Dry heat, scorching winds are plant killers

By Cathie Draine, Journal columnist | Wednesday, July 04, 2007

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At the request of the plants in the garden, I state definitively and against all typological protocols, that HOT  is a four letter word.

This heat is ghastly … and a stiff wind (forget “breezy”) from the southeast doesn’t help any. Before we lapse into carping, whining and working ourselves into a further sweat, let’s consider what constitutes heat stress for plants.

The term, heat stress, generally describes a condition where the temperatures of both air and soil are hot enough for sufficient time that     irreversible  damage is caused to plant function or development. Plant scientists commonly agree that permanent cell damage can occur in plants when the air temperature is between 86 and 92 degrees. Trees and shrubs generally have optimum growing conditions (think: effective photosynthesis) in temperatures between 70 and 86 degrees. When the air temperature rises into the mid-90s, the plants begin to experience physical stress.

Early and intense heat can also rush a plant into premature fruit or seed development without the benefits of the nutritional support of appropriate photosynthesis.

A plant’s temperature usually runs just above air temperature. Plants dissipate heat by long-wave radiation, convection of heat into the air and transpiration or water loss from the leaves.

There are numerous studies that seem to restate the obvious: plants can dissipate tremendous heat loads if their soil is kept moist and cool (think: compost in the soil and mulch on the surface) and if the plant is placed so it is not the victim of reflected heat from pavement, walls and buildings.

Our mixture of intense heat and exceedingly dry air increases the water vapor pressure deficient (dryness of the air), which causes the stomates, the microscopic openings in the leaves, to close. This is the plant’s reaction to control rapid water loss. But this action limits transpiration, which limits the plant’s ability to cool itself.

This sad scenario begins because the process of photosynthesis slows or ceases in heat. Respiration increases as the plant desperately tries to stabilize its functions. The stomates close and CO2 capture ceases. The cooling effects of transpiration slow and internal temperatures rise.

Physiological change occurs as cell membranes fail and leak. Meanwhile, the plant is experiencing continued water loss. Obviously, growth slows or stops. The plant begins to starve as a result of rapid use of food reserves and inability to call on more.

Because its vascular system is now compromised, toxins caused by cell collapse accumulate, and proteins break down. Plant tissue temperatures rise above the thermal death threshold of roughly 115 degrees, the plant cells collapse, cellular membranes melt like butter, the plant’s vascular system and chemical integrity fails, toxic materials accumulate in the plant and death occurs.

What can we do?

We cannot control the direction or force of the wind, but we can take note that the mornings and evenings are generally calm. Aha! that might be time to water.

The most important part of the garden to receive water is the soil. Forget sprinklers that blow in the air. Turn off the automatic systems. This situation needs our most alert observations, attention and our hands on the controls.

Our air is so dry that a high percent of water from traditional sprayers will be sucked up by the dry air and never hit the ground, let alone penetrate and soak the rhizosphere -- that vital root zone. Hand water and give each plant a good drenching; use soaker hoses if you have good level areas.

Keep soil temperatures cool to insure that not only are the plant roots shielded from heat trauma but also all the fungi and bacteria in the soil remain alive and productive. Do this by applying mulch, mulch, mulch.

In my opinion, there is no place in any garden for cloth weed barriers and landscape fabric. Put absolutely nothing on or in your soil that will not ultimately decompose and feed it.

Fabric “barriers” are traps for wind-blown weed seed and, depending on the material stacked on top, can impede the movement of soil gases and water in the soil       

To help protect your garden in the heat, cultivate very lightly, water well, cover the soil with pads (6-8 pages) of soaking wet newspaper and cover that with wet mulch (dried lawn grass, old hay, old straw, city yardwaste compost, coffee grounds or other organic materials) 2-inches to 3-inches thick between plants or rows, graded to about half an inch near the plants.

Begin by applying the mulch in a thin layer, water it well, apply more mulch and water it. Do this until the mulch is as deep as your forefinger.

From then on a good, slow soak on that mulch will protect the soil from rapid fluctuations in temperature. The microbial community in the soil will begin to consume the mulch, so by fall you will need to add more.

Doing serious and aggressive deadheading and cutting back will ease the stress on the plants right now. Remove spent blooms from plants. Cut the stem as close to the base of the plant as possible.

In many perennials like penstemon, perennial geraniums and “Walker’s Low” catmint, cut the plants back to the basal rosettes — the small clump of new growth at the crown of the plant. If you don’t know what can tolerate this kind of pruning, call the Pennington County Extension office (394-2218) and speak with a Master Gardener, Google your question on the Internet or start asking gardening friends.

Container plants are at great risk for overheating. Try to protect them from radiant heat from walls. Put the pot into a larger pot and insulate the space between with packing peanuts or bubble wrap or wadded newspaper.

Put mulch around the base of your plants in containers. Do whatever you can think to do to prevent the top of the soil from heating to killing temperatures. Many of us keep a mulch of straw in our pots of tomatoes, peppers and herbs.

The extreme heat, wind and dry air create desperate conditions for plants. We can’t control much of this, but we should know the enemy, death by heat. To lose plants from ignorance or neglect is an unnecessary tragedy, and none of us need be guilty of that.

Cathie Draine is a member of the South Dakota State University Co-operative Extension Master Gardeners and the Garden Writers’ Association. She lives and gardens in Black Hawk. She may be contacted at cathiedraine@rap.midco.net.