Q. The landscape people are into variable emitters and think I am a little looney sticking to drip emitters.  How do you determine how many gallons to give large trees — like African sumac, bottle trees, Swan Hill olives, Yew pines and chaste trees? A. There are three things we have to consider when watering; how much water the plant needs, how frequently it needs it and the time of day when we will apply the water. All this needs to be entered into the irrigation controller. All plants of a similar size will be watered with a similar amount of water. Larger plants require deeper watering and more applied water In your example, you have two different categories of trees to consider; the desert trees like bottle tree and chaste tree AND non desert type trees like the African sumac, olives and yew pines. If they are all the same size then they will get a very similar amount of water. The difference in irrigating these two types of trees is the FREQUENCY or how often they are irrigated. True desert trees can be watered less often and should be. The nondesert trees can be watered more often.   If watered too often, many desert trees can have problems. If the soil drains of water freely then the usual problem we see is luxurious, unrestrained growth. Have you ever had a neighbor or friend brag, “My mesquite tree last year grew 8 feet! (i.e., I must be an extremely good gardener if I can get a tree to do that!). Well, my friend, it just means it is getting alot of water. Most desert plants respond to excess water by putting on lots of growth! As these trees get larger and larger, they will demand more and more water.   The smaller plants are easiest to do. I like to ask people, “What size container would you use to grow that plant?” Some people can visualize this while others have a hard time at first. Just think of the plants you see at the nursery. Generally speaking, I like to make sure the plant is getting at least half of the volume of its container to a maximum equal to the size of its container. It is better to estimate too much than not enough.  Different sized nurseery containers   (Disclaimer: nursery containers do not hold their namesake. For instance, a five gallon container DOES NOT hold five gallons. I know its dumb, but use its namesake anyway.)   You will apply all of this water in one hour. So if you need to apply five gallons of water, it will need to be applied in one hour. So the total amount of water applied to the plant would be five gallons per hour.   Then there are the types that are variable and can be twisted open to give you more or fewer gallons per hour. You have to figure make the conversion and convert it yourself. It is not terribly hard to do, particularly in drip irrigation. Each emitter is labeled or color coded to the gallons per hour that they emit “Flag” type drip emitter. Usually the flag allows you to turn it on or off, you can usually pull it to clean it, and the color refers to how much water it emits per hour. They are very simple to use, clean, inexpenive and accurate. I have used them in simple drip systems with no problems for over 20 years. There are also many others depending on your needs.   The hardest emitters to figure out are the types that can be adjusted to different amounts of water. Many of these are adjustable between 0 (shut off) to 10 gallons per hour. It seems simple. You just twist the emitter open and it delivers more gallons per hour.   I am sorry to you out there that like these emitters. I do not share the same feeling. When I see them, I just cringe. This is a variable output drip emitter that varies from “completely closed” to “I have no idea”. Landscapers LOVE them. Of course! It doesnt require any knowledge and no design is needed! Wonderful option for the ignorant. But in actuality it begins to defeat the purpose of drip emitters: precision. Also many of these variable flow emitters are not pressure compensated. If it is not pressure compensated, then opening one emitter and allowing more gallons to flow can affect the number of gallons flowing on all the other non pressure compensated emitters on the same line. This can mean you have to twist open or twist close each emitter along the same line perhaps multiple times to get the flow that seems to be appropriate.   Not only that but these variable output emitters frequently emit so much water so quickly at the higher settings that it results in water puddling and running off to low spots. This is exactly contrary to the reason we should use drip emitters. So you can see that I am not terribly fond of these types of emitters. With many different types and sizes of plants along the same line the next difficulty for most people is to figure out what size (gallons per hour) to match up with each plant along the line. So this is how I do that.   The first thing I do is determine how many hours or minutes the valve will be left open for watering. Frequently, for drip irrigation, the shortest time you should use is one hour. “Yikes” you might say because most people want to irrigate fifteen or twenty minutes. The problem with these short irrigation times is that it may force you to use the variable output emitters. Or it results in water applied so rapidly it does not penetrate the ground and instead runs and puddles somewhere it is not supposed to go.   Assume a minimum of one hour for the irrigation time. In some cases you might water for two or three hours on