Plants Have Ambivalent Attitude Towards Water

Plants have an ambivalent attitude toward water. They need water to grow, yet they will die if water is prevented from draining past their roots.
Roots will grow through rock to find water, yet plants use less than 10 percent of the water they extract from the soil.
More than 90 percent of the water absorbed – 98 percent in a corn plant – moves from root to stem to leaf, where it escapes into the atmosphere through minute pores called stomata. This process of water loss in plants is called transpiration, a kind of botanical sweating.
To illustrate plants’ inefficiency of water use, consider this: In relation to their size, sunflowers take up and transpire 17 times more water than human beings. The water we imbibe is recirculated many times throughout our bodies; with plants, it’s in one end (the root) and out the other (the leaf).
In the landscape, people often are not much better than plants at using water efficiently.
Sprinklers are customarily allowed to run two or three times longer than is useful. For instance, a clay or compacted soil, which is typically found under Los Angeles lawns, can only absorb 0.2 inches of water in an hour.
Conventional spray sprinklers – the kind most frequently encountered here – deliver 0.2 inches of water in about five minutes. No matter how long you leave the sprinklers on, the soil can absorb no more than 0.2 inches of water; after just five minutes, puddling and runoff occur.
Lawn grasses need 2 inches of water per week during the summer. This means that 10 sprinkler applications (0.2 x 10 2) per week, at five minutes each, are necessary to properly water a lawn. A simple way of doing this would be to water twice in the morning – say, at 5 and 9 a.m. – five days a week.
With rotary sprinklers, where water is emitted by stream(s), the rate of application is slower, and sprinklers may be left on for up to 40 minutes at a time. Still, you will have to water 10 times a week.
The problem here is that an almost daily watering schedule, where the soil surface is kept moist, encourages superficial root growth. If you forget to water once or twice, the grass – without deep roots to seek water from below – could wilt or even die.
The solution is to decompact or soften the soil so that it can absorb and store more water with each irrigation. As a result, roots will grow deeper and watering frequency can be reduced. Soften the soil with monthly doses of compost, with quarterly applications of gypsum, and with mechanical aeration three times a year.
In planter beds, the situation changes. Properly sited and mulched, virtually all annuals and perennials will grow fine if they are slowly soaked with a hose once a week. It is important to remember sprinklers were invented for the convenience of human beings, not for the benefit of plants. The problem with sprinklers is that they cannot possibly distribute water evenly to all parts of a plant’s root zone. In a planter or flower bed, foliage gets in the way of the water being applied. In a mature landscape, plants may block sprinkler spray altogether. If you must use irrigation apparatus in your planters, flood bubblers or soaker hoses will probably give you the best results.
Early morning is the best time to water for two reasons: Humidity is high and wind is low. More water is lost through transpiration when the air adjacent to leaf surfaces is dry. The humidity inside the leaf is 100 percent; the air on the outside acts like a blotter, absorbing interior leaf moisture when stomata are open. The lower the humidity of the air, the faster leaf moisture will be sucked out. Wind lowers humidity. Normally, a layer of water vapor, lost through transpiration, surrounds the leaf. Wind blows away this insulating layer, drying the air that now pulls yet more water from the leaf.
If a plant is stressed from lack of water, it should be watered immediately, no matter what the time of day. There is a widespread belief that watering in the heat of the day causes leaf burn. With the notable exception of ivy, virtually all landscape plants, as well as lawn grasses, will not be damaged by mid-day watering. In certain avocado groves, in fact, enormous overhead sprinklers have been installed to cool off trees on scorching hot days. In propagation nurseries, shoot cuttings are rooted under mist that comes on at regular intervals throughout the day, leaves are kept constantly moist until roots are formed.
Perhaps, some day, micro-misting systems will be available that, using a minimum amount of water, could keep landscape plants constantly cool and significantly reduce water loss from transpiration.

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