And in many regions and many seasons, plants can fend for themselves getting water, but sometimes they could use help.
Before you touch that hose spigot, however, do what you can to help plants eke the most out of natural rainfall and water.
Add compost, leaves and other organic materials to your soil to help it retain water. Laid on top of the ground as mulch, these materials slow evaporation from the surface; they also keep the surface loose so water seeps in rather than runs off.
Weeds suck water from the soil, so rip them out to leave more water for your plants. And finally, contour the surface of sloping ground with low mounds or terraces to catch and hold water.
Water needs vary with soil type and weather. Sandy soils need most frequent watering. Low humidity, wind and heat all make plants thirstier.
Individual plants also vary in their water needs. Those that are lush-growing use the most water, and plants recently set in the ground need help until their roots venture out into surrounding soil.
A reliable way to tell whether the soil is moist or dry is to dig a hole and feel the soil for moisture. Or, instead of pocking your garden full of test holes, you could periodically check for wetness by probing the soil with an (inexpensive) electronic moisture meter.
Even easier, though less precise, is to play the averages. Monitor rainfall and apply water so plants receive a 1-inch depth of water per week, which is what an average plant needs in an average season.
A rain gauge or any straight-sided container can tell you how much rain has fallen, and then you can water to make up the difference.
That inch-depth of water is equivalent to about a half-gallon of water per square foot, so if you want to figure, instead, how many gallons a plant needs, estimate the number of square feet covered by its roots and multiply by one-half.
One exception to the 1-inch per week (or a 1/2 gallon per square foot) rule is for plants in containers. Such plants may need water every day -- perhaps even twice a day -- during their peak summer growth.
For plants in the ground, you'll be applying that inch of water either with a sprinkler or through drip tubing.
If you're sprinkling, water once a week, preferably early on a sunny morning.
With drip irrigation, use a timer to spread that inch of water as much as possible over all daylight hours of all seven days of the week.
This is, after all, how plants use water. Drip irrigation typically uses only about 60 percent of the water used by sprinklers.
Don't worry about diseases from the frequent watering with drip irrigation; it does plants no harm because leaves stay dry.
Don't be too zealous: Overwatering wastes water and, by suffocating roots, is as harmful to plants as underwatering is.
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