This Question on Drip Irrigation Made My Head Hurt!

Q. I have a LOT of different shrubs and trees on only 2 valves. I don't know how to water them.
The shrubs have 2 drip emitters for each shrub and they range from 2 each at 2gph (gallon per hour; gallon = 4 liters) for plants like photinia, red autumn sage, fountain grass, jasmine vines, rhaphiolepsis etc.; 2 each at 4gph for euonymous, abelias, boxwood, honeysuckle vines etc. The trees have more emitters. My landscaper told me to water 6 days per week for twice for 20 minutes each day (equals 4hrs/week). The nursery people advised me to water only 3 times each week for 45 minutes each day. Another nursery advised 6 days each week. So, what to do?

My biggest problem is my 8 year old magnolia tree, trunk diameter about 5", has 4 each a t 4gph emitters. It used to be very full, now it's about 10 ' high; the leaves are dark brown, dry and falling off.  All help would be appreciated.

A. You even confused me! In their defense I would have to say it's possible that all three could be right. Because plants are adaptable to different situations there can be several right answers to one irrigation question. Nurseries are there to provide service, the best answers they can muster up.

I am an educator so let me take a stab at it from an educator's point of view. I would like to give you enough information so you can solve your own problem with irrigation.

But in my opinion, two valves are not enough to give you the flexibility of different watering schedules with all your plants and different microclimates.

Bear with me on this. Let's all agree for the most part that as plants get larger they will require more water. Let's also agree that large plants will use more water, and considerably more water, than smaller plants. The larger the plant, the more water it needs and should receive.

Three irrigation valves. They are basically on and off switches for water.

Irrigation valves are basically an on and off switch for water; when the valve is open, water flows. When the valve is closed, water stops flowing. Since you have one valve in the front and one valve in the back, these switches open water to all of your plants in the front at the same time and the valve in the back does the same for plants in the back.

There are three basic questions that must be answered when irrigating;
1) how long to water,
2) how much should be applied, and
3) when to apply it.
The valves basically solve the question how long to water (on/off).

Button type drip emitter

Flag type drip emitter.

The drip emitters solve the question about how much to water. An irrigation clock answers the question when/how often to water. The irrigation valves allow water to flow a length of time and the emitters determine the amount of water applied to each plant during that time. The length of time the valve is open combined with the size of the emitter determine the amount of water delivered to each plant.

This is where the confusion begins. To make it as easy as possible to irrigate let's hold one of these variables constant. Arbitrarily, let's hold the length of time the irrigation valve is open: one hour. 
Just for the sake of argument. It could be 30 minutes, it could be 90 minutes, but let's just hold it at 60 minutes.

If we make this decision first, how many minutes to open the valves, it can make our other decisions much more simple. So we now agree the valve will be open for 60 minutes for our drip emitters. For me, this is a common length of time to leave the valve open for drip emitters.

To determine how much water each plant will get we have to size our drip emitters; the gallons per hour we want to use. Because of plugging, it can be dangerous to give plants only one emitter. If that emitter plugs, chances are we will lose the plant in a short period of time during our extreme summer heat.

Three different button drip emitters color coded for three different amounts of water per hour (gallons or liters per hour)

To determine how much water to give the plant at each watering (or when the valve is on) we look at its size. The smaller plants of course require less every time the valve is on. So for the sake of argument let's do this. Let's give a plant 1 gallon of water every time the valve is open (in this case one hour) for every foot of its mature size. Example only!

• A very small plant may get 1 to 2 gallons. 
• A medium-sized plant may get 3 to 6 gallons. 
• A large shrub may get 8 to 15 gallon every time it's watered.

The larger the plant, the more emitters it will need under its canopy.

• A very small plant may require one to two emitters. 
• A medium sized plant might require 3 to 4 emitters. 
• A large shrub might require 6 to 8 emitters. 

So now you will take the number of gallons you are giving this plant and divide it by the number of emitters you will provide for each plant. When you do this, you will determine the number and size of the emitters you will give to each plant. So for instance a medium sized plant may get 3 to 6 gallons at each watering delivered by 3 to 4 emitters. So the size of the emitters might be 1 to 2 gallons per hour. But I would keep all the emitters going to one plant at the same size. 

 

It doesn't make much difference if it's one or 2 gallons more than you calculated. What is important is that you apply enough water during one irrigation to water the entire root zone of each plant (plus a little extra to keep any salts in our city water flushed out of the rootzone).

This is a really bad picture of the rootzone of plants. They dont grow like this. But it does demonstrate that you need to water the entire rootzone each time you water.

So now we have answered two of the questions; how long to run the valve and as a result of that how much water each plant will get because you have selected the correct size and number of emitters.

The next and last question is probably the most difficult to answer. Remember, you have elected to set the time that the irrigation valve remains open to 60 minutes. This is constant. This does not change ever. Every month it is the same.

If you are even tempted to change the number of minutes to 30 minutes or 10 minutes or 90 minutes, slap your hand. Slap, slap, slap. When you are changing the schedule of your irrigation system, the only thing you change from this point on is how many days per week or per month or every two weeks the valve comes on. Now raise your hand and repeat after me. I (state your name) promise to never change the number of minutes on my irrigation clock ever, ever again.

Generally speaking, and this may be modified because of the type of soil you have in your landscape, you'll irrigate about three times a week in Midsummer, twice a week in the late spring and early fall, once a week in early spring and late fall, and once a week to every 10 days during the winter. The irrigation schedule I gave you above could apply to most fruit trees.

Exceptions. Exceptions would include cacti and some succulents and true desert plants. True desert plants should be on a separate valve or valves. Lawns, bedding plants and your vegetable garden should be on separate valves as well but generally lawns, bedding plants (flowers) and annual vegetables are irrigated similarly...not the same...but similarly.

Here are opuntia type cacti in production for fruit (tunas) and pads (cladodes). They get watered every three weeks in midsummer (115F; 45C) and produce well.

Now your Magnolia. Your Magnolia is in deep doodoo. It is not supposed to be here. It is supposed to be in San Diego, Louisiana, East Texas, Mississippi, Alabama, Georgia and South Carolina. If you look around Las Vegas there are no old magnolias. It doesn't like it here. That being said, it is here and what are you going to do about it?

One of my old axioms states,
“Plants that are not desert-adapted but which are planted in the desert, will require more time, energy and money to maintain them than desert-adapted plants.” 
Your Magnolia tree will progressively require more and more water as it gets older and larger. Besides struggling with our soils and our climate, the tree is simply not getting enough water.

I would recommend that you either put a donut shaped moat around the tree and use either more emitters or different emitters so that water fills the moat each time it's irrigated or you switch the tree to a different form of irrigation that will deliver more water in one hour (such as a bubbler). When you switch to a different form of irrigation that applies more water in one hour because of your valve, you will have to retain the water around the tree or it will run off the landscape and not wet all of the roots.

Here is a basin around a fruit tree. The basin is being filled with a hose used to adjust it so it is level. A bubbler (near the hose) is used to fill the basin with the irrigation system.

Some people recommend a coil of drip irrigation tubing with built in emitters every 12 inches. This type of tubing can be easily connected to existing main drip irrigation lines. They operate at similar pressures to your existing drip emitters. These will need to be flushed once a month so make sure you add a flush valve or flush cap at the end of this line.

Remember that all drip systems need to operate at the correct pressure and must be filtered. So make sure you have pressure regulators and filters on the system. Make sure you can flush the ends of the lines once a month.