Natural Resources Canada
www.nrcan.gc.ca
Common menu bar links
Drought
The past five summers were dry throughout much of Ontario. Let's use this column to consider what these dry conditions mean to your trees and shrubs. I'll try to explain in simple terms how to help your plants during a dry year.
It is important to know that 99% of the water taken up by a plant is lost through evaporation from small holes in the surface of the leaf. These holes or structures are called stomates.
Similar to the way that environmental conditions influence how your wet laundry dries hanging on a clothes line, they also influence how fast evaporation occurs in leaves.
Shade can greatly reduce the water needs of a plant by lowering the surrounding air temperature. The shaded side of a tree loses 25% less water than the sunny side.
A tree in the forest shaded by many neighbouring trees loses considerably less water than a tree standing in full sunlight and exposed to wind. Think of that single tree standing all alone in your front yard.
Humidity affects water use too. Remember the wet clothes hanging out to dry. We all know they dry more slowly on a humid or damp day. The humidity in a city where there are large areas of concrete and pavement is much lower than the humidity over turf grass or other living plants.
In dry air, water evaporates more easily and plants dry more quickly. Plants increase the humidity in their immediate environment by releasing more water from the holes in their leaves.
Wind greatly increases the rate of evaporation from leaves. Think about your clothes on the line one more time. Plants dry much faster on windy days than on dry days.
Temperature has dramatic effects on increasing water loss from plants. Obviously, a plant will lose water more quickly on a hot day. Water uptake and loss is slower on cool days.
Evergreens often show dieback or "burn" due to early spring drying. On a hot spring day, the top of the plant can become very warm, while the roots are locked in cold soil. The water rapidly lost by the needles, cannot be replaced by the cold roots and needle tissue dies due to desiccation or drying.
In nature, most plants avoid drought by growing only where there is adequate moisture to sustain their needs. Marsh plants are not found on uplands, and upland species are not found in deserts.
Trees have many ways of dealing with drought conditions. They can conserve available water or improve their water uptake from dry soil.
Water conservation by a tree can involve careful regulation of the holes, or stomates on their leaves. Because 99% of water leaving a plant goes through the stomates, rapid closure of them at first sign of drought is an effective means of conserving moisture.
One of the reasons green ash is more drought resistant than trembling aspen is because the ash closes its stomates more quickly in response to dry soil than the aspen does.
Small, thick leaves with a thick waxy covering enhance water conservation. Plants with thick leaves loose water more slowly than those with thinner leaves. Smaller leaves provide less surface area from which water can evaporate.
What can you do to help your trees and shrubs stay healthy during dry weather? Always choose those trees and shrubs which would be found naturally in the same environment as your yard or garden.
When watering, always water deeply and thoroughly. Roots will only grow where soil is moist. They will not extend into dry soil. Frequent shallow watering confines roots to the upper regions of the soil, leaving plants with shallow roots prone to rapid drying between waterings.
Since most of the "feeder" roots are located in the furthest extent of the root system, it is pointless to water the trunk of the tree. Water at the drip line of the tree and beyond.
Mulches will moderate soil temperature and enhance soil moisture retention. All mulches are not the same and you will want to consider which are best before applying them. The most effective are made from organic material such as tree bark.
Enhanced root growth is important. An increased level of phosphorus (2nd number in formulation on fertilizer bag) will stimulate healthy root growth.
Competition from aggressive shallow rooted plants with fibrous roots will reduce water availability to other plants. Turf grass, ground covers and weeds will rob valuable moisture from deeper rooted trees.
Concrete, pavement and large buildings not only increase air temperatures but limit water availability. Trees and shrubs in highly urbanized settings will lose more water than trees in natural park-like settings.
Chuck Davis is employed as a forest mycology technician at the Great Lakes Forestry Centre.
Direct inquiries to: glfcweb@NRCan.gc.ca.