Watering Santa Rosa Plum in the Mojave Desert

Q. I read your gardening articles faithfully, and I just searched your blog for some help, but I would like more information.  I just purchased a Santa Rosa plum, but I am not certain how much I should water it.  I haven't yet planted it, and I want to make sure to give it enough emitters as it is located in the front yard which is mainly desert landscape plants and therefore do not require as much water as this will (I think). 

Santa Rosa plum

The tag indicated how much water in inches, and I wasn't certain how to convert that into gallons.  Also, although there is rock mulch in the front yard, I am more than happy to remove that in this are to give the plum the best chance.  Is that necessary?  Finally, your article on planting fruit trees from your blog mentions a "slurry" but I don't know what that means, and I wasn't able to find a definition on your blog.  I would really appreciate some help.  Thank you. 

A. Plums are not desert plants and should not be irrigated as if they were desert plants. Mixing desert plants with non-desert plants on the same valve can lead to watering desert plants to frequently or, the flip side, watering the non-desert plants not often enough. But this may be a moot point. In this town desert landscaping means surrounding any type of plant with rocks. This is kind of like calling tofu, meat. Just because people put rocks around plants does not make it desert landscaping but does give it the appearance of desert landscaping.

Hopefully your Santa Rosa plum will be on a valve that has other drip emitters on it. The first thing to do is determine your current run time for that valve on your irrigation clock; the number of minutes or hours that station or valve operates. It will be more difficult to irrigate your plum if the valve irrigates other things on it that are not on drip. Drip irrigation is designed to give small amounts of water over long periods of time, usually hours. Other types of irrigation such as bubblers or sprinklers are designed to operate in minutes, not hours. Mixing the two together on one the valve makes it very difficult to accurately apply water to either one.

I like to see true drip irrigation operate a minimum of one hour. Let's assume that the valve for your plum will operate for one hour. (Now I am going to give you some misinformation and tell you a couple of lies so that I can make my point. I will clear up the lies and misinformation later.) So, if the plum was purchased in a 5 gallon container, then make sure it gets irrigated with 5 gallons of water in that one hour. If it was purchased in a 15 gallon container, then give it 15 gallons in one hour.

As a tree in a 5 gallon container gets bigger, you will need to increase the amount of water applied to it in one hour to perhaps 10 gallons or 15 gallons next year. You can do this by adding additional emitters; perhaps a second one next year and the third one maybe three years from now. If the plum is really growing fast you may have to add emitters sooner. This is a judgment call.

Mature plums at the Orchard are getting 30 gallons every time they are irrigated. This 30 gallons is applied once every 10 days to two weeks during the winter to as frequently as twice a week in midsummer. As it gets hotter, the frequency of applied water changes, not the amount per application.

Let's say you now have discovered a problem. The valve which will irrigate your plum runs for 15 minutes, not one hour. And everything on that valve is designed to be irrigated in 15 minutes. This means we have to enter the dark side of irrigation; bastardizing drip irrigation to get it to do something it was never intended to do.

Adjustable drip emitter: the dark side
of drip irrigation.

Now we enter the realm of adjustable drip irrigation emitters. These emitters can be adjusted with flow rates of zero (shut off) to a lot. Oh, they like to say it is 0 to 10 gallons per minute but in actuality who really knows? I don't care what they call it but this is no longer drip irrigation. But in some cases it will get the job done and you may have no other choice.

Plants surrounded by rock mulch use more water than plants surrounded by wood mulch. Any type of mulch will help but rock mulch will get hotter than wood mulch and drive water use up. Some plants should never be planted in rock mulch. Your plum is one of them. Others include roses, lilies, Photinia, mock orange, heavenly bamboo and many others. It might be okay for a couple of years but I will guarantee its health will decline living in rock mulch after just a few years. Organic mulches like wood mulch will help your tree a lot. This wood mulch should cover an area not less than 6 feet in diameter around the plum tree and be at least 4 inches deep. Be sure to keep wood mulch a foot away from the tree trunk the first four or five years.

There is 7 1/2 gallons in one cubic foot of water. 1 inch of water covering one square foot is a little bit more than half a gallon, actually 6/10 of a gallon. Since rain is measured in inches of precipitation and sprinklers are also measured in inches of precipitation per hour there is a tendency to give watering advice in inches rather than gallons. They leave it up to you to determine how many square feet you are going to irrigate. A penstemon might be irrigated over an area of one square foot. A mesquite tree might be irrigated over an area of 300 ft.². This then translates into a different number of gallons.

A slurry is a mixture of water and soil the consistency of mud which can be poured. Think quicksand, smoothie. I hope this helps.