Proper irrigation can be difficult to figure out

Q: Could you tell me where I could find irrigation instructions in gallons rather than minutes or hours? I am having difficulty calculating how much to irrigate larger established plants and trees.

A: I am going to dance around this subject a bit because it is not a simple question to answer. Determining the amount of water to give larger plants is difficult . It is really more of a “guesstimate.” This is because we are forced to think about “minutes of water” by our irrigation controller, rather than in gallons, which is a volume.

I have a hard time recommending drip emitters for most plants that get much taller than 20 feet. I feel that too many emitters are necessary to provide enough water to the plant over a few hours. I would rather see plants this big in irrigation basins with bubblers .

Shrubs as tall at 10 feet, groundcovers, vines, perennials, flowers and vegetables are much easier to “guesstimate.”

To try to get a handle on how much water the plant needs, I ask myself : “What size container would be necessary to support a plant of that size?” When we start getting into big trees, the container needed is hard to visualize. The volume of water may be in the 30-50 gallon range or more.

Generally speaking, we want to add enough water under the canopy of a large plant so that the water penetrates to a depth of about 18-24 inches. Much smaller plants and turf need water only to a depth of about 12 inches.

We also want to distribute the water to at least half the area under the canopy. So, we need to use enough emitters to get the water distributed to an area that size.

Another way to irrigate large plants is to put other plants close to, or under the canopy of the large plant. This helps give the larger plant enough water through the irrigation of the plants under the canopy .

Still another method is to put a grid of “in-line” drip tubing to water an area and cover it with mulch. This is effective for watering small areas with a high density of plants with similar watering requirements.

In-line drip tubing for smaller areas is generally around ½ inch in diameter with built-in emitters spaced 12-18 inches apart. The drip tubing itself is constructed in a grid pattern and should be constructed in a way so it can be “flushed” about once a month.

Q: I have no idea if I am overwatering or underwatering. My red tip photinia plants are all brown and dropping leaves left and right. My sago is turning yellow. I have a drip system and it is set for twice a day, every other day, for 10 minutes. Is that enough?

A: Usually drip irrigation is measured in gallons per hour, not in the number of minutes it operates. The controller forces you to think in minutes, which causes problems . Each emitter operates at a specific number of gallons, or partial gallons, it delivers in one hour.

Drip irrigation does not need to come on twice a day. The only reason to operate an irrigation system twice a day, or operate several cycles in a day, is to reduce water running off or puddling .

The only times I have seen runoff occur is when people are using sprinklers, those so-called drip emitters that are variable with a dial you can open up, or have leaks or breaks. In true drip irrigation, water drips at a specific location so slowly that it enters the soil without running off to a different location. Whenever you apply water faster than the soil can take it in, you will get runoff .

With sprinklers or those so-called drippers, you need to have it come on several times just to keep the water in one spot. I will point out that there is nothing wrong with applying water in split applications, (breaking a watering cycle into several smaller ones). But you must give the soil a chance to breathe between these cycles.

With true drip irrigation it is better to apply all of the water needed in one application. Your drip irrigation should not be on the same valve as your lawn. Lawns should be irrigated separately from woody perennials such as trees, shrubs, ground cover and vines.

You will have to convert your minutes of irrigation to gallons. In your case, if your drip emitters are applying 2 gallons each hour, then the plant is getting one-third of a gallon every 10 minutes. Since you water twice in a day, this totals two-thirds of a gallon each time you water.

Will two-thirds of a gallon be enough for a photinia? No. A good-sized photinia might require five to 10 gallons each time you water. Of course, the larger the plant, the more water should be applied .

In this case you would change the emitters to deliver more water, add more emitters and/or increase the number of minutes. You should not just simply water more often.

Photinia have another big problem if they are growing in rock mulch. In rock mulch, they tend to yellow and begin to scorch around five years after they have been planted. The soil under the rock mulch has become mineralized and the plants cannot take up adequate nutrients to satisfy their needs.

Supplement these plants by adding a good fertilizer once or twice a year and an iron chelate to the soil once in late January or February. Use a quality fertilizer specifically made for trees and shrubs and add an iron chelate such as iron EDDHA to the soil.

Sago palms have other problems. If a Sago palm is planted in poor soils or in a very hot location with lots of reflected light, it can begin to yellow. It can yellow in the spring if we had very low winter temperatures.

If it is yellowing because of a lack of iron, then the iron chelate mentioned above will correct it. If the yellowing is because of watering too often, then you may correct it by changing your watering cycle and adding wood mulch to the soil surface.

Bob Morris is a horticulture expert living in Las Vegas and professor emeritus for the University of Nevada. Visit his blog at xtremehorticulture.blogspot.com.