By Island County Extension Agent, Donald B. Meehan
WATERING GARDENS & LANDSCAPE PLANTS
Each summer and fall my office gets its share of plant problems caused by insufficient watering. The symptoms are easy to determine: wilting, brown tips on evergreens, leaf scorch on Rhododendrons, and excessive needle drop on conifers, to name a few.
The problem usually arises from a perception about living in Western Washington that it rains all the time. Fortunately, it doesn't rain all the time and even when it seems to, the quantity we get can be quite low. This winter season was a good example. It followed an exceptionally dry summer period last year. This summer has also been very hot and dry. Summers in Western Washington typically call for extra watering of landscape plants.
Water loss takes place in a variety of ways even when there is adequate moisture. It can percolate past the root zone, evaporate from the soil surface, and it can be lost through leaf transpiration from the plant. Water which evaporates from the surface will be replaced by water deeper in the soil through capillary action. This, too, will eventually evaporate and can eventually lead to deep soil water loss. Large trees can lose hundreds of gallons of water during a hot summer day from transpiration. The transpiration process allows the tree to cool itself and is linked to the role of photosynthesis.
Too much water on a plant will prevent roots from getting adequate oxygen. This causes the roots to die and symptoms such as wilting, yellow and drying foliage, leaf drop, and twig dieback can appear. All are symptoms of lack of water as well.
When watering plants you want to fill the entire root zone with water. Then allow it to partially dry out before the next soaking. How dry will be somewhat dependent on the type of plant and its size. Large trees and shrubs can be allowed to dry several inches down but small or newly established plants will require more water before much drying occurs.
Getting good water penetration will be dependent on the soil type. Sandy soils will get penetration down to 12 inches with one inch of water applied. Clay soils, on the other hand, will only see penetration down 4-5 inches. Soils which have organic matter incorporated will hold more water and often will accept water more readily.
When applying water it is important to do it slowly so run-off isn't created. A very dry clay-loam can take as long as 120 hours to become completely wet to 12 inches. Whereas a sandy loam may only take 4 hours. Having a trowel or shovel handy to check the soil at the expected root depth is handy.
I think it may be worthwhile to address some of the common problems with watering. Once soil becomes very dry it is harder to re-wet. That is why containerized plants that completely dry out need to be soaked. Once the soil becomes completely dry, roots attempting to grow in it begin to die.
Shrubs and trees planted near house foundations, under eaves and are in southern or western sun exposures will need more water. This comes about for two reasons. First the south and west exposures are hotter in the summer. Plants transpire more and use more water. Temperature-sensitive plants,such as many of the Rhododendron hybrids, need a cooler home. If you can't live without a Rhody in those locations, find a variety which can stand the heat. Plantings along house foundations often find less water due to rain gutters on the eaves and often the presence of foundation drain tiles below the soil surface designed to keep the area free of soil moisture buildup.
Mounds or berms in landscapes create a water holding problem. Since they have more soil exposed to the air a greater degree of evaporation takes place. Mulching can be very beneficial since it reduces the evaporation rate and often will reduce run-off.
Many native woody plants should not receive summer water. They are drought tolerant once they have become established. As a rule of thumb, we don't consider a landscape plant completely established until three seasons have passed. It is especially important to keep water away from the crowns and larger roots of Madronas and Western Dogwoods. Summer watering of these species can lead to root rot problems.
Lawns are a unique problem when it comes to watering. The deeper the wetting, the deeper the roots will grow. People who give their lawns frequent, light sprinklings will have shallow grass roots. This will be all right as long as the frequent sprinklings continue. We encourage people to deep-water lawns to avoid or reduce summer stress and to reduce the incidence of fungal diseases and moss build-up.
Thatch accumulation on lawns can prevent rapid penetration of water. Overhead sprinkling, spread out over a longer period, but applying the same amount of water will help to overcome this problem. Aeration of the lawn by removing plugs will help a great deal, as well.
You can generally tell when you have applied enough water to your lawn by noticing how it feels when you walk on it. When it is dry it will tend to be a bit springy. This feel will disappear when adequate water has penetrated the soil. Needless to say, you still need to dig down once in awhile with a shovel to see what kind of penetration you are really getting.
There are many points I have not covered here which you may find useful. If you want more information , contact our Cooperative Extension Office for a publication called "Watering Home Gardens and Landscape Plants", available for a small fee. We will be glad to mail it to you.
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