How much water do your plants really need?

Water issues are a major concern around the country. Population growth and increased urbanization put increasing pressure on water supplies, making it increasingly important to use water more efficiently.

It’s not just the quantity of water that matters, water quality is important as well. Excessive irrigation inevitably results in leaching of water and fertilizer. This results in runoff that can end up in the ground or in surface water if it is not captured on site. Fertilizer runoff, especially nitrogen and phosphorus, can result in algae growth in ponds and lakes. Many growers face strict fertilizer runoff regulations. The best method to minimize runoff is to water more efficiently.

Less water needed

Although the question about how much water your plants really need is simple, the answer is not. Surprisingly, there is little information on plant water needs. The last few years, researchers at the University of Georgia and University of Maine have been studying the water needs of annuals and perennials. The good news is that many plants need much less water than most people expect. For example, we have grown petunias in 4-inch pots from seedlings to marketable size with about a half gallon of water applied over 40 days. The plants were not given just enough water to survive, but all the water they needed to grow. However, they were only watered when they needed to be and no excess water was provided.

Two soil moisture sensors were used for each group of 12 petunia plants, which were irrigated when volumetric substrate water content dropped below 40 percent.Determining water needs

Since each greenhouse environment is unique, and weather changes daily and differs from region to region, it is important to have some idea of your plants’ water needs. There is a simple way to determine this.

Thoroughly water the plants in the morning and let them drain for at least 30 minutes. Then weigh the pots, come back 24 hours later and weigh the pots again. The decrease in weight is the amount of water that has been used by the plant. Water use will differ from day to day.

The main environmental factors that affect water use are light, temperature and relative humidity. Plant size also plays an important role with larger plants needing more water than smaller ones. Determining daily water use several times during the production cycle, both on warm, sunny and on cool, overcast days will provide you with valuable information for water management.

In order to use this information to water more efficiently, you also need to know how much water your irrigation system delivers in a given amount of time. With drip irrigation, this is simple. Put a few drippers in a beaker, run the drip system for a specific period of time and measure how much water was applied per minute.

For sprinkler systems, an easy method to determine how much water each pot receives is to line the inside of the pots with plastic bags, and then put a second pot inside the plastic bag. Trim the excess plastic. This creates beakers that are the same size as the pots the plants are growing in. Run the irrigation system for a specific period of time, and then weigh the pots before and after irrigation to determine how much water the plants receive per minute.

Now that you have determined how much water the plants use and how much water is applied per minute, you should be able to make better decisions about how much water to apply. However, dealing with day-to-day fluctuations in plant water use due to changing weather conditions may make frequent adjustments necessary. If you prefer to not have to make these adjustments manually, it is possible to automate your irrigation system in a way that assures that the plants receive only the water they need.

Sensors improve efficiency

We have developed an irrigation system that can water plants when needed with the appropriate amount of water. This system uses sensors to measure substrate moisture. Soil moisture sensors have become available that are relatively inexpensive ($60), reliable and low maintenance.

Among these new sensors are EC-5 and 10HS soil moisture sensors from Decagon Devices Inc. The EC-5 sensor works well in 4-, 5- and 6-inch pots. The 10HS sensor is suitable for container sizes of 6-inch and larger.

In many cases, the sensors can be connected to computerized greenhouse control systems to directly control irrigation. Sensor operation is simple. When a plant is transpiring, it takes up water from the substrate, which dries out. Since the sensor measures the substrate water content frequently, the greenhouse environmental computer notices when the substrate water content drops below a particular set point and can turn on the irrigation system. The duration of the irrigation period can be either a set amount of time or it can be sensor-controlled. For example, the irrigation system can be turned on when the substrate water content drops below 40 percent and turned off when it has reached 45 percent.

Plants control watering needs

By irrigating based on substrate water content, the plants essentially are in control of their watering. On warm, sunny days the plants use water quickly, which results in a quick drop in substrate water content and more frequent irrigation.

Another approach to using soil moisture sensors for irrigation control is to use them like a cut-off switch. A timer can be set to water at a specific time. At that time the irrigation valve opens only if the sensors detect that the substrate water content is below a designated set point. If the substrate is still moist, the sensor prevents the irrigation valve from opening. If the sensor does allow irrigation to occur, it can then turn off the irrigation automatically when the set point for substrate water content is reached.

A tablespoon per day

We have studied how different irrigation set points affect a variety of plants. Petunias were grown at substrate water levels ranging from 5-40 percent for three weeks. In a peat-lite substrate, a 5-10 percent substrate water level is the lowest most plants could survive and 50 percent is near container capacity. Plant growth increased with increasing substrate water content, although there was little difference between 25-, 30-, 35- and 40-percent treatments. Even in the substrate maintained at 40 percent water level, there was no leaching.

A higher substrate moisture set point resulted in more frequent waterings, so the amount of water that the plants received increased with increasing substrate moisture levels. Over a three week period, plants received anywhere from 3½ to 22 ounces.

A substrate moisture content of 20 percent was enough to grow quality plants. For a three week period, these plants received about 16 ounces of water per plant. This is a little more than 1 tablespoon per day. Water use was not constant during the study; small plants used 1 tablespoon per day, while large plants used slightly less than 2 tablespoons per day. Overall, there was a good correlation between plant growth and the amount of water applied. The study indicated that controlling irrigation can be an effective method of controlling growth. 

Marc van Iersel is professor and Jongyun Kim is a PhD student, University of Georgia, Department of Horticulture, (706) 583-0284; mvanier@uga.edu. Stephanie Burnett is assistant professor, University of Maine, Plant, Soil and Environmental Sciences Department, (207) 581-2937; sburnett@maine.edu. This research was supported by the Fred C. Gloeckner Foundation.