Arbor Business: Trees and Drought Stress - Dry & Thirsty

In the midst of the 2002 growing season, there are concerns about drought and the impact of potential restrictions on water use in landscape settings. The current drought follows several years of lower than normal precipitation that have lead to deficiencies of both surface and subsurface water. Also, drought stress symptoms have been evident on many trees during the past few years.

Even though we can’t control the weather, we can cope with drought and minimize its impact on trees.

ROOT DOWN. To manage drought stress, contractors must understand a tree’s root system, which has four types of roots: framework roots consisting of primary and secondary woody roots, transport and storage roots, nonwoody feeder roots and roots hairs. Almost 99 percent of this root mass is in the top 3 feet of the soil. The feeder roots and root hairs, which are in the top 12 inches of the soil, are responsible for water and nutrient uptake. Unfortunately, since they are sensitive to drying they are the first part of the root system drought affects.

When feeder roots and root hairs become nonfunctional, they can no longer provide sufficient water to the top of the plant. In addition to direct damage to the root system, drought triggers other factors like those that influence the number of leaves that will emerge the next year.

SPOTTING SYMPTOMS. Drought symptoms manifest in different ways depending on the plant species and water deficit severity. Symptoms often are not evident in the tree top until sometime after the event has occurred – even as much as one to two years later. These symptoms include loss of turgor in needles and leaves, wilting, yellowing, premature leaf or needle drop, bark cracks, and twig and branch dieback. Leaves on deciduous trees often develop a marginal scorch whereas needles on evergreens turn brown at the tips. Other symptoms include canopy thinning, poor growth and stunting. 

In addition to direct root damage, drought weakens plants and predisposes them to secondary invaders and opportunistic pests such as fungal tip blights, vascular wilts, root rots and needlecasts. Many drought-stressed trees also show increased sensitivity to deicing salts, air pollutants and pesticides. In short, drought threatens a tree’s health and, in extreme cases, can result in death.

PLANTING PROBLEMS. Native plants growing naturally in forested areas are usually adapted to regional and seasonal fluctuations in the amount of precipitation, and only unusually severe drought causes problems. However, planted landscape trees often show symptoms of drought and severe water stress. Planting practices are frequently key sources of drought problems since people often plant trees in unfavorable sites, don’t prepare the rootball properly or plant too deep or too shallow.

It is important to match the needs of the plant with what the site has to offer. This helps maximize plant vigor from the time of planting. Avoid spacing trees too closely since crowding increases competition for available water and promotes disease.

Proper preparation of the rootball is also critical to plant growth. With balled-and-burlapped material, the burlap should be removed or shredded and folded down. If a wire basket is present, it should be removed or the top 1/3 cut off. With a container grown woody plant, the rootball should be moist – not wet – and scored, cut and teased apart before planting. This is especially important if the root mass is very tight and dense.

Planting holes should be dug two to four times wider, but no deeper than the rootballs to be planted. In most cases, the soil dug from the planting hole with minimal amendments should be used as backfill. This method promotes faster root growth into the planting site since the soil is receptive to lateral root growth and minimizes the transitions between soil types as the roots grow into the planting hole and the planting site.

Remember, 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 percent of their original root mass after digging. For container-grown plants, 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 quickly and are difficult to rewet. This situation creates moisture stress in the rootball regardless of the availability of water in the surrounding soil. This problem often continues until the roots grow beyond the rootball and establish themselves.

Complete establishment in a new planting site often takes woody transplants two years. Thus, these plants should be given extra care and attention during periods of drought.

PREVENTION STRATEGIES. While there is no cure for drought, its effects can be minimized by following some preventive measures:

• Water in periods of soil moisture. Trees and shrubs require approximately 1 inch of water weekly. 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 to 18 inches. The length of time required to deep water will vary depending on soil type and water pressure – clay soils usually require more time than sandy soils. Frequent, light surface watering will not help the tree and can actually cause harm by promoting surface root growth. A deep soaking just before the ground freezes in the fall will also increase winter hardiness of drought-stressed plants.
• Select native plants or match plant species to site conditions. Be mindful of drought-sensitive trees (dogwood, some oaks, ash, birch) vs. drought-tolerant ones (most pines, many Prunus, larch) when selecting plants.
• Mulch to maintain soil moisture. Properly applied mulches are helpful in drought situations. Mulches are usually applied 1 to 3 inches thick and spread evenly out to the drip line of the tree. Keep the mulch 6 to 12 inches away from the trunk.
• Prune any dead or weakened tissues to avoid secondary problems.
• 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. – Sharon Douglas

The author is agricultural scientist, department of plant pathology, Connecticut Agricultural Experiment Station, New Haven, Conn., and can be reached at Sharon.Douglas@po.state.co.us.