March 2014: How to Know When to Water and for How Long

How to water-tape in box gardenI often get the question: How much should I water? The answer: It depends. I know, I know that sounds overwhelming so I will give you just the basics and a practical method that you can use now.

The Basics

Watering length and frequency is determined by 3 factors: Soil, weather, and how much water the plant uses (transevaporation.) These factors are modified by conditions of shade either on the plant or on the soil.

 
 
Soils

Soil texture can be clay, silt, sand or a variety of combinations of all 3. For more information on soil texture and fun, easy ways to test your soil click here.

Clay and silt will absorb water slowly and spread out from the water source near the surface and eventually go deeper. If you add water quickly it will just runoff. The watering strategy is to water longer and infrequently. If you see water running off, stop irrigating for an hour let it absorb and then resume the water cycle. This is a very useful technique for lawns.

Sandy soils and light box garden and potting soil mixes drain water immediately with very little spreading out to the roots. The watering strategy is short frequent cycles. For example, in these extremely porous soils, you may have to run a cycle twice a day for a few minutes. And, if you are using drip irrigation, the emitters need to be close to the plant.

You cannot deep soak these porous soils and benefit shallow rooted plants. A hazard particular to box gardens is when excess irrigation water settles in the bottom and can't drain out fast enough. When this happens, you will smell a swamp-like stench when you dig deep into the box indicating a lack of oxygen. This condition will kill roots and waste water.

The combinations of clay, silt and sand create different loams that absorbs the water rather than draining or running off and spreads it quicker and deeper into the root zone. Your watering strategy for these soils will fall somewhere in the middle.

Water absorption ability and rates in all soils mentioned above can be modified by mixing organics (compost) into the soil. Adding a layer of mulch will help reduce evaporation.

Weather

Weather will change your garden watering cycles. Adjust your watering as the days get longer or shorter, with changes in temperature and humidity, wind, and rain.

Plants

Different plants will absorb water at different rates. Vegetables and annuals usually have a higher rate of consumption and need the upper layer of soil moist because of their shallow root systems. Perennials, shrubs, and trees tend to have deeper and wider spread root systems so deeper and less frequent watering is the general rule.

 

So How Do You Know if Your Plants Need Water?

In vegetable and annual beds the premier monitoring tool is your gardening hands in the dirt with gloves off. Poke around with your finger at different soil depths including the surface and downward in increments of inches. You want to get to where the roots are or should be. The soil moisture should feel similar to a wrung-out sponge.

If you are using drip tape, check for moisture between the emitters when your water cycle is done. It should feel moist, just below the surface.

Young seedlings need to have consistent moisture from the surface to just beyond the tip of their root system. As they grow larger and the roots grow deeper, the surface moisture is not as critical as the root zone.

In established planting beds, around shrubs and trees, or in the lawn, a long screwdriver comes in handy to gage soil moisture if you don't have rocky soil. Push the screwdriver into the soil. If the soil is dry you will meet a lot of resistance. Get to know your yard's soil moisture by doing it several times over the growing season. For help with lawn watering schedules through the summer, check out the Utah Lawn Weekly Lawn Watering Guide based on real time field data.