Drought and the Landscape
Drought stress or water deficiency remains both a complex and confusing issue to many people. Short-term or seasonal droughts may occur in one season and result in scorched leaves, daily wilts and recovery in the evening, slowed growth, and leaf loss. These seasonal droughts may be alleviated with irrigation and by adequate rainfall in the later part of the season. Long-term (multiple year droughts) can become devastating with greater plant stress, increased disease and insect susceptibility, loss of branches, and death of the fine tender roots that the tree relies on for mining water. Our confusion as growers come from understanding the symptoms of drought stress, mechanisms for managing moisture stress, planning to avoid stress, and recognizing that brief rainfalls or down pours do not reduce the potential drought.
Most often, our response to drought does not occur until we see signs of drought stress. Then we consider what we should do. In order to reduce that tendency the following describes some of the signs and symptoms of drought and some methods for controlling or reducing drought stress.
Progressive water stress symptoms in plants include:
Leaf scorch: Leaf tissue away from the main veins browns and dies due to loss of large amounts of moisture without replacement by the roots due to low soil moisture on deciduous plants. This may occur due to lack of available moisture and due to reflective hot surfaces such as trees planted in inadequately small planting pits of parking lots. Scorch may be confused with anthracnose; however, moisture stress induced necrosis rarely crosses over leaf veins on the inner section of the leaf area. Leaf scorch tends to be most severe in the upper branches of the tree or shrub in contrast to anthracnose that is evident in the lower branches. Maples and dogwood readily show leaf scorch symptoms. Needle tip die back is a common symptom to moisture stress in conifers.
Interveinal necrosis: The leaf tissue surrounding major veins remains green but the tissues between the veins turn brown. This can be confused with nutrient deficiency, specifically micronutrients such as iron. Knowing the pH and soil fertility in your planting beds combined with keeping track of rainfall rates can help you to differentiate between the two issues.
Midsummer defoliation (leaf drop): This is commonly preceded by scorch and necrosis mentioned above. Defoliation will begin at the top of the tree and move downward. Other issues that may be confused with drought stress induced leaf drop are verticillium wilt and girdling root. Although both may affect water uptake, the reasons are clearly different. Both of these problems will be manifested in trees if water stress is present. Being aware of verticillium wilt will help you to determine if this soil borne disease is a problem. Investigating the root flare may help you in determining if girdling root is a problem on your tree.
Unabscised dead leaves remaining on the tree: Oaks and other deciduous trees may show complete browning of foliage and the foliage remains attached. If the leaf loss occurs too rapidly for the abscission layer to form, the tree will remain in full leaf but brown.
Progressively small leaves: Plants under drought will show progressively smaller fully expanded leaves compared to a non-stressed tree.
Realities of drought stress symptoms:
1) In a single year, moisture stress symptoms may not appear until late in the summer after extensive hot and dry windy weather.
2) Extended drought stress (more than one season) can result in crown decline, twig die back, small branch die back in the upper crown and progressively larger branches can succumb or are vulnerable to breakout under strong wind conditions. Suckering may occur on the trunk and upper branches of heavily stressed trees, cambium death and cankers may also occur resulting in the girdling of the tree and total tree death. Often cankers may be the direct result of moisture stress or may occur with the development of disease which produce cankers while the tree is severely stressed and susceptible. Another symptom of extended drought stress is heavy seed loads the year following the drought.
3) Often the symptoms of drought stress are delayed. Water deficiency may cause extensive root injury in the late summer and fall. The current year's foliage may not reveal any symptoms. Conifers are an example of a plant that by the time it expresses symptoms of stress the plant is already in dangerously poor health. In sum, the symptoms and effects of the drought may not appear until the following year when rainfall is normal.
Tips for avoiding water stress situations:
1. Keep track of rainfall amounts at your location
2. Supplement with an efficient watering system such as drip irrigation
3. Scout your plants for signs of water stress and use indicator plants to assist you in measuring need for supplemental watering
4. Good indicator plants include Viburnum tomentosum var. plicatum 'Doublefile Viburnum', Azalea, Cornus sp. (Dogwood), Forsythia, Acer palmatum (Japanese maple), Cercis canadensis (Redbud), Hydrangea sp., annuals, and herbaceous perennials.
5. Keep an eye on trees near normally wet areas (streams, lakes, low areas). Once their access for water has been diminished, these plants will have a great susceptibility for damage because their root systems are not sufficiently developed for mining water outside of their root zone.
