I know you've seen them. Little caps on the end of vertical irrigation pipe that can be adjusted to control flow with a twist (or screwdriver). They are called flood bubblers and are used widely in the Eastern Sierra to deliver water to woody plants outside of a lawn sprinkler system.
Unfortunately, I see more tree problems caused by this type of irrigation than any other single cause of tree death in our area. Many dead or sick trees in our area have been irrigated (insufficiently) by flood bubblers.
What are bubblers for?
Bubblers have a few important uses and benefits. They are good at watering young trees during establishment since the water is concentrated near the root ball. In the right types of soil, they can also be good at watering shrub beds or large tree wells if the output can exceed the soil's infiltration rate. They flood the soil with water. When there is a berm or wall around the bed, and if you can fill it, bubblers are a fine choice.
In general, as a source of water there really is no problem with their use. They are cheap and low maintenance. The lack of spray helps to prevent water lost to evaporation while they are running.
The issue is how we use them locally over the long haul.
Bubblers are not drip systems
The purpose of a bubbler-type emitter is to deliver a high output rate of water to a concentrated area, like a single, young tree. I often see one bubbler per tree. The shape of the bubbler takes away some of the kinetic energy of the flow and allows a gentle flood that hopefully will not disturb or erode the soil.
Many bubblers here are used as a substitute for a drip emitter. This is the beginning of the problem. Bubblers are designed to deliver about 2 gallons per minute, whereas a single drip emitter is usually closer to ½ gallon per hour.
Instead of filling the root zone with water, only a small amount is delivered frequently, often several times a week in that same small spot. In practice I mostly see bubblers used to irrigate trees and shrubs outside of a lawn.
There are two issues that arise with this practice:
- Trees grow! And as they grow their water requirements and root area increase. You should be aiming to water the whole area under a tree, not just at the base. While it's possible to crank up the flow to deliver more water, this doesn't solve the problem of having an insufficient area irrigated under the tree.
- Most of the Eastern Sierra has coarse, well-drained soils. In our soils, the frequent, low-volume application of water only saturates a very small area much smaller than is needed. This constrains a tree's root system and eventually leads to chronic stress.
Drip systems are designed to deliver a predictable, slow rate amount of water from each emitter on the line. The key word is each. Drip lines usually incorporate many emitters to deliver water slowly to the root zone. A properly designed drip or microsprinkler system can irrigate the entire region that needs water.
Perhaps if additional bubblers are added to a row of trees along with more output from each bubbler, things wouldn't be too bad. This, however, is not how they are used.
What to do if you already have bubblers
It's not the end of the world if you have bubblers in your landscape, as long as you don't wait until your trees or shrubs begin to die to address the situation. The first step is to recognize that flood bubblers are not drip emitters.
The simplest solutions involve swapping the bubbler head with something different.
An easy, but perhaps less water-efficient, change you could make to your system would be to replace the bubblers with spray heads after the trees are established. Spray heads come in a variety of patterns, and some can be adjusted. These are an easy way to increase the coverage area. The will heads have the same threading as your bubblers. As long as you can deliver enough flow and pressure, this conversion works well. Of course, you'll need to alter your irrigation schedule.
Because you will now be hitting the trunks with water, which is less than ideal, adjust your system to deliver more water each irrigation, but less frequently. This will allow bark to dry.
If the line of plants is long or irregular, another approach could be to replace the bubblers with drip conversion heads. These will supply several connection points for ¼” drip line. At the ends of each you can either have drip emitters or microsprinklers. If you only have a few trees consider installing a clever set up called a T.R.I.C. (Tree ring irrigation contraption).
Either way, the idea is that you switch to covering a larger area with more water but less frequently.
Alternatively, you could open up the bubblers' nozzles to allow more output, assuming you can deliver enough flow to overcome the rapid water drainage of our soils and cover the root area. That may require installing some sort of containment. In most situations, I'm skeptical this can work.
In landscape beds and containers, especially in commercial situations, bubblers make some sense. The area to irrigate is usually small, and there is almost always some sort of impermeable “wall” around the bed that confines growth. This is likely their best use case. When there is pavement covering a substantial part of a tree's root system, such as a parking lot, this may be your only means of providing enough water.
Final thoughts
In theory, there is nothing wrong with flood bubblers when they are used as designed. Our local issues arise when we try to use them as an alternative to a real drip system in coarse, desert soils on mature trees...forever.
To determine how much water to provide trees and shrubs in the Owens Valley, there is a handy fact sheet available at this link. Your goal with tree and shrub irrigation should be to fill the root zone with water, but no more than, and replenish it when about half that water is used by the plants.
Don't assume that because of the prevalence of flood bubblers in our area that this is the best way to irrigate landscape trees. It's a useful tool for young trees and containers, but not usually enough for larger, established trees.
If you have questions about replacing your bubblers, contact the Inyo-Mono Master Gardeners at immg@ucanr.edu.
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