Minimizing the Long-Term Effects of Drought on Trees and Shrubs
Plant Science Day 2002 Short TalkDr. Sharon Douglas
Department of Plant Pathology and Ecology
The Connecticut Agricultural Experiment Station
123 Huntington Street, P.O. Box 1106
New Haven, CT 06504
Symptoms of drought stress can be seen on many woody plants in landscapes, woodlots, and forests throughout the state. And as we entered the 2002 growing season, Connecticut residents had many questions and concerns about the drought situation. Although the official "drought advisory" for Connecticut was lifted by the Department of Environmental Protection in June, we still need to be concerned about the after-effects of the record breaking 18-month-old drought. Some of the questions we are fielding include "Should I plant new trees or shrubs? Should I fertilize my lawn and plants? What are symptoms of drought? What can I do to prevent damage to my plants"?
We can start today’s discussion by asking the question, what is drought? Drought is a meteorological and environmental event. It is defined as the absence of rainfall for a period of time long enough to cause depletion of soil moisture and damage to plants. And, although drought is a normal, recurring feature of our climate, it can result in significant problems with plant health.
In addition to the recent drought, we’ve also been experiencing abnormally low levels of precipitation here in CT since 1995– for example, 1995 and 1999 were extremely dry years, especially during the growing season. Dry weather during periods of active growth is particularly harmful to plants since water demands are the greatest at that time. Up-to-date information on precipitation levels measured here at Lockwood Farm can be found posted on the Experiment Station web site (https://www.ct.gov/caes).
To understand the impact of drought on woody plants, it is helpful to understand the water needs and relations in the plant. Without water, there is no plant life. Water is necessary for nearly all biological and biochemical processes within plant cells. Water is also necessary for uptake and transport of nutrients. For example, transpiration is the primary mechanism driving the movement of water from the soil, to the roots, to the xylem, to the leaves, and to the air.
Drought and dry soil conditions have physical and physiological effects on plants. Physical effects can be primary and secondary. The primary physical effects are damage and death of the roots. If ones examines the root system of a woody plant, one sees a hierarchy of roots. This includes framework roots, transport roots, storage roots, non-woody feeder roots, and root hairs. The feeder roots and root hairs are the most important part of the root system for uptake of water and nutrients from the soil. Unfortunately, they are also particularly sensitive to drying and are the first portion of the root system to be affected by drought. It is important to understand that 99% of the root system of a woody plant is located in the top three feet of the soil, and a good portion of functional root system (non-woody feeder roots and root hairs) is in the top 12 inches. When feeder roots and root hairs dry, shrivel, and become non-functional, a water deficit develops in the plant because these roots can no longer provide sufficient water to the top of the plant.
In addition to direct damage to the roots, a significant secondary effect of drought is that it weakens plants and predisposes them to secondary invaders and opportunistic pests. These include diseases such as cankers, vascular wilts, and root rots. Many drought-stressed plants also show increased sensitivity to de-icing salts, air pollutants, and pesticides.
Drought also has important physiological effects on plants. Drought triggers many changes in the metabolism which can substantially alter the physiology of a plant. These changes include levels and types of hormones produced by the plant and alterations of basic processes such as photosynthesis. Drought also influences factors that determine the number of leaves that will emerge the next year, flower production, and opening and closing of the stomates.
What are the symptoms of drought?
Symptoms of drought are quite variable and are manifest in many different ways depending on the plant species and the severity of the water deficit. However, it is generally agreed that the symptoms of drought are often not evident in the tree or shrub until sometime after the event has occurred-- even as much as one to two years later! As you can imagine, this can make diagnosis very difficult. As a consequence of this delay in symptom expression, symptoms of drought stress can be seen now, will continue to develop as this season progresses, and will be also be evident next year. Symptoms 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 .
What plants are affected?
Native plants growing naturally in woodlots or forested areas are usually adapted to regional and seasonal fluctuations in the amount of precipitation and only unusually severe drought causes problems for them. However, most of the plants that we are dealing with in our yards and gardens are landscape plants (not naturally occurring plants)-- those that we have selected and we have planted. As a consequence, they often show symptoms of drought and severe water stress. Why? We often plant in the wrong site, we don’t prepare the rootball properly, we plant too deep or too shallow, or we mulch too thickly.
Symptoms of drought can develop on a wide range of deciduous and evergreen trees and shrubs but they are particularly severe on seedlings and new transplants. This is because their roots occupy the uppermost layers of soil where the most rapid drying occurs. In addition, recent transplants typically lose important feeder roots during the transplant process. For example, balled and burlapped trees are estimated to contain only 5-15% of their original root mass after digging. For container-grown ornamentals, the medium in which the transplant is growing can be a key factor--many of the soilless mixes used for container stock are highly porous, dry out very quickly, and are very difficult to re-wet. This situation creates moisture stress in the rootball, regardless of the availability of water in the surrounding soil. Additionally, 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.
In addition to seedlings and new transplants, established trees and shrubs are also affected by drought, especially in marginal sites such as those with pavement over their roots, street trees, those in pockets of soil on ledges or in sandy soils, or those that just haven’t been planted right!. Once stressed by drought, these trees quickly decline and often die.
