What Plants Need: Water

It's no secret that water is a necessity for a healthy landscape. Even desert plants need water occasionally. The questions are, how much and how often.

Watering The Garden

It’s no secret that a consistent supply of water is a necessity for a healthy garden or landscape. Even desert plants need water occasionally. The questions are, how much and how often. Although there is considerable variation from plant to plant and location to location, the general rule of thumb that American gardeners use is one inch of water per week, but that’s just the beginning.

How Much Water Is One Inch?

At first glance, one inch of water doesn’t seem like much. It’s not uncommon to receive an inch of rainfall in a single thunderstorm. A few interesting water facts:

  • One gallon of water= 231 cubic inches (covers about 1.6 square feet, an inch deep)
  • One cubic foot of water= 7.48 gallons (covers 12 square feet, an inch deep)
  • The roof of my house is about 1500 square feet, and so an inch of rain on my roof= 125 cubic feet, or 935 gallons of water
  • My front yard is approximately 5000 square feet, and so an inch of rain there= 416.67 cubic feet, or 3,116.69 gallons of water

To put those numbers in perspective, according to my water bill, my household uses an average of about 3500 gallons of water per month. An inch of rain adds up quickly!


How Often Is Watering Necessary?

The starting point of knowing when to water is knowing how much rain has fallen recently and how well established the plant in question may be. On the day any plant is planted, it needs to be watered. The initial watering not only soaks the roots, but also begins the process of rebuilding soil structure around the roots by removing air pockets. On the other hand, if a tree or shrub has been in place for a few years and the precipitation has been more or less average, there should be no reason to irrigate.

The “inch per week” rule really comes into play with shallowly rooted herbaceous plants such as annuals, perennials, vegetables, herbs and lawns. Many of these plants rely completely on the moisture in the uppermost layer of the soil which can dry out quickly after several hot days without rain. This is especially true if the soil is of a coarse, sandy texture, or if the plants are types adapted to wetter-than-average conditions. For these reasons, it is important to pay attention to rainfall, both frequency and volume, as well as the moisture level in the soil. Use these patterns to guide your decision whether or not to irrigate.

Check out tips on how to efficiently water your garden. 

Tools Of The Trade

Use a rain gauge to monitor rainfall. They can be purchased inexpensively, or may be had for free by attaching a used tuna can to a post. Measuring up from the bottom, use a permanent marker to mark the one inch depth. Empty the gauge after each rain event. A single one-inch rain event may not be sufficient to thoroughly soak the soil, whereas several quarter-inch events may do just the trick. To be sure, probe the soil to see how the moisture level is at depths of four to ten inches. You may simply dig down and look/feel the soil, or purchase a moisture meter that will gauge it for you.

Watering Plants

Watering properly is important to be effective without waste. When watering by hand, use a garden watering can with a “rose” or diffuser on the spout to break up the stream. The same is true when watering directly with a hose – use a device with a diffused or shower-head spray pattern. Keep the pressure low so that the soil surface tension is gently broken, allowing the water to soak in rather than running off.

When using a sprinkler, use a rain gauge to measure the amount of water being applied and monitor for runoff. Keep in mind that sprinklers are most effective for shallow-rooted plants. Use soil soaker hoses,  bags or similar devices for watering trees.


Garden Water Conservation

Water conservation is absolutely critical for gardeners! In my humid region, we receive 48.5 inches of precipitation in an average year – that’s nearly four inches short of enough to provide an inch of water per week. The U.S. average is only 38.67 inches per year. Where does the rest of the needed water come from? One of two places: surface water such as streams, rivers and lakes, or groundwater (i.e. wells) which deplete the water table. Lots of people taking water from either of these places can lead to serious stresses. Water is a finite resource and there is no guarantee that the averages will hold or that the rain will come when the garden needs it. Here is a list of the best ways to conserve water in the garden and minimize the need for added water:

  • Select regional natives and other plants that are adapted to the local climate.
  • Amend planting beds with compost and or organic soil conditioner. High levels of organic matter lead to a healthy water holding capacity within the soil.
  • Keep plants well mulched. Mulch breaks up the soil surface capillary structure, reducing evaporation of soil moisture. Plant-based mulch (like bark or straw, as opposed to rubber or other synthetic materials) breaks down over time, adding to the organic content of the soil.
  • Minimize the use of overhead sprinklers. More than forty percent of the water delivered by overhead sprinkler systems is lost to evaporation and wind drift. Use drip irrigation, soil soaker devices and hand watering whenever possible.
  • Irrigate in the early morning, before 9 a.m. When the humidity is higher in the morning, water is least likely to evaporate before finding it’s way into the root zone.
  • Irrigate when the plants need it, and water deeply to minimize watering frequency. It is better to apply a half inch of water twice a week, than an eighth of an inch every day. This less-frequent approach promotes deeper rooting and allows more flexibility to skip watering if a storm blows through.
  • Expand shrub borders, perennial beds and other garden features to minimize watering grass.

More in the “What Plants Need” series:

What Plants Need: Soil

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