Over-watering can kill healthy plants PDF Print E-mail
Written by KATE COPSEY Master gardner, columnist   
Thursday, 05 July 2012 08:59
These two photos show the same hydrangea. In the top photo the leaves are down-turned, which is a frequent sign of lack of water. However the bottom photo shows the same plant later in the evening without any water added. Many gardeners mistakenly water when water is not needed. (Photos by Kate Copsey)
Without a doubt, even with the weekend storms, it has been hot and dry out there and both field crops and your garden plants are showing some stress from lack of water. However, more garden plants and shrubs are killed by well-intentioned homeowners over-watering them, than are killed by under watering them.
The problem stems from the homeowner misreading the signs.
When a plant or shrub is in need of water, the leaves turn downward and curl up. In situations such as with houseplants, this is a clear sign that you need to give the plant some water. In very hot weather though a similar appearance is not a sign of dehydration but rather a natural conservation method from the plant.
The underside of the plant leaf is filled with small openings called stomata, which allow the movement of water and gases to and from the plant's system.
The stomata are located on the underside of the leaf. When the temperature soars and water is limited, the stoma closes to preserve water and prevent the plant from dehydration. 
When all the stomata close together it shrinks the lower surface area slightly making the upper layer curl downward. This happens with all plants on hot days but is particularly noticeable with large leaf shrubs, such as hydrangeas and rhododendrons. When the sun goes down in the evening, the stomata open again and the natural leaf contour returns to normal. 
When a homeowner sees the wilting plants, and waters it, the plant still looks unhappy and wilted. Continual attempts to make the plant leaves return to normal during the day eventually cuts off the air pockets in the root zone which is crucial to the plants ability to draw in nutrients and the plant dies from too much water.
So a good rule of thumb is to ignore the plants during the day, and wait until evening, or first thing in the morning, to observe your plants.
If they look wilted at that time, you need to water. If the shrub looks normal, then the plant has sufficient moisture to survive happily for another few days.
Other signs that the shrub or tree is dehydrating though include leaves that are turning yellow or getting brown on the edges. These signs can be remedied by running a very slow hose for an hour or two and letting the water seep into the root zone.
Remove the hose after a few hours and let the shrub or tree absorb the water for a few days before assessing it, and deciding if more water needed. Note that yellow leaves may drop anyway, but new ones will start growing, so don't water every day expecting the yellow leaf to return to green again.
All plants and shrubs, including grass, need about an inch of water each week to thrive, but they can do very well on much less than that. When the hot midday sun burns in a perfect blue sky and the heat builds, the plants as well as humans start to look wilted but they both recover when the cooler evening temperatures arrive.
Last Updated on Thursday, 05 July 2012 09:44

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