How Much To Water
By Ed Browning, Master Gardener coordinator

What are the water requirements of plants? The common practice of watering is probably to water a little bit every day. Some plants are high users of water, but there are some things to consider before making this part of your daily routine.

A research project in the early 1970s at the University of Missouri’s Southwest Missouri Research Center at Mt. Vernon has shown that water evaporates from a pan an average of 0.2" per day in May up to 0.28" in July. However, looking at the maximum evaporation, 0.46" was recorded in July.

Evapotranspiration, or that water which is transpired from plant leaves (transpiration) and evaporated from the soil surface (evaporation), makes up the total amount of water used by plants. Environmental factors such as temperature, wind velocity, solar radiation and humidity have an effect on evapotranspiration or ET. Additionally, water availability depends on the soil type and it’s ability to hold water. Evaporation from the soil also depends on ground cover or canopy provided by plants.

Going back to the study at the Southwest Center, ET was considered to be 80 % of pan evaporation, which converts the above numbers to a plant use of 0.16" average per day in May to a maximum of 0.37" per day in July and the data shows nearly the same amount in August.

What does all this mean? It implies that during the hottest, driest part of the summer plants will use as much as 2.5" of water per week. If a soil has the ability to hold 1.5" in the topsoil, then you would need to supplement with an inch of water. The next week, if there’s been no rain, you would need to supplement with 2.5" of water because the soil moisture has been depleted. We normally recommend irrigating/watering 1" to 1.5" per week in one or 2 applications rather than ¼" or a smattering every day which seems to be the most common practice.

So, now you ask why not water every day. First of all, root systems need oxygen. If you water every day, there isn’t enough oxygen in the soil to keep a root system healthy. Secondly, giving a plant a little water daily forces the root system to stay near the soil surface rather than growing deeper to seek water. Then when it does get dry, there’s not enough soil moisture for the roots to keep the plant going. Additionally, some gardeners tend to over-water and root rot develops.

Potted plants are a little different situation. Because the root system is generally confined to a small area, often times the volume of plant tissue above the pot exceeds the root volume in the pot and the total amount of water stored in the pot is not enough to sustain the plant very long. Therefore, potted plants may need watering more frequently. However, I would propose that a good soaking watering every other day, possibly every third day, is still better than every day. Protecting potted plants from wind and shading them so they only get part-day sunlight—preferably early morning—will help extend the watering interval.