Plants, like animals, need water to live. A seed must absorb water before it can germinate. Roots can take up nutrients only when water is present in the soil; water transports nutrients throughout plants. And water is essential to photosynthesis.
However, how much water your plants need and how frequently they need it depend on a number of interrelated factors, including soil texture, the plants themselves and their age, and the weather.
Your soil's ability to absorb and retain water is closely related to its composition. Clay soils absorb water slowly and drain slowly as well, retaining water longer than other soils. Sandy soils, in contrast, absorb water quickly and drain just as quickly. Loam soils absorb water fairly rapidly and drain well, but not too fast. You canwork in organic amendments to help clay soils absorb water faster and drain better and to make sandy soils more moisture retentive.
Once their roots are established, different sorts of plants have widely differing water needs. Plants native to semiarid and arid climates, called xerophytes, have evolved features that allow them to survive with little water and low relative humidity. They may have deep root systems, for example, or leaves that are small, hairy, or waxy. The majority of familiar garden plants, however, are adapted to moist soil and high relative humidity. Called mesophytes, they usually have broad, thin leaves. Keep in mind that all young plants, including xerophytes, require more frequent watering than mature plants until their root systems become well established. And many annuals and vegetables require regular moisture throughout the growing season if they are to bloom well or produce a good crop.
Weather affects water needs as well. When it's hot, dry, and windy, plants use water very rapidly, and young or shallow-rooted ones sometimes cannot absorb water fast enough to keep foliage from wilting. Such plants need frequent watering to keep moisture around their roots at all times. During cool, damp weather, on the other hand, plants require much less water. Water needs are lower during winter as well, when the days are short and the sun is low on the horizon.
Because soil texture, plant type and age, and weather are all variable, following a fixed watering schedule year-round (or even all summer) isn't the most efficient way to meet your plants' needs. Always test your soil for moisture and look at your plants before you water. To check the soil around new transplants and in vegetable and flower beds, dig down a few inches with your fingers or a trowel; if the top 1 to 2 inches are dry, you probably need to water. In lawns or around established trees and shrubs, a soil sampling tube (shown at right) is useful: it allows you to test moisture at deeper levels without digging a hole that could disturb roots. Leaves can also can tell you when it's time to water. Most will look dull or roll in at the edges just before they wilt.
When you do water, aim to soak the root zone of your plants. As a general guideline, the roots of lawn grasses grow about 1 foot deep; roots of small shrubs and other plants reach 1 to 2 feet deep. While the taproots of some trees and shrubs may grow more deeply into the soil, most roots tend to concentrate in the top 2 to 3 feet. Watering below the root zone only wastes water.
To check how far water penetrates in your soil, water for a set amount of time (say, 30 minutes). Wait for 24 hours, then use a soil sampling tube or dig a hole to check for moisture. You'll soon learn to judge how long to water each plant to soak its root zone thoroughly.
Soil Texture & Water Penetration.
Applied to sand, 1 inch of water penetrates about 12 inches. Applied to loam, 1 inch of water reaches about 7 inches. Applied to clay, 1 inch of water soaks only 4 to 5 inches.
Giving plants too much water, especially in clay soils, can cause as many problems as supplying too little. Roots absorb oxygen from the air found in pore spaces between soil particles. During irrigation or rainfall, water displaces the air in these spaces; then, as the water drains away, evaporates, and is taken up by roots, the pore spaces fill with air again. But if water is applied too often, the pore spaces never have a chance to drain. They remain filled with water, and air is not available to the roots. The lack of oxygen makes roots susceptible to various water-mold fungi, which in turn can lead to rot. Overwatering also compacts the soil and literally washes some nutrients beyond the reach of roots.
Using a Soil Sample Tube.
This device allows you to check soil moisture at deeper levels than you can reach with a trowel, without disturbing plant roots too much. Push the tube into the ground, pull it out, and examine the soil in the sample. If it is dry or only slightly moist, it's time to water. If the top layer is damp and the rest is dry, you need to water longer to ensure deeper penetration. A soil sampling tube is also useful for detecting compacted layers of soil and checking how deeply roots penetrate.