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August is a tough month for many ornamentals, and it’s getting worse. Record breaking heat and drought conditions are putting the health of landscape plantings at risk more than ever before. Once stressed, plants become more susceptible to a host of other pests and diseases, which can be costly or troublesome to abate. Here are perspectives on recognizing and addressing summer plant stress from experts in three different states: Dr. Sharon Douglas from The Connecticut Agricultural Experiment Station; Mike Hrivnak, head agronomist with Canopy Lawn Care in Cary, NC; and Peyton Ellas, owner of Quercus Landscape Design in Springville, CA.ornamentals summer stress

Why Can It Be Hard To Tell If A Plant Stressed?

Douglas: Symptoms of drought manifest in different ways depending on the plant species and the severity of the water deficit. One important aspect of drought is that the symptoms are often not evident in the top of the tree or shrub until some time after the event has occurred—even as much as one to two years later!

Hrivnak: Often you may not realize some plant varieties are experiencing heat or drought stress until it’s too late. Why? Because when many bushes or shrubs are desiccated, they still look green. The gardenia is a good example. It appears to hold its color well, even if it has received too much sun or too little water. However, as soon as it’s watered, it begins to turn brown and wilt.

Ellas: Most signs are not apparent immediately; rather, it may take a few days to a few months. With some plants, stress symptoms are visible only after weather has cooled down and the plant is able to “take stock,” shed dead cells, and start to regrow.

What Are Visible Symptoms?

Hrivnak: Some ornamental tree and shrub varieties may show more obvious signs of heat stress. Hollies are fairly heat and sun tolerant, but when stressed, will begin to drop their leaves. Most evergreens also do well in the sun, but quickly begin molting when they receive too much heat and too little water.

Douglas: Obvious symptoms in trees and shrubs include loss of turgor in needles and leaves, drooping, wilting, yellowing, premature leaf or needle drop, bark cracks, and twig and branch dieback. Leaves on deciduous trees often develop a marginal scorch and interveinal necrosis, whereas needles on evergreens turn brown at the tips. Trees and shrubs can also exhibit general thinning of the canopy, poor growth, and stunting. In extreme cases, drought can result in plant death.

Ellas: Most of our ornamental plants are not adapted to high temperatures. In mild cases, heat stress will limit flowering and cause leaf tip browning, or older leaves will turn brown and fall off. In severe cases it will cause symptoms ranging from sunburn to leaf and twig death.

Why Do Landscape Plants Die?

Ellas: Most plant stress research is done for agricultural crops, so there are still a lot of questions about how high heat, in both soil and air, affects many ornamental plant species, especially those popular in the last five years or so. In many cases, we are still left with making something of an educated guess on whether or not our plant is suffering from 1) too little water; or 2) too much water; or 3) heat stress. But as our climate continues to warm, with extreme high temperatures—especially at night—becoming more common, we will want to consider heat stress as a factor in our gardens, related to, but distinct from added water.

Check your plants in the early morning. If they look better in the morning, and worse later in the day, the problem is most likely heat stress rather than a watering issue. A good example from the vegetable garden is summer squash. In ornamental plants, differences in geraniums and sages are fairly easy to see.

There four kinds of heat stress to plants.

  1. High daytime air temperatures is probably the easiest for plants to recover from as long as they are adequately watered. But while overwatering can help species adapted to high water use deal with heat stress, it encourages root and crown rot in low and moderate-water-use plants, which can be fatal.
  2. High nighttime air temperatures is harder to overcome because it affects plants’ ability to take in and release water and gases, and thus limits photosynthesis, needed for survival.
  3. High soil temperatures. Soil temps rise and fall, but more gradually and in a delayed fashion, as air temperatures change. Usually, soil temperatures at common root depth (20″) is about 20˚cooler than air temperatures. Most plants grow best in soil temperatures of 65˚ to 75˚F. In our area of CA in July and August, average soil temperatures can be 85˚F. Even in September, the average soil temperature is 80˚F or above.
  4. All of the above, plus wind (even fairly light but steady), is the worst kind of heat stress. Most plants don’t do much more than survive during this phase. Even cacti might turn to mush and collapse, as they cannot open their stomates to transpire and cool themselves.

Douglas: Symptoms of drought can develop on a wide range of deciduous and evergreen trees and shrubs. It can be especially severe on seedlings and new transplants because their roots occupy the uppermost layers of soil which dry rapidly. In addition, recent transplants typically lose important feeder roots during the transplant process. For example, balled and burlapped trees are estimated to contain only 5% to 20% of their original root mass after digging. For container-grown ornamentals, the growth medium can be a key factor—many soilless mixes used for container stock are highly porous, dry out very quickly, and are very difficult to rewet. This situation creates moisture stress in the rootball regardless of water availability in surrounding soil. This problem often continues until the roots grow beyond the rootball. Contrary to popular opinion, it often takes woody transplants two years to become completely established in a new site. Thus, these plants should be given extra care and attention during periods of drought.

ornamentals

A shrub dying of summer heat stress. (Photo: Canopy Lawn Care)

Combatting Heat & Drought

While there’s no cure, effects of summer stress can be minimized by the following:

Know Plant Tolerance.

