What you need to know about watering.

What you need to know about watering.

What you need to know about watering.

All things green.
Aug. 30 2006 7:33 AM

The Gardener's Shadow

Everything you need to know about watering.

Click here for the latest installment of Slate's guide to planting a beginner's garden.

Illustration by Mark Alan Stamaty. Click image to expand.

Most people water their gardens too little. Our natural inclination is to give plants a refreshing shower, something we would appreciate on a hot day.

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But the truth is that plants don't care much one way or the other about water on their leaves. They take up water through their roots, which are, though this seems almost too basic to mention, under the ground. Getting water down to the roots is the single most important thing you can do to keep your plants alive.

Try this: When you think you've watered enough, take a trowel and dig down into the ground four inches. Because most of us don't water thoroughly the first time, the soil will probably still be dry. This was a humbling and useful revelation for me as an apprentice gardener. Now soak the ground again.

Gardening books give blanket advice: Water infrequently and deeply. The traditional rationale, rather moralistic, was that shallow watering allowed the plant to get used to finding water near the soil surface, thus making it lazy and ill-prepared to go deep in case of drought. But the real rule is this: Water when needed. The answer to when it's needed is, "It depends." Observant gardeners begin to see that different plants have different needs at different times.

For example, what constitutes "watering deeply" depends on the plant. Is it around for the summer or for 80 years? Many plants that give us showy summer color and fruits and vegetables have naturally shallow roots. The roots of common annual flowers like petunias or marigolds go down no more than 3 or 4 inches. Pull up a tomato plant and you'll see that the underground base of that giant green sprawl is only 6 inches deep. It takes an amazingly small root to maintain a monster zucchini vine. These aren't redwoods; they have to support themselves for only one growing season.  

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The monolithic rule to water deeply and infrequently doesn't always work for perennial plants, either. (Perennials, which include grasses, ferns, and many flowers, die down in winter and sprout back up in spring on the same root structure. Annuals, by contrast, produce seeds and die.) Perennials should be watered with care the first year after planting. But after that, if the soil is in good shape and the plantings are well-selected for the site, the plants can usually get along on their own.

Vegetables and annuals do fine if they're cared for every few days by an attentive gardener with a hose whose nozzle slows the flow to a gentle shower. (Here's a note on how to handle the hose so you don't feel like a klutz.) Hand-watering is relatively labor-intensive but has its advantages for both plant and person. Someone once said that the best fertilizer is the gardener's shadow. If you're the watering system, you notice wilting, yellowing leaves, and insects. And you get to feel good about the plants that are doing well. One of the great pleasures of gardening is watering in a summer twilight, running the hose over your toes, picking a cherry tomato, squashing a couple of slugs and a caterpillar. True, many garden books call for watering in the early morning, the time when the least water is lost to evaporation, but it's OK not to rise at reveille.

If you're a gardener who doesn't want to be in the vegetable or flower patch often, an inexpensive soaker hose can work pretty well. This is a black rubber hose with small holes through which water seeps. You attach it to your regular hose and faucet, snake the hose along the ground through your garden bed, and turn it on every few days to deliver water slowly, with little wasted. These days, sprinklers that shoot water up in an arc and are fun to run through are frowned on (and in some places banned, including parts of formerly rainy Great Britain) because so much water gets lost as it flies through the air. 

You can also buy a drip irrigation system complete with a computer control that turns on water without human intervention. Assembly instructions with phrases like "backflow preventer" and "compression fitting" and "pressure regulator" tend to make my blood run cold, as it does at Home Depot. So, I hired an irrigation professional for my vegetable garden.

The green things you should water deeply and infrequently, as the traditional rule holds, are trees and shrubs. They're the big ones in which you have a financial, aesthetic, and emotional investment. The feeder roots of trees and big shrubs fan out like the base of a goblet, so soak an area well out from the trunk. In a drought, work hardest to save your trees.

All green things, big or small, short-lived or long-lived, lose water rapidly when it's hottest. Water taken in through the roots gets sucked up through the stem to the leaves, where it departs through small openings. Wilting occurs when the plant keeps releasing water out of those pores even when the sun has baked the soil dry and there's no water coming in to the roots. In summer days of three-digit heat and water-use restrictions, I fear we're all at risk, plant and human alike. But as long as you pay attention to your plants, you'll develop a sense of what needs water and when. Meanwhile, here are five brief basic rules, in ascending order of importance, that you can follow and not go far wrong.

  • Lose the sprinkler, unless you have a lawn. If you have a large planted area to water, use a pulsating sprinkler that shoots in one direction and can be aimed precisely. 
  • Use mulch to keep the soil moist. 
  • Make watering convenient. Don't hide the faucet behind a prickly hedge, or plant your ferns and azaleas six hose-lengths away.
  • Anything you plant, even if it's allegedly drought-tolerant, needs a lot of water at planting time and frequent watering through the first season. Don't plant and walk away praying for rain.
  • Finally, a shortcut for the distracted: If you water thoroughly once a week through the summer and into early fall, you and your green things will be okay. And if there's a delicious soaking rain that lasts half a day or more, rest and be grateful.