During an entire year, a meager three-hundredths of an inch of rain falls on Arica, Chile, yet halfway across the Pacific in the Hawaiian Archipelago, Mount Waialeale receives a sopping 460 inches. Here in the Hudson Valley, all of northeastern US, in fact, we have a congenial climate for growing plants—at least those plants we enjoy in our gardens—during our spring through fall growing season.
We average about four inches of rainfall each month throughout the year, and this amount complements nicely the inch of water per week recommended for most garden plants. That, of course, is an average. Early in the season, when the air is cool and plants are small, less water is needed than in the heat of summer when lush growth is slurping up soil water to cool the plant and pump nutrients. Coil texture also plants a role, with sandy soils less able to cling to water for future plant use than, say clay soils.
But generally, four inches of rainfall each month is more than enough water to supply the 33 gallons used by a tomato plant, the 54 gallons needed by a corn plant, even the 1800 gallons quaffed by a single large apple tree!
So much for theory. Problem is that rain is not always in the right place at the right time. Four inches of rain dumped from the sky on the Fourth of July, with none again until the first of August, would meet July’s quota. But much of that water might run off the surface of the soil or down through the soil beyond the reach of roots.
One way to remedy this feast or famine situation is to help all water falling from the sky get into the soil. Small catch basins built up around individual trees and shrubs keep rainfall in place for these plants. Organic mulches like straw or leaves help water percolate into the soil by preventing raindrops from pounding, then sealing, the surface.
Once water is in the ground, those organic materials also hold it there. They do this by preventing evaporation from the soil surface and, mixed with the soil, by acting like a water-holding sponge.
Once you have gotten water into the soil and held there, why waste it on weeds? A full-grown ragweed plant sucks about two gallons of water per day from the soil, water that could be put to better use plumping up juicy, red tomatoes. Timely weeding keeps water in the soil for the plants we choose to cultivate.
Okay, for all your water conservation efforts, sometimes you just gotta water your plants. There are two approaches here: water either deeply and infrequently with a sprinkler or bucket, or water lightly and frequently with a drip irrigation system. Either way, the amount of water to use translates to the same as our average rainfall, one inch (about a half-gallon per square foot) applied once each week, or one-seventh of an inch applied every day or through the day.
I’m a big fan of drip irrigation. It makes more efficient use of water (about 60% savings as compared with sprinkling), keeps leaves dry to limit fungal diseases, and is easily automated with an inexpensive timer.
When your time or energy is running short—often the case when hand watering and using sprinklers—you might want to first care for those plants that need water the most. Most apt to suffer from thirst are annual flowers and leafy vegetables like lettuce, spinach, and cabbage, as well as any perennials or trees you set out just this spring.
Roots of full-grown trees and shrubs run deep into the soil, so these plants can wait longest for water. Roots of perennial flowers, and vegetables like asparagus, beans, peas, cucumbers, melons, and tomatoes, also can tap moisture a few feet down in the soil.
When time or water is short, don’t worry about your lawn. Lawn grasses tolerate drought by going semi-dormant and tawny, then perks up when moist conditions return.
Lee Reich, PhD is a garden and orchard consultant; he also hosts workshops at his New Paltz farmden (inquire at leereich.com/workshops). He details how to grow blueberries—and other fruits—in his book Grow Fruit Naturally.