Water has registered on many a gardener's radar, but it can still be a confusing subject. Smart gardeners want to know: how much (and when) to  water in the garden this summer?

Know what you are watering weekly.

It's a general watering rule of thumb: plants need 1 inch of water per week, whether it's from your rain barrel (see sidebar below), a soaker hose, or rainfall courtesy of Mother Nature.

Of course the weather creates exceptions to the rule. The regular rule is 1 inch of water when daytime temperatures are in the mid-70s Fahrenheit. If temperatures soar into the 90s, double the amount to 2 inches per week, and add more if a heat wave persists. How to keep track?

• Set a day of the week that you prefer to water—say Saturday morning—and adjust the 1-inch rule according to the week's weather you (and your plants) have just experienced.
• Sunny days or cloudy this week? Moisture evaporates out of soil quickly on sunny days.
• Windy days or still this week? Wind causes soil moisture to evaporate quickly, too.
• Cool nights or warm this week? Cool nights slow evaporation.

For some gardeners, an electronic or written garden journal is a helpful tool for plotting rainfall and keeping track of weekly conditions.

Water deeply not daily.

It's common for everyone—even experienced gardeners—to overwater.  How do you know when you've met the 1-inch rule above? Measure the water, check the soil.

• A rain gauge plus a simple watering gauge (see sidebar at right) do the water math for you.
• An inch of rain or water percolates downward 6 inches into clay soil, and 8 to 12 inches into loamy soil. Use a soil knife, a trowel, a stick, or your finger to dig into the soil after watering—make sure the water's getting where you want it.
• Dry, cracked soil? Water can run off the surface if the soil is hard and compacted. Mulch and soil amendments (see #5) can help.
• Soggy, saturated soil? Hold off on watering again until absorbed. Compacted soil has less air, oxygen, and room for roots but, as above, mulch and organic matter amended into the soil can improve absorption.

Just as a long, steady, soaking rain is better for the soil than several quick sprinkles, deep and infrequent watering is better for your garden beds than a quick, daily drink.

Watch for plants signaling "Water me."

Plants are largely water: 80 to 90 percent of an herbaceous plant and 50 percent of a woody plant is water.

Here's a basic look at how a plant works: Water percolates into the air pockets in soil, and is absorbed by a plant's roots. Pulled upward through the stem, water fills out the cells in tissues and eventually evaporates out through the leaves. When a plant's cells are full of water, leaves look open and flat. When there is less water in the cells, leaves droop or wilt. Yellow, curling, and dropping leaves can indicate extreme water need.

Most roots spread wide beneath the plant, often reaching just 12 to 18 inches deep into the soil. Consistent, deep watering helps develop strong roots, anchoring plants in the soil. Water plants all the way out to the drip line to ensure that the entire root zone gets a drink.

Water plants in the morning so that tissues fill out before the heat of the day and to discourage fungal and bacterial growth overnight.

Water plants when they need it most.

Baby seedlings and transplants need special water attention, since their root systems are not yet settled into the soil. Water them gently and daily to keep the soil moist until roots take hold and you see signs of growth.

Vegetable gardens follow the same general watering rules above, although there are vegetable stages when consistent watering is critical. Pay special attention when

• pods are forming on beans
• fruit is forming on summer squash or zucchini
• tomatoes are flowering and setting fruit
• cabbage is heading
• sweet corn cobs are swelling
• harvesting lettuce (seven to ten days prior) and heading vegetables such as summer cabbage and cauliflower (two to three weeks prior)

Of special note: broccoli, celery, and spinach should never dry out!

Conserve water in the soil.

Like a lightweight blanket, a layer of mulch protects soil from the elements and steadies the ground's temperature, so soil stays moister longer. Even rocks placed near or around plants can help soil hold onto its moisture.

Organic matter—compost made from leaves, grass, garden trimmings, and vegetable peelings—feeds the soil and nurtures the microscopic life that lives within it. Compost improves soil's ability to hold water as well as its overall quality. It can be lightly dug into the soil near plants, or side dressed above the root zone. By watering wisely, you help to conserve the most precious resource on our planet, while also raising a healthy and productive garden.

Karen Zaworski is a garden writer and photographer who lives and gardens in Oak Park, Illinois.