6. In the landscape, consider designing with water use in mind and target non-irrigated areas with drought tolerant species. Design with plants that need regular irrigation closer to the house, moderate water requiring plants further from the house, and drought tolerant plants away from the house.
7. Mulch landscape beds to maintain moisture with 2 to 3 inches of well-composted organic matter. Do not allow the mulch to directly contact the trunks of trees, shrubs, or herbaceous plants.
8. Designing group plantings can provide for greater water management. Beware of over-planting; over-planting can place further stress on the soil moisture available to the plantings.
Tips for watering in the landscape:
Immediately after planting, trees should be provided with 1-2 gallons/inch trunk caliper through a slow, soaking system such as drip irrigation, irrigator tree watering bags placed close to the flare or collar of the tree. Watering approximately 2-3 times per week for at least the first 8-10 weeks after planting should be sufficient. Shrubs and smaller container trees (1-3 gallon pot size) require 3-5 gallons of water 2 times a week. These rates should be modified based on the amount and frequency of your rainfall. Note that drought tolerant plants or low water use plants are effective once they are established. Newly planted plants are NOT immediately drought tolerant. They must be allowed to establish and require supplemental water during their establishment process.
After the initial planting and immediate establishment process, the frequency can be reduced to once a week and the area of water coverage should be increased to assure that the growing edge of the root ball is receiving moisture (water beyond the drip line). A rate of 1 gallon/square foot of soil surface area within the root zone of the plant should be covered. Watering should be slow assuring moisture reaching a depth of 8 inches. Do not allow runoff. If runoff is visible then reduce the rate of watering and keep track of the amount of water going onto the area. More is not better in this case. Trickle irrigation systems are the most efficient watering method. You must keep track of time and not forget you are irrigating.
It may take two to three years for a woody plant to establish in the landscape. This means that during dry periods supplemental watering may be necessary. Keeping track of rainfall will help you in determining if additional water is necessary.
Of course, the primary problem with drought is the water shortage, usually from low rainfall. Complicating this even further are sprinkler bans and/or voluntary water conservation programs. The establishment of a watering priority management program can minimize drought effects on your landscape plants.
Priority #1: Newly Planted Trees, Shrubs, Lawns. Newly planted trees and shrubs and newly sodded or seeded lawns should receive most of the supplemental water during drought conditions. Because these plants have not had sufficient time to develop deep root systems and adapt to their new surroundings, they depend on surface water for their survival. They can also be quite expensive to replace, so they should receive top priority. Mulch applied around the bases of shrubs and trees keeps the soil moist by preventing rapid evaporation.
Priority #2: Young Trees and Shrubs. Because young trees and shrubs are not as well established as older trees and shrubs, they will require more water to survive. Typically trees and shrubs planted within the past two years will require irrigation during drought, thought not as much as newly planted trees and shrubs. During severe drought conditions, plantings up to five or six years of age may also require supplemental water.
Priority #3: Flowers, Gardens and Older Plants. Because of the relatively low cost of establishing gardens and flowerbeds, these should receive less care. Mulching helps retain moisture in the soil around the roots. Certain old trees and shrubs are more susceptible to drought than others. Birch and dogwood, for instance, tend to be very susceptible to drought and will require more water. (Drought-resistant varieties are discussed elsewhere in this publication.)
Priority #4: Lawns
Older, well-established lawns can tolerate long periods of drought by becoming dormant. Dormant lawns can be damaged by foot traffic. During drought conditions, lawns should be watered regularly or allowed to go dormant. Irregular irrigation that caused lawns to repeatedly enter into and recover from dormancy can severely weaken them.
|Large Trees 30+ft at maturity
|| Celtis occidentalis
|Tulip Poplar/Yellow Poplar
|Small Trees (10-30 feet)
|| Acer buergeranum
||Magnolia x soulangiana
|| Pinus species
A Representative List of Landscape Plants in Size Categories that Require Limited Supplemental Irrigation Once Established (Not a complete list)
|Large Shrubs (8 feet and up)
|Rose of Sharon
||Juniperus chinensis (several cultivars)
|Medium Shrubs (5-8 feet)
|| Abelia x grandiflora
|Small Shrubs (2-5 feet)
|| Deutzia gracilis
|Aaronsbeard St. Johnswort
|Blue rug juniper
|| Liriope spicata
|Indicator Plants (Plants that tell you that the soil is dry)
||Viburnum tomentosum var. plicatum