The Right Amount of Water

Water. Every plant needs it. Everyone knows it. Being essential, then, also makes it essential to learn about.

The Green-thumbed Secrets

Simplicity and obviousness of its import, however, do not equal ubiquitous knowledge, especially of the finer points. What, then, are the secrets that green-thumbed gardeners know that make their vegetables do just a little bit, or a lot, better? The good news is that none of the secrets are at all hard to learn. In fact, these are probably the easiest concepts to understand and put into practice. Plus, for the most part, nature does it for you.

The Basics

How Much?

First, your garden fruits and vegetables need about an inch of water per week. There are some exceptions to this. Onions, for example, do a little better with half again or twice that, and peppers can do well enough on three-quarters or half of that. Still, that amount—simply an inch per week—will get you a WONDERFUL garden, no matter the crop. 

Water Deeply

Second, all garden crops do much better when watered less often but deeply—as opposed to several shallow waterings. That is, you’re better off watering an inch once per week than a quarter of an inch four times a week. Shallow, light waterings leave only the top few inches of soil moistened, if that, encouraging a flush of shallow root growth to absorb the bursts of available water and leaving the plants susceptible to being seriously damaged when the top few inches of soil dry out during hot, windy days (more on this below). Watering them deeply, by contrast, encourages their root systems to grow deeply, empowering your plants to be better suited for meeting their own needs by getting the water and nutrients that are available deeper in the soil. 

Other Benefits of Deep Watering

Deep watering has other essential benefits, as well: It avoids soil “deserts” and reconnects and feeds the natural capillary action process.

Capillary Action

Natural capillary action pulls water up from the deeper water table toward the surface, making it available to your plants’ roots. We’ve all seen capillary action at work. It’s what happens if you immerse just part of a porous material in a liquid (e.g. cloth or cardboard in water or a lantern wick in kerosene): The liquid rises up the material well beyond the height of the liquid itself. It’s not drenched all the way up, but it’s moist. Your soil works the same way, wicking water toward the surface from the deeper concentrations closer to the water table. Along with ground-shading mulch, this is one of the main processes that keep your soil moist between rains.

After extended periods of dry weather, however, especially if it’s hot or windy and the soil is exposed, the natural capillary action can’t keep up with evaporation near the surface, and the soil’s top layers start to dry out. If dry conditions continue, this dryness extends deeper and deeper into the soil, often reaching several inches deep during prolonged dry periods.

Light Watering: Soil “Deserts” and Plants’ Roots

Since light waterings only wet the top few inches of soil, they leave an area that remains dry between the wetted few inches on top and the deeper soil that is still moistened from below via capillary action. This dry area acts like a desert roots need to cross to bring water and nutrients to the plant from still-moist areas deeper in the soil.

Plants’ roots cannot withstand drying out, not even for a few minutes. If they do, they die. When the soil first starts to become a little dry, roots become stressed and start limiting their new growth. If dry conditions continue, roots become weakened and eventually die, the smallest ones dying first. When soil is watered lightly during dry conditions, the “desert” that is created is typically between a few inches and a foot or so deep—right in the heart of a plant’s root zone, which should be a thriving area of root growth. Even short periods of lack of ideal moisture here forever diminish the plant’s potential as it lessens or eliminates the bulk of the essential smaller feeder roots and root hairs and shrinks the few remaining major roots that survive to transport moisture and nutrients across the “desert.”

Deep Watering: Filling the Desert, Reconnecting Capillary Action, and Replenishing Ground Water

In contrast, when watered deeply and this region of soil is kept evenly moist, this area becomes a dense tangle of lush root growth where primary and secondary roots and plentiful root hairs expand laterally into an ever-larger system, greatly strengthening the plant.

The dry area “desert” created by light watering during dry weather also acts as a capillary break. The top few inches of moisture has its own capillary action, as does the moisture deeper in the soil, but, since the dry area acts as a capillary break, they’re disconnected from each other, leaving the middle, “desert” area deprived of help from the soil’s natural wicking of water. When deep waterings fill this “desert” with moisture, they reconnect the surface moisture with the deeper source of soil moisture, allowing capillary action to once again work throughout the entire area and keep the soil evenly moist.

Light watering also fails to feed the groundwater supply that feeds capillary action. Deep waterings, in contrast, not only make water available to deeper plant roots but also allow water to filter down and replenish this deeper water supply. 

The Upshot

These are simple concepts that empower you to make much better decisions about your watering. Simply making sure your plants get about an inch of water per week will give you a wonderful garden. Plus, watering your plants deeply is very important for their health—and shallow watering can be quite detrimental. These two pieces of knowledge are the most important green thumb secrets about watering. There are others, however.

The Next Step

You may be wondering, for example:

Still, do I really even need to water?

Rain literally falls from the sky exactly where you need it almost once a week.

Aren’t there gardeners who rarely, if ever, water?

Perhaps you live in a place where you get around 32” of rain a year. That’s not an inch per week. Plus, it probably comes disproportionately throughout the year, being heavier at times when you have few, if any, plants in the garden. However, many people who live in such areas never water, not once, and have amazing vegetables and fruits. What’s the story? Well, an inch per week is ideal, but the honest answer is that many gardens do just fine with a little less on average. But, there are also a few tricks of the trade for sure. 

Next up, we explore those tricks.