Soils vary with their ability to accept water so some experimentation will be necessary. If too much water is standing around an hour after you stop running your drip system, reduce the watering duration by 15 minutes. If the plants show signs of stress before the next watering time, then increase the watering duration.


Extra Care Saves Money

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 an eye towards a cool fall.



Gary Petterson is president and horticulturist of Gardener’s World located at 3401 E Baseline Road in Phoenix, the South Mountain area. Contact Gardener’s World at 602-437-0700. Sign up for free email garden tips and get class information at www.gardenpro.net




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