Subsurface PVC Watering System
I am not a big fan of drip irrigation.
It's hyped on just about every garden channel or website as "THE" perfect solution to getting water to your plants but for me, it seems expensive and does the job wrong. Putting water on the surface invites weeds and is wasteful, as wet soil evaporates the moisture you want into your plants. Wet moist soil on the surface is not what you want because it can breed mold, insects, and a variety of plant illnesses. And then, watering the top few inches of the soil only doesn't encourage the deep roots your plant really needs to survive hot weather or windy conditions.
A better idea would be to get the water down several inches before it touches the soil and roots. And to soak it well. Then, provide a period where the upper soil dries, forcing the roots to burrow downward seeking the left water. In this post, we will look at a way to do that. A way that is easy to make and the parts are cheap to buy.
Let's get started.
The Science Behind The System
Good soil is porous. It has lots of organic material in it and allows the oxygen and nitrogen that plants need to filter into it. Good soil also retains water. When it rains, or when you spray it down with the hose, the water soaks into the soil and is retained. Unfortunately, the surface is exposed to the Sun. It will begin to dry out as the moisture evaporates. Plants have adapted to this by developing deeper roots. These taproots then seek out the remaining water in the deeper levels of the soil and bring them up for the plant to use. This action also helps to keep the plant upright in windy conditions. This is healthy for a plant and we want to encourage it.
The first diagram, "Typical Rain Cycle", is a rather simple example of how this process works. When it rains, the entire soil structure is made wet. Then as the week goes by, and the Sun evaporates the surface water, the soil becomes dry and the remaining water is only available at deeper and deeper levels. Water slowly depletes in the sub-surface reservoir from plant action.
In daily drip irrigation, the hose system deposits a small amount of water at the plant base each day. This feeds the plant via the surface root system but doesn't penetrate very far. Not like a deep soaking does. It also wastes water because a percentage of drip irrigation is subject to evaporation.
Both cycles do encourage weeds and plant competitors. As a gardener knows, moist soil is the perfect medium for the wind-born and animal-borne seeds of weeds carried into our beds to sprout. Drip irrigation though, provides that perfect condition all week, though I will admit it just does it around the plant base. Not the best place to encourage it though. A dry and uninviting surface will decrease your weed problem, but how to do that while still providing for your garden plants?
How about we water our plants with man-made irrigation, not on the surface, but several inches down instead?
As you can see, the water we introduce between rain cycles should never reach the surface. At a depth of 4 to 5 inches, any wicking by the water should be minimal. Watering at this depth replenishes the sub-surface reservoir and feeds the plant without promoting weed growth.
The right picture shows the irrigation loop sitting in a dug out trough before it is covered. Once the excess is pushed back, and in this case, I added 2-3 inches of topsoil bought from the local store, you should get the desired depth. Why did I buy some soil and not just dig it out? I actually added quite a bit of organ material last Fall to these beds. This would have been the ideal material for weeds to grow in. By adding a few inches of cheap soil, I made a rather poor nutritious layer and saved some money. When I put my seed starts in, I will dig a small hole for them, and allow their roots to get started in that rich layer while giving any weeds a poor place to get started.
Ok, now that we have looked at the why, let's look at the how.
Building A Sub-Surface Irrigation System
We're going to be using 3/4 inch regular PVC pipe and fittings. This is a common type of plumbing and irrigation material because of its durability, ease of use, and assembly, as well as its cheap cost. You can get it just about anywhere.
Note: Looks like I goofed up and didn't get a picture of the parts separately. I swear I took one, but don't see it in my camera folder. Since I need to complete the other beds as soon as it stops raining here, I'll get one then and post it to this thread. Until then, here are the parts on the completed loop.)
These beds are 3x4 feet. Since they are two beds attached, you'll need to put a watering loop into each. Larger or different sized raised beds or ground garden rows will take different configurations.
Cut the longest parts first. PVC pipe comes in 10-foot lengths, so use the off fall to cut your shorter ones out. You do not need to use glue to put this together. The couplers and pipe will press together snuggly. I do take a hammer and tap the entire thing together to firmly seat the pieces.
A couple of questions, and answers.
1) "Why do you have three long pipes?" - I'm using a modified "square foot garden" placement for my plants. This method subdivides your bed into 1x1 foot squares. This bed will have 12 squares in three rows. I wanted to have the water come out under or next to each row. So I made three watering lengths. Here is a picture of the squares with their dividing twine. After I got the loops into the ground, I put I-bolts into the sides of the metal beds and strung nice bright orange through them.
You can see how high the soil level is now that I added more fill dirt on top, in the second photo.
2) "What are the two pipes sticking up for?" - Those two pipes are how we fill the loop and introduce water into the ground. I put two, on opposite sides of each other, so that air has a way to exit the loop. The other pipe also gives me a good gauge of how much water to turn on. When I fill, I turn the hose on until I see water coming out the other pipe, then back off just a little. This tells me that all of the water from the hose is going into the ground. Too little pressure and all of the water may just soak into one side of the loop. You want the entire bed to get soaked.
You can gauge how much water you are adding, and how long to run the hose if you do a test at the start. Put the hose into the fill tube, turn the hose on until the water is coming out the other end, THEN take the running hose out of the fill tube and put it into a five-gallon bucket. Time how long it takes to fill the bucket. Divide by 5 and that will tell you the gallons per minute (total seconds divided by 60, divided by 5). Keep a track of the amount of water you add and what dates you add it. You are keeping a garden record book, aren't you? Increase the amount or shorten/lengthen the time between watering as you observe the plants, or if you get rainfall.
One of the nice things about using 3/4" pvc is that the size of the inside of the couplers is just big enough for the tip of a common garden hose. Be sure to check though, you may have a larger hose. Nothing worse than trying to retrofit this system once you have it all in the ground.
Once you have the loop finished, turn it over and drill 1/4" holes approximately 2" apart all along the underside of the loops. This is how the water will get out and into the ground. I did put holes in the end pipes, as you can see in the left picture. I might not do that in future loops. I don't think it matters very much, it just means the ends of the beds will get a little more water.
When you get the drain holes finished, turn the loop over and set it on your bed. Scoop out the dirt along where you want to bury the loop, then put it into the depression and cover it. As I said, I added a few inches of fill dirt because the bed soil was very high in organic material and all those sticks and twigs made digging down hard. More decayed and loose soil would be easier to dig into and place the loop. Either way, that is about it.
The cost is about $20 or less. Lengths of 10-foot pipe are less than $5 each. Fittings are less than a dollar each. The thing will last years if not longer. I have several that have been in the ground a decade and when I dug them up recently they showed little to no wear. I don't think you could get this from a drip irrigation system. Nor match that price.
Consider going low-tech for your beds. Your plants and your pocket book will love you.