Douglas: Select native plants or match species to conditions: Drought-sensitive (dogwood, ash, birch) versus drought-tolerant (most pines, junipers, and larch).

Ellas: Now that we’re planting “climate right” and “low-water-use” plants, they should look great all the time, right? All plants have a range of temperatures in which they thrive, based on the climate they evolved in. However, most plants have tolerance outside their preferred range. This is especially true of popular landscape plants. But tolerance is a range from barely surviving to looking okay, but not spectacular.

Throw Shade.

Douglas: Select the appropriate site and follow good planting practices: Drought stress can magnify even subtle improper planting practices (planting too deep, too shallow, etc). When planting a tree, try to anticipate water needs at maturity.

Hrivnak: Bushes and shrubs are all too commonly planted in the wrong location. Make sure you pay attention to which areas of your landscape receive the most sun. Which areas remain shaded all day? Which areas receive part sun and part shade?

Ellas: Once the weather cools, move the plant to a shadier location, or plant a screening tree or shrub on the south side of a prized plant. This doesn’t work for all plants, but the small reduction in sunlight and high temperatures might help plants that don’t thrive with summer watering.

Mulch Matters.

Douglas: Properly applied mulches can be very helpful to maintain soil moisture. Mulches are usually applied 1-3″ thick and spread evenly out to the drip line of the tree. It is also important to keep
the mulch 6-12″ away from the trunk. Mulches applied too thickly
or too close to the tree base (in a “volcano” or “pyramid” style) can be harmful.

Ellas: Use organic mulch to regulate soil temperatures, especially on cooler climate plants. Popular stone mulches like cobbles and decomposed granite can significantly raise air and soil temperatures around plants, or prevent temperatures from cooling off enough at night. Rock mulches are fine for desert-adapted species, but many succulents aren’t true desert plants and shrivel up in summer heat, even in shade, when surrounded by rock. Consider living ground covers, even vines, which act as a cooling mechanism.

Water Smart.

Douglas: Water in periods of low soil moisture. Trees and shrubs require approximately one inch of water per week. Special attention to young trees is important. For most soil types, water is best applied at one time as a slow, deep soaking of the entire root zone to a depth of approximately 12-18″. The length of time required to deep-water will vary depending on soil type and water pressure: clay requires more time than sand. Frequent, light, surface watering will not help and can actually cause harm by promoting surface root growth. A deep soaking just before fall ground freeze will also help.

Hrivnak: Be sure to water plants appropriately. Don’t overwater. Many ornamentals and shrubs are susceptible to root rot. Overwatering in an attempt to compensate for heat stress can damage root systems. A good rule of thumb is to water plants at the same time you’re watering the lawn, which is convenient for landscapers.

Ellas: Make sure soil is well hydrated, but not saturated, especially below 20″, in advance of a heat wave. Then avoid overwatering  the soil surface during the heat wave. In CA, if ceanothus, manzanita, sage, or buckwheat have entire brown branches, but others look healthy, it’s probably root rot. If the plant is losing leaves or drying out uniformly, another deep soaking may help. Also, hose plants off every three to five days during a heat wave, ideally in the evening or early morning when there’s no breeze. This temporary cooling can help a plant go from barely surviving to tolerating well. Plus it keeps mites and pests away. This is hand hosing and if no water hits the ground, that’s okay and even preferable for some plants. In CA, deep water plants and trees year round.

Pruning Prudence.

Douglas: Eventually you’ll want to prune dead or weakened tissues to avoid secondary problems.

Ellas: Prune off dead plant material after the heat wave, not during. Try to tolerate a little brown or wilted leaves while your plants are experiencing heat stress. Every cut is a wound, which is yet another added stress.

Feed Or Not?

Douglas: Maintain plant vigor by following good cultural practices: It is generally accepted that trees under stress should not be fertilized. However, applications of biostimulants, mycorrhizae, or similar compounds can be beneficial and can help to stimulate root growth and regeneration.

Dr. Douglas is an Emeritus Scientist with the Department of Plant Pathology and Ecology at the Connecticut Agricultural Experiment Station in New Haven, CT.

Ellas is the owner of Quercus Landscape Design, a design & build firm specializing in California native plant based landscapes in the Southern Sierra, southern Central Valley, and inland CA. Her book, Gardening with California Native Plants, was released in January of 2019.

Hrivnak is head agronomist with Canopy Lawn Care in Cary, NC. A graduate of North Carolina State University’s Agronomy and Turfgrass Management program, he has more than 20 years of  turfgrass management and knowledge. Hrivnak is a North Carolina Certified Turfgrass Professional, a Certified Sports Field Manager, and a board member on the Turfgrass Council of North Carolina.

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