Every gardener has heard the term "water deeply," meaning that it is better to water deeply once a week than to water superficially more often. What does it mean to water deeply, and how can you tell if you've accomplished it?
What Does Watering Deeply Mean?
There is no hard-and-fast definition for watering deeply, but it generally means that the water can soak at least eight inches below the soil surface. The reason behind watering deeply is that most plant roots are not sitting close to the soil surface. They have worked their way down into the soil, in search of water and nutrients. This helps protect the plant in times of drought because the soil surface will dry out much quicker than it will below ground, where the soil is cooler. Since you cannot control the rain, there will be weeks when your garden will get much more water than it needs and weeks when it will be your responsibility to see that your garden is watered.
Another common gardening recommendation is to make sure your plants get at least one inch of water every week. One inch does not sound like a lot, and it isn't. That's a minimum. It is better for the plants if the soil gets a good soaking down to at least the eight inches mentioned above. That's because one inch of water will evaporate or dissipate quickly, whereas a thorough soaking several inches below the soil surface will linger long enough for your plant's roots to get a good drink.
You can try to get around this by giving your plants a little bit of water daily, rather than a good weekly soaking. If you have a drip irrigation system where you are guaranteed that the garden really will get a daily watering, that's fine. However, it is not a practical plan if you are watering by can or hose. Plants that are used to getting frequent water will not develop the deep root system that is needed for the plant to survive periods of drought, so making your plants dependent on daily watering and then missing a few days will cause long-term problems. Once a plant is water-stressed, it can take weeks to recover, and in the case of annuals and vegetables, every week counts.
How to Test How Much Water Your Garden is Getting
How quickly water runs through the soil and how much is absorbed for the roots to access will depend on what type of soil you have, the weather conditions, and how fast the water is being applied.
Water runs through sand much more quickly than it penetrates clay. That's why soil amendment with organic matter is advised for sand and clay. The organic matter is great at holding onto the water just long enough for the plants to get at it. A three-to-four-inch layer of mulch will help conserve whatever moisture is there.
However, there's a simple test to get an approximate idea of how much water is falling on your garden. Water your garden and then wait half an hour. Dig down into the soil with a trowel. If it's not wet eight inches or more below the surface, it might be that you haven't watered enough or it could be that you watered too fast and the water ran off elsewhere. It is probably both.
Next time, try a more gentle stream of water for a longer period. A gentle soak for an hour or two is better than puddling soil around your plants and moving on. It primes the soil to absorb more water and allows the water to spread out in the soil.
It can take a few tries to get it right, and you don't have to be obsessive about getting exactly eight inches. The point is to make sure the soil is absorbing and holding the water long enough to hydrate the plants. Once you master the concept of watering deeply, your plants will stay healthier in whatever weather comes their way.
Water Wisely: Start in Your Backyard. University of Minnesota Extension