Proper management of water resources in gardens and landscapes depends upon understanding how plants regulate their water use and retention. This process is intimately connected with photosynthesis, in which plants use the energy in sunlight to combine the carbon in atmospheric carbon dioxide (CO2) with hydrogen from water (H2O) to make both energy and the carbon compounds of life.
Plants carry on photosynthesis during the daylight hours, which means they must take up large quantities of water. This is because water is their source of hydrogen, but also because the way they take up carbon dioxide results in the loss of water through a special form of evaporation called “transpiration.”
How A Plant Drinks
Consider the simplest of plants: A single leaf, a stem and a root. The root terminates in tiny “root hairs,” which are capable of taking up both soil water and the minerals dissolved in that water. Special tubular conductive tissues run from the root hairs through the root and stem to the leaf. “Xylem” is the water-transport tissue that carries water and minerals only from the root to the leaf. “Phloem” is a transport tissue that carries the carbon compounds of life from the leaf to all parts of the plant. Viewed through a microscope, Xylem looks like a simple tube, while Phloem retains its cellular structure. The leaf itself has two special tissues: A spongy tissue called “Parenchyma” and special pores called “stomata.”
Photosynthesis takes place in the Parenchyma cells. Embedded in the Parenchyma, usually on the underside of the leaf, are the stomata. A single stoma consists of a cavity opening to the atmosphere and two “guard cells.” When the guard cells close, the stoma is protected from the atmosphere. When the guard cells open, the stomata is open to the atmosphere and both air and water vapor can diffuse into and out of the cavity. There is no special mechanical equivalent to “breathing.” Air, as well as water vapor, simply diffuses into the stomata past the open guard cells or diffuses out of the open stomata.
The diffusion of water vapor into and out of the stomata is called “transpiration.” This loss of water vapor through transpiration is why plants need large quantities of water during the daylight hours. The plant has to take up both the water needed for photosynthesis and the water lost through transpiration while the stomata are open.
Watering at night?
Many recommend watering at night, because they believe it saves water, as evaporation from the wet soil is less and because they recognize the plant is less physiologically active. It is true that less water is lost through transpiration at night, because the guard cells are closed, and because less water is needed when photosynthesis quits at night. It is also true that evaporation of water poured onto the surface of the soil is somewhat less at night because it is cooler.
The problem is that wetting the surface of the soil at night usually provides insufficient water to supply the amount which will be lost through transpiration during the day and the additional water requirement needed to supply the hydrogen for photosynthesis.
A better way to conserve water is to supply exactly the right amount of water directly to the roots during the day.
The How-Tos
On a large scale, this is accomplished by drip irrigation – provided the equipment and piping employed meet a standard of manufacture sufficient to prevent water-loss through leaks and failed piping. Such equipment is not found in a hardware store. It is expensive, although the water-saving payout is offset by the reduction in water required.
On a small scale, as in a garden or patio or with large pots of plants, this is usually impractical. When I was involved in testing products we developed through agricultural biotech, we built our own very sturdy copper-line drip irrigation system in the test greenhouse. We used a water flow meter to accurately measure the amount of water used. This was far cheaper than having employees trot around with watering cans.
More useful for small gardens or large pots is a simple length of one-inch plastic pipe, closed at each end with a cap. At the bottom end tiny holes are drilled above and through the cap. Bury this pipe in the soil to the depth of the root ball, so it slowly leaks water directly to the roots and sticks up above the soil to around eight inches. At the top end, simply remove the cap and pour in water to the top once per day, then replace the cap. For a three-foot diameter pot three-feet tall, about one pint of water per day is usually sufficient during most of the year (there are eight pints in one gallon).
A catchment basin should be provided to capture run-off from accidental over-watering. The goal is to adjust the water amount to minimize water in the catchment basin. With this system, one can water at any time, although mid-morning is best. If, during August, you notice the leaves drooping slightly during the afternoon, increase the water by pint increments for the dry period.
This article first appeared in the June 15, 2016 issue of the Rossmoor News, author Wayne Lanier.