What are the strategies for minimizing the long-term effects of drought?
Since we can’t control the weather and there is no cure for this problem, there are some steps that we can take to cope with drought and minimize its impact on plants. Today’s discussion is geared towards woody plants but can also be helpful for any plants in the landscape or garden.
Most plants, including trees and shrubs, require approximately one inch of water per week. 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 8-10 inches. The length of time required to "deep-water" will vary depending on soil type and water pressure: for example, clay soils usually require more time to deep-water than sandy soils. Frequent, light, surface watering will not help plants and can actually cause harm by promoting growth of surface roots. If possible, it is helpful to use efficient systems for delivering water such as soaker hoses, directed sprays at the roots, and trickle irrigation. Overhead watering isn’t very efficient since substantial amounts of water are lost through misdirection and evaporation and it can promote disease problems. However, if it is necessary to use overhead irrigation, watering early in the day (early morning) helps to limit some unnecessary losses and minimize the risk of disease. Once again, new transplants will need special attention, especially during the first 2 years after planting.
2. Selecting the Plant and the Site:
It is important to match the needs of the plant with what the site has to offer as closely as possible since this helps to maximize the vigor of the plant from the time of planting. The old axiom "the right plant for the right site" is especially pertinent in periods of drought. Factors for consideration include the amount space for root development, amount of light, hardiness zone, drainage, exposure, and soil texture and pH. Select drought-tolerant plants, if possible. Correct choices at the time of planting can greatly reduce the need for supplementary water during the life of the plant. For example, if you know you have a very dry, sandy soil, avoid planting drought-sensitive species such as dogwood, hemlock, some oaks, ash, or birch and select more drought-tolerant species such as most pines, many Prunus or junipers. Avoid very dry sites with thin soil profiles or highly exposed sites where water loss from wind and sun can exacerbate a drought situation.
3. Planting Methods:
Use the correct spacing for each type of plant. Avoid crowding since crowded plants just don’t grow well and crowding also increases competition for available water.
Digging and preparing the planting hole:
The old saying "dig the $10 hole for the 50 cent plant" certainly rings true, especially during drought. The "old method" for planting called for digging the hole substantially deeper but no wider than rootball. It also called for the addition of many amendments to the soil. This method has been replaced by a "new method" based on what we now know about the way roots grow. This new method is applicable for both woody and herbaceous plants. With this method, the planting hole is dug 2-4 times wider but no deeper than the rootball to be planted. In most cases, the soil dug from the planting hole is used to backfill the hole and minimal amendments, if any, should be added. However, if the soil is substantially lacking in organic matter, additional organic matter can be added to help with soil moisture retention. This method eliminates settling over time (especially for large trees and shrubs), promotes faster root growth into the planting site since the soil is receptive to lateral root growth, and it minimizes the transitions between soil types as the roots grow from the rootball, into the planting hole, and into the planting site.
Preparing the rootball:
Whether you’re planting woody or herbaceous material, proper preparation of the rootball is critical to plant growth. Preparing the rootball is unfortunately often overlooked but it is particularly important during periods of drought. With balled and burlapped material, the burlap should be completely removed or at least shredded and folded down at the base of the rootball. If a wire basket is present, it should also be completely removed or the top 1/3 cut off. With container-grown woody and herbaceous plants, the rootball should be moist but not wet at the time of planting. It should also be scored, cut, and teased apart before planting. This is especially important if the root mass is very tight and dense.
Properly applied mulches help with soil moisture retention and have the added advantages of weed control and soil temperature moderation; mulches also help to minimize disease spread by splashing. Organic mulches include materials such as shredded bark, wood chips, cocoa hulls, and lawn clippings that have not been treated with herbicide. The soil should be moist when you apply the mulch—if it is dry, you should water before putting down the mulch. In order to be effective, mulches need to be applied correctly. Mulches that are applied too thick or too close to the base of the plant ("volcano" or "pyramid" mulches) can be harmful. Improperly applied mulches cause many problems: when applied too thickly, the mulch impedes water penetration and smothers the roots. When applied too close to the stem, the mulch creates conditions favorable for the development of stem and crown rots. The rule of thumb for mulching: mulches should be applied approximately 1 inch from the base of herbaceous plants and 6-12 inches from the base of woody plants. The thickness depends upon the coarseness of the mulch. For example, fine shredded bark ~ 1 inch, coarse shredded bark ~ 2 inches, pea gravel ~ 3 inches, and bark nuggets ~ 4-6 inches.
5. Maintaining plant vigor by using sound cultural practices:
It is generally accepted that woody plants 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. Maintain the health of the plants by removing any dead, damaged, or dying branches—this will help to minimize problems with secondary invaders and opportunistic pests.
Although the drought has certainly affected many woody plants throughout Connecticut, it is important to realize that there are steps we can take to minimize its long-term impact on plant health.
If you have trees, shrubs, or plants with problems and you are uncertain as to the cause, my office, The Plant Disease Information Office in New Haven, can assist you with diagnosis and with outlining strategies to deal with the problem.
(A number of fact sheets with more detailed information on this topic are available upon request or on the CAES web site.)