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While plants can certainly die of thirst, we can also send them to an early grave by overwatering them. "When we overwater, the soil becomes saturated," writes Joe Lamp'l in The Green Gardener's Guide: Simple, Significant Actions to Protect & Preserve Our Planet (2008, Cool Springs Press), "forcing out vital oxygen and literally drowning out plants." While it's critical to provide ample water to new plants, notes the garden writer and television host, you can reduce the amount of water you apply as soon as they become established. In fact, they should require supplemental water only in the absence of rainfall. There's also the environmental impact to consider, Lamp'l says:

If you're an average user of water in the home landscape, by watering only when you need to and only as much as your plants require, you will reduce your water consumption around 25 percent, or by an average of 11,000 gallons of water per year for each household. If all U.S. homeowners do this, it will reduce national water consumption by about 700 billion gallons per year.

His rule of thumb: In the absence of rain, provide one inch of supplemental water each week, or whatever is necessary to make up the difference. (Use a rain gauge to help you determine how much irrigation is needed to boost what nature has provided.) If you're still uncertain about your plant's water needs, here are two easy ways Lamp'l says you can find out for sure:

1. The finger test: You can't tell what moisture conditions are like below the soil just by looking, especially if the surface is covered by mulch. Poke your inger into the soil around the base of the plant, down to the second knuckle, Lamp'l says. Pull it back out and take a look at your finger. If it came back with soil stuck to it, then the soil has sufficient moisture. If your finger remains relatively clean, however, the soil is dry and in need of immediate thirst quenching.

2. The dig test: Before watering, Lamp'l says, simply make note of the moisture level of your soil, 6 to 12 inches below the surface. Your goal: To determine how long it takes for your irrigation system, whether it's a soaker hose, drip irrigation, watering wand, or overhead system, to soak the soil to the target depth.

Assuming that the soil is dry when you start, he says, begin the irrigation process and note the time. Several minutes later, depending on the flow rate of your system, turn off the irrigation and dig down to the target depth. If your soil is saturated at this point, then this is the amount (or less) you'll need to achieve your target irrigation volume. If it's still dry, on the other hand, keep watering until you can determine the optimal time required to get the moisture down to the target level.

Difficulty level: Easy to moderate