Used to be, they had to. It was less than a hundred years ago that garden hoses came on the scene. Before that, rainfall was pretty much all plants got, except in arid regions where periodic "flood irrigation" was used.
Still, plants sometimes could use help getting water, especially these days, when more of us are trying to eke more vegetables out of less land.
MAKE THE MOST OF WATER
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.
Next, find out if your plants need 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.
IS WATER NEEDED?
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 "one-half 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 of growth in summer.
NOT TOO MUCH, NOT TOO LITTLE
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 some sunny morning when it's early enough that the air is still calm yet late enough that leaves soon dry, lessening chances for diseases.
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 — one reason for the good "bang for the buck" you get with water merely dripped slowly into the ground near a plant. (Drip irrigation typically uses only about 60 per cent 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.
No matter what your proposed method of watering, keep in mind that many plants grow fine with little or no supplemental watering over much of the season in all but arid regions. Don't be too zealous: Overwatering wastes water and, by suffocating roots, is as harmful to plants as underwatering is.