Moisture Meters and Rebar Tell You When to Water and How Much to Apply

Q. Our water bill gets high in the summertime. I suspect we're overwatering but don’t know. What strategy can we go through to determine when plants are getting just enough water. More than enough is hard to determine.

A. There are 2 pieces of information you need when irrigating: how many minutes to run the irrigation timer and how often. This is the basic information that's entered into an irrigation controller in a variety of methods. Irrigation controllers have all sorts of whistles and bells but that 2 pieces of information is what is needed.

This requires a small investment on your part in the beginning. The two questions that need answering are when to water and how long (minutes) to water. 

How often to water

You will need some sort of moisture meter that measures soil moisture and a steel rod for determining how long to water.

This is an inexpensive moisture meter you can buy for under $10 at any hardware store for nursery. They are made for use with houseplants and so they probably won't last very long when you try to push them in our soils. But they are fairly accurate. Most houseplants should be watered when the meter shows a "6". For houseplants use distilled or RO (reverse osmosis) water. 

            There are two types of soil moisture meters available. One is inexpensive you can buy at box stores for houseplants for less than $10. A better one can be bought online for $40 – $75, can be pushed into more difficult soil and lasts longer. I have purchased both the Reotemp and Lincoln soil moisture meters and the Lincoln lasted. The Reotemp fell apart after a few tries.

This is the Lincoln soil moisture sensor and has had a solid workout for two years in tough Las Vegas soils.

/The Reotemp soil moisture sensor broke after a few tries. The tip came off. Same problem was reported by another person to me. The tip was not secured to the sesnsor rod and it fell off.

All of them have the same scale for moisture readings, 1 – 10. After calibration, recently watered soil will read 10 on this scale. Irrigate days later when the scale reads six. The expensive one lasts longer and can be used in more difficult soils, but it gives you about the same reading as the inexpensive one.

How much to water

How much water to apply or how many minutes on an irrigation controller requires a steel rod about three feet long. Use a 3/8-inch diameter steel rebar that is 3 feet long. They can be purchased at the major box stores for about one dollar. Shortly after the irrigation, push the steel rod into the wet soil in several spots.

This is what the steel rebar looks like if you go looking for it in the store. You can get it at any box store/hardware store. Get the 3 eighths inch diameter rebar and select one that's about 3 feet long. They will have them in various lengths. If you want to get fancy you can sharpen the end of it into a point on a grinder and bend the top over into a handle. But using it as is works just fine. Pushing this into the wet soil will tell you how deep the water has penetrated. It slips into wet soil easily but when it hits the dry stuff it's hard to push. Lawns and flowerbeds should be irrigated to a depth of 8 to 12 inches. Large trees should be irrigated to a depth of about 2 feet. https://www.homedepot.com/p/1-2-in-x-20-ft-Rebar-REB-4-615G4-20/202532809

            Steel bars slide easily through wet soil until they hit dry soil. Trees and large shrubs should have wet soil down to at least 24 inches. 12 inches is usually enough for most other plants including lawns and vegetables.

            Water long enough, or apply enough gallons, to make the soil wet to the desired depth for all the plants on that circuit or valve. If some plants aren’t getting enough water while others are, add more emitters to those that aren’t.

            The first two seasons you might have to measure soil moisture and use the steel bar five or six times to get a “feel” for when to water. But after the second year you will start recognizing a seasonal pattern to irrigating plants in your landscape and you will not need them as often.

I bought both the Lincoln and Reotemp moisture meters on Amazon. The rebar I bought at Home Depot.