There's an Art to These Things
As with most things in this life, finding the correct balance is everything. Life is art, and in art there are no absolutes. Believe it or not, this is true of drip irrigation too. Although the majority of emitters sold through most retailers are rated at 1 gallon-per-hour (GPH) that does not mean that 1 GPH is the correct rate for every application. Since the purpose of drip and micro-irrigation systems is to apply the optimal amount of water, at the most efficient rate for any given plant, it is important to know how to figure the GPH rate that works best for your particular plants, and that is all about finding the right balance. To address this, agriculture and drip irrigation design experts consider the "soil, plant, water" relationship when creating a new system. They must work to balance the sometimes conflicting needs of soils vs. plants to arrive at the most efficient watering rates. As you can imagine, in huge agricultural applications growers cannot afford to waste water, or invest in unneeded irrigation capacity…neither can you.

The first factor we will consider for determining the correct drip rate (measured in gallons-per-hour) is soil type. Depending on whom you ask, that can be twelve or more types! The US Department of Agriculture lists seven types, ranging from very coarse sand to clay. For non-commercial applications, most people condense this down to three key types: sand, loam and clay. Knowing the soil type is important because water moves, percolates, at different rates through these different soils, with sands having the fastest percolation rates and clays the slowest. A simple visual means of identifying the soil type is to note its wetting pattern. When an emitter dispenses drops of water, a distinctive wetting pattern is created on, and below, the surface of the ground depending on the soil type. In sandy soil, the water percolates downward very quickly, creating a small area of moisture at the surface, while clay soils tend to spread the water horizontally, creating a larger wetting pattern. You want to create a small, compact wetting pattern, so by watching the way the water spreads as it drips you will be able to see if you are applying too much or just enough. For the most part, sandy soils require drip rates in the 1-3 GPH range, while in clay soils a drip rate of just 0.5-2 GPH might be suitable.

"The Rest of the Story"
Now let's talk plant anatomy for a bit. The root zone of the plant is the area at the base of every plant that contains all of its roots and root hairs. The entire root zone is normally about the same size as the plant's canopy. The larger the root zone, the more water the plant needs for consistent growth. The root hairs are the thin filaments extending out from the main roots, and they are the part of the root system where water is absorbed. The root hairs of most landscaping plants are relatively shallow in the soil, residing in the top few inches. This may create a conundrum: you could find yourself with a plant that has a large root zone, which requires a lot of water, but on the other hand, the root hairs are shallow. So, if you are using traditional irrigation methods, watering at high flows, the water percolates right past the shallow root hairs and much of it is not used by the plant at all. A good rule of thumb to remember is that the top 25% of the root zone absorbs 40% of the water, and the bottom 25% absorbs only about 10%. One of the principle advantages of drip irrigation is that drip allows you to apply small amounts of water, at precisely timed intervals. This limits percolation through the soil, and creates a state of consistent moisture for the plant. The shallow root hairs absorb as much water as they need, when they need it. The result is greatly improved plant health and reduced water waste.

The Inner Truth
What does it all mean? It means that for the average homeowner, the best way to figure out what drip rate is right for what plant is to understand the basic root system characteristics of your plants, and know the type of soil they are growing in. For instance, a large plant with a large root zone in clay soil could use an emitter with a medium flow rate, whereas, the same plant in sand might require a high flow rate to accomplish the same thing. Finally, you need to consider time. In drip irrigation longer run times (the number of minutes the system actually waters), at slow rates, appropriate to the soil type, create an area of uniform moisture throughout the plants root zone, without ever saturating the soil with water. This is the perfect growing environment for plants, and minimizes water waste and evaporative loss.

Getting to this state of perfection requires a little knowledge, a little experimentation, and a little desire. It is as much art as science, but once these few factors have been taken into account it is easy to achieve just the outcome you need using standard 1 GPH emitters, by adding additional emitters, if necessary, to increase the drip rate for specific plants. If you prefer, you can buy higher drip rate emitters. Like every other part of life it boils down to finding your drip irrigation systems perfect inner balance…oohhhmmmm…