Dogwood leaves are quick to show symptoms of drought stress with curling of the tips, flagging of the entire leaf, and dead leaf edges.
Because Kentucky had subnormal amounts and uneven distribution of rainfall during the last five to 10 years, you may be worried about your landscape plants. Having invested time and money in these plants, now you need to take protective measures to keep them alive.
Landscape plants probably suffer more from moisture-related problems than from any other cause. Because of water, plants experience feast or famine, flood or drought, air or suffocation. Plants are 70 to 90% water, which is essential for plant growth, manufacture of food, and nutrient transport.
However, too much water can be a problem. Excess water often causes a plant’s decline and death, because when water fills the soil’s pore spaces, roots can no longer get air, and they die.
During 1988 Kentucky experienced its sixth dry year. Starting in 1983, each year has had untimely, prolonged dry spells, so if you put plants in your landscape during the last 10 years, they have struggled to survive, and many have died. Also during these years, many old, well- established trees growing in shallow soils have died because they could not adapt to long periods without rain. Sensitive plants begin to have problems after three weeks without rain, and the past few years have included many such dry periods.
Plants generally fall into three categories relating to their capacity to get and conserve moisture (Harris 1983):
Many people plant trees in soil with undesirable characteristics, which affects how plants respond to drought or to too much watering during drought. To understand this plant response and how you can deal with it, you need to know a little about how plants get their water.
Plants get most of their water directly from soil surrounding their roots. The soil’s physical characteristics largely govern the amount of water that it can hold and the amount available for plant use. By the same token, these same physical soil properties govern how much air is in the soil surrounding plant roots. Both water and air exist in the pore space surrounding individual soil particles and aggregates. In an ideal situation, about half the volume of soil surrounding roots should be solid mineral or organic material and about half pore space. Ideally, about half the pore space should hold air and about half, water. Under such conditions, plant roots can get both the air and water they need. In contrast, when soil is waterlogged (most of its pores filled with water) roots cannot get enough air; when soil is dry (most of its pores filled with air) roots cannot get enough water.
The soil’s natural characteristics at any specific site govern how much pore space it has and how much water can be stored in pores. Some soils have a high available water-holding capacity (medium-textured soils like fine sandy loams, loams, silt loams, and silty clay loams), whereas others have lower capacity to provide plant-available water. Coarse-textured soils (sands, loamy sands) cannot store enough water to last longer than a few days after rain or irrigation, while fine-textured soils (silty clays, clays) hold much of their stored water so tightly that plants cannot extract it fast enough for optimum benefit.
When new houses or buildings are built and when the area around them is prepared for landscaping, many changes affect the soil. The naturally occurring physical relationship between the amount of solid material and pore space in soil is often modified. Most often, soil is compacted. That is, the amount of pore space is lowered because more solid material is compacted into the same volume of soil. Machinery traffic or even too much human traffic can cause compacting, and it takes place more easily when soil is moist or wet rather than dry. Because areas around foundations are often backfilled with compacted, clay-textured soils, or because topsoil is often scalped down into clayey subsoil, you often have no choice but to use such soils for your plantings.
To minimize soil structure breakdown and compaction, try these suggestions (Harris, 1983):
When plants do not get enough water, leaf scorch can occur. In this situation, leaves and tips of young shoots begin to wilt. If they are not turgid again by the next morning, lack of moisture may seriously affect the plant. First, leaf tips and margins can begin to brown; then this condition spreads into areas between veins. After that, oldest leaves on weak branches turn brown and begin to fall. Species most susceptible to leaf scorch are flowering dogwood, maple, horse chestnut, ash, elm, and beech.
Lack of moisture usually affects all the leaves on one or more branches. Leaves affected first and most severely are those exposed to afternoon sun and prevailing winds. Older leaves; leaves that are small, thick, and rigid; and most conifer leaves may not wilt visibly but may turn brown entirely or just at the tips or margins. When leaves drop as the plant tries to conserve water, reduced foliar cover per unit of ground surface area helps the plant conserve water further.
During 1975-1977, California suffered a severe drought, with annual rainfall below 10 inches. Most trees that were lost had shallow root systems or were growing on shallow or poor soils. Trees and shrubs planted in deep, well-drained, undisturbed soils that had well-developed, extensive root systems survived.
When an area has repeated dry periods, even if lack of rain occurs during different seasons, the long-term effects are substantial:
Plants can be injured or totally killed by low temperatures at any time of year, but especially in spring and autumn, in the coldest part of winter, and when low temperatures follow warm winter periods.
A. If rain is lacking for three weeks or more, and if you put in plants during the last 10 years, they need supplemental water. If the plant has been transplanted in the past 13-26 weeks, it will need water every five to 10 days.
Q. How often do I water?
A. If plants are well-established and in a well-drained soil, a thorough watering once every two weeks will usually keep them alive. If the soil is shallow or structurally poor, however, you need to water once every one to two weeks. Conditions conducive to water loss (day temperatures in the 90s, night temperatures above 70, reduced humidity, etc.) will require more frequent watering.
Q. How much do I water?
A. Try to put a minimum of 3/4 to 1 inch of water on at a time. Be sure not to overwater dense, clayey soil. Remember, after a clayey soil is wet, it doesn’t dry out nearly as fast as a medium or coarse-textured soil, so it doesn’t need watering as often.
Q. How do you know whether once a week or once a month is often enough for supplemental water on older, more established plants?
A. Let the plant tell you. Flagging or drooping leaves is one of the first symptoms of water stress. If this occurs late in the afternoon, don’t get alarmed—this is a normal happening. But if leaves are still flagged early the next morning, start watering the plant. The number of irrigations and the amount of water required during a dry summer depend on the soil’s water-holding capacity, the plant’s rooting depth, and environmental conditions.
These 10 plants showed widespread decline and death during the last 10 years in Kentucky:
Dwarf Alberta spruce
Some native plants in Kentucky have a good record of survival during the last 10 years. These plants have potential for poor landscape sites (shallow soils, compacted soils and other disturbed soils):
Southern red oak
Black jack oak
Black gum, Black tupelo
Harris, Richard W. Arboriculture: Care of Trees, Shrubs, and Vines in the Landscape. Prentice Hall, 1983.
Swan, J.B., J.F. Moncrief, W. B. Voorhees. Soil Compaction: Causes, Effects, and Control. University of Minnesota Extension Service Bulletin: Ag-BU-3115, 1987.