Plants use water in three main ways: to maintain their turgidity, to transport water-soluble nutrients and to photosynthesize. A turgid plant is erect, not limp. When plant cells have enough water, they maintain their shape. When cells lack water, they shrink and the plant wilts, or loses turgidity. By the time a plant wilts, many plant processes have stopped and the plant will take some time to recover.
For photosynthesis, plants have small pores in their leaves called stomata. These pores open to exchange gases: carbon dioxide in and oxygen out. As the oxygen goes out so does water vapor. This evaporation cools the leaves but also draws in replacement water from the roots—like sucking water through a straw.
When the water enters the roots, it carries soluble nutrients along with it—compounds that are in solution in the soil, such as fertilizers
Water also assists photosynthesis, the process of making food for the plant. Water combines with carbon dioxide to form sugars and gives off oxygen and a little water, too. If you have ever lain under a tree on a warm day, you may have felt moisture dripping from the canopy. That's some of the excess water, and this process partly explains why shade trees have such a welcome cooling effect.
When a plant does not have enough water to fill all these needs,it has to adapt.
A water-stressed plant goes through a series of steps to conserve its resources. One of the first thingsit does is to change the chemical content of its cells. This action thickens the content of the cell—like putting antifreeze in a car—so the cell is less likely to lose water and the plant retains more turgidity.
This more concentrated cell solution also helps the roots increase their water uptake, if the soil has any water. Water-stressed plants will also close their stomata: no gas exchange and thus no water loss.
For most plants, closing the stomata also shuts down photosynthesis. With no new food being made, plants use up their reserves. Many plants will move food resources from non-essential functions like growth and fruit production to essential processes that keep them alive. Root activity stops and cells stop growing and multiplying. Plants initiate senescence—the onset of old age—and foliage, branches and roots begin to die. Next, plants prepare themselves to drop these dead parts. First, they drop any fruit, then leaves and eventually branches. At some point, the whole plant may die.
The effect these responses have on the plant depends on its stage of growth when the stress occurs. In the early spring, plants are growing roots and pushing out new buds; mild stress can stunt this growth. Under-developed root systems can cause problems later, if plants do not have sufficient roots to take up water and nutrients in summer. Poor bud development in spring can restrict growth during the rest of the year and sometimes into the following year as well.
If the water stress occurs while the plant is flowering, flower buds may drop, open poorly or not develop into fruit. Once the flowers have been pollinated and the fruit begins to develop,water stress can stop cell division and expansion. The result is very small fruit or fruit that will not ripen. Peaches and other stone fruits that are stressed during seed development may form split pits that can cause fruit to split open and spoil.
You can reduce water use and still keep plants alive, but they may not be as lush or productive as in past years. The key to helping plants survive with less water is good observation skills. Many plants will manageif you slowly reduce the amount or frequency of water you provide.
Know your soil type and build healthy soil. Healthy soil holds more water. Mulching keeps roots cool and reducessoil evaporation. Be sure to keep mulch a few inches away from the base of plants and trees.
Change your watering regimen to adjust for day length, temperature and wind. Day length is important because plants will actively photosynthesize as long as the sun is out. Plants need more water during hot, windy, sunny days in the middle of summer when days are long.
Knowing how drought-tolerant your plants are is also important. Plants that have evolved where summers are dry will usually tolerate more stress or may have means of adapting toa dry season. If you must let some plants die, consider replacement costs and value to your landscape. Keep your trees and big shrubs alive and let the little guys go.
Workshop: Napa County Master Gardeners are conducting a workshop on “Structures in the Garden” on Saturday, June 7, from 9:30 a.m. to 11:30 a.m. Do the beautiful flowers in your garden need extra support? At this workshop you will learn how to make your own support systems for the garden. Workshop location is the Master Gardener Demonstration Garden,
Connolly Ranch Education Center, 3141 Browns Valley Road in Napa. Online registration (credit card only); Mail in registration (cash or check only).
Master Gardeners are volunteers who help the University of California reach the gardening public with home gardening information. Napa County Master Gardeners ( http://ucanr.org/ucmgnapa/) are available to answer gardening questions in person or by phone, Monday, Wednesday and Friday, 9 a.m. to Noon, at the U. C. Cooperative Extension office, 1710 Soscol Avenue, Suite 4, Napa, 707-253-4143, or from outside City of Napa toll-free at 877-279-3065. Or e-mail your garden questions by following the guidelines on our web site. Click on Napa, then on Have Garden Questions? Find us on Facebook under UC Master Gardeners of Napa County.
Napa County Master Gardeners welcome the public to visit their demonstration garden at Connolly Ranch on Thursdays, from 10:00 a.m. until noon, except the last Thursday of the month. Connolly Ranch is at 3141 Browns Valley Road at Thompson Avenue in Napa. Enter on Thompson Avenue.