1. Insufficient rain (drought), or irrigation
Drought, quite obviously, can be caused by less than normal rainfall, but it can also be caused by hot or cold temperature extremes where the top several inches (cm) of soil may freeze, and roots may be unable to take up water. Reduced water uptake in the winter can make trees, in particular evergreens, more vulnerable to drying out.
Be sure to water in the winter if the ground is dry, including soil in containers and raised planters.
2. Not enough soil volume in the planting area
Most trees, and especially urban trees, and trees in containers, become stressed when they have a disproportionately small volume of soil for their roots. Reduced soil volume leads to soil that dries out rapidly as the root system absorbs moisture, often causing a chronic water deficit for the trees.
General signs of water stress include:
- Reduced growth
- Poor flower or fruit production
- Limp, slightly curled or scorched leaves
- Abnormal changes in leaf color
- Various soil texture characteristics
Sandy soils tend to drain rapidly making the soil drier than silt loam soils. Clayey soils tend to have dry to very wet moisture extremes depending on rainfall amounts. Depending upon your climate and soil type, you can have very dry soil conditions.
How Trees Adapt to Lack of Water
To survive prolonged water stress, trees must be able to prevent or reduce water loss from certain tissues. Common leaf adaptations include:
1. Thickened waxy layers on leaf surfaces, increases in leaf thickness, and coverings of short hairs (pubescence).
2. Some trees reduce moisture loss by closing their leaf pores (stomates) or decreasing leaf surface or size of new leaves, both of which decrease the amount of water loss.
3. Narrow or spiky leaves of conifers enable them to survive not only the droughts of hot summers but also the cold induced droughts of winter.
4. An extreme form of dealing with water deficits is leaf drop.
5. Some trees adapt to dry conditions by developing massive, spreading roots, or deep roots, either of which can enable the tree to absorb larger volumes of water.
Avoid Soil compaction
Soil compaction over a tree’s roots, due to equipment operation, material storage, or paving, can prevent moisture from reaching roots.
To improve the soil and its water retention, incorporate organic matter into the top 6 to 9 inches (15 to 23 cm) of soil of an entire planting site at a rate of 3 cubic yards (2.3 cubic meters) of compost or pine bark per 1,000 square feet (93 square meters).
In sandy soils increase the rate to 5 cubic yards (4 cubic meters) per 1,000 square feet (93 square meters).
Note: Do not incorporate sand into clayey or compacted soil — compaction will increase and drainage decrease.
Drying winds can seriously increase the effects of drought, and in fact, wind can damage trees suffering from lack of moisture at any season.
In coastal regions, winds carry both salt and sand, causing abrasions to leaves and stems as well as drying. In exposed coastal gardens, trees tolerant of both wind and salt spray include:
- Russian olive
- White poplar
- Eastern redcedar
- Japanese black pines
Six Dry Site Solutions
1. Install efficient irrigation – Drip irrigation provides more efficient water use than overhead sprinklers. Drip irrigation ranges from inexpensive “soaker” hoses to computerized systems.
2. Reduce fertilizer use during droughts – Fertilizers are salts that can further stress or kill trees if water is inadequate. Salt concentrations can build up in the soils due to decreased water availability. Even controlled release fertilizers may release much more rapidly during hot weather, causing excessive salt levels.
3. Apply mulches – Application of mulch can improve the water retention of trees. Mulches also reduce the impact of water droplets hitting the soil surface, which help reduce soil erosion and crusting which increases water penetration into the soil.
Mulches should be applied 2 to 4 inches (5 to 10 cm) deep depending on particle size (larger particles require a thicker layer ).
4. Water in early morning or evening – Don’t water in the middle of the day when evaporation losses are highest. Instead, make sure to water in the early morning or in the evening when trees are less stressed and water uptake is more efficient. Avoid light sprinkling — slowly soak the root zone if you water with a hose.
5. Plant trees in groups – instead of individually to increase the amount of unpaved surface around each tree.
6. Select trees tolerant of dry sites – See our charts below for drought tolerant trees that are good for dry areas:
Authors: Imran Ramzan, Arslan Shehroz1, Muhamamd Zunair Latif2.
Author * Department of Agronomy, University of Agriculture Faisalabad
1&2 Co-author Department of Plant pathology, University of Agriculture Faisalabad