Subsurface PVC Watering System

  • Posted on: 14 April 2022
  • By: David Trammel

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.


ClareBroommaker's picture

How about bamboo with joints burned out for pipes? (Just imagining how to continue such a system without plastic pipe factories.)

I'm satisfied with surface watering, though. I figure that imitates nature. Watering from above also helps carry air into the soil, which is needed.

mountainmoma's picture

I also surface water, I usually have deep mulch which helps to not have the quick evaporation that causes salt to rise up. My goal is to keep the soil moist, and I live in a very dry place in the non-rain months, which is most of the year. If I used drip I would end up like I was growing in pots. Pots of moist alove soil surounded by desert, better to have an area that is alvie, the whole garden

I have to take exception to this statement as it is not true in my experience.

"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."

I have used drip irrigation in my home garden and community garden plots for many years and the drip system doesn't work the way this author says it works. The water that the emitters produce seems to fall straight down from the drip line and as it accumulates, it forms a cone that spreads down through the soil in a deep cone shape; a narrow pointy end at the surface and a widening wet zone well below the surface of the soil. Plant roots have to reach down for this moisture.

When we plant seeds for our veggies, we usually plant them right under the drip line since they need water right around them to sprout. If we plant them further away from the drip line, we have to water by hand to get them started. Once they are up, the plants will find the subsurface water cone and will be fine. We usually plant transplants further away from the drip lines and we will hand water them for a few days to settle them, but they will also find the water cone and need no further surface water.

If you wish, you can bury the drip lines, but that makes maintenance very difficult and makes it much easier for damage to occur if you can't see the hoses. Granted, the drip lines are made of plastic, but will last for a long time with care. I have repaired them before with electricians tape.

There are a couple of writers who don't use drip irrigation; Carol Deppe who believed that under ground cone of water with the surrounding drier soil made a kind of pot that limited plant growth. She watered with sprinklers and felt that also washed the plant leaves off to their benefit. Steve Coleman advocated that you plant everything farther apart and not worry about irrigation at all. If the plants are not very closely spaced, then there is less competition for the available water and the plants will grow and produce just fine. I am not sure I believe him, but he used this system in eastern Oregon I believe.

My own experience has shown that burying the drip lines is a mistake and you will have trouble with them in the future. I think mulches are fine as they are easy to move aside to get to the drip lines if necessary or to plant. With a mulch, you probably don't need to run your drip system as often.

Just saying, your mileage may vary.

mountainmoma's picture

yep, I am a Carol Deepe fan. hand watering and surface watering is less expensive too and less plastics to buy.

I do know alot of people who garden with drip and enjoy setting it up each season....

I also love the beneficial plants next to my raised beds that get watered with the sprinkler, the herbs and flowers.

more than one way to garden. I also personally would not bury PVC in the garden beds, I would rather use drip. It seems having buried watering like that would mean I couldnt use the garden fork to loosen soil, I could hit it. And I dont buy PVC. With drip, you pull it aside off the bed if you need to loosen with a garden fork between crops

Sorry for the misspelling of Carol Deepe's name. I was too lazy to get up and look at her book to get it right.

Since the garden fork is my favorite digging tool when I have to dig one of my beds over I just move the drip lines to the side then move them back when the bed is ready. I don't have to worry about putting a hole in my irrigation system.

David Trammel's picture

MM, you make a good point about potatoes and root veggies being hard to fork up. I haven't planted many of those until now, and when I did it was in containers. I do plan several smaller beds for those types of plants so I guess the best option would be a single tube buried into the soil, which has holes for distribution drilled at a starting depth of 2-4 inches, alongside each plant.

The concept would be similar to the water bulbs you see used now for plants.

Sweet Tatorman's picture

The OP writes "I am not a big fan of drip irrigation". I am in the opposite camp as I find it the easiest and more economical way to irrigate on the scale of my garden which is on the largish side for a home garden. I use thinwall drip tape with emitters spaced at 1 foot and the closest two parallel runs of tape are placed is 2 1/2 feet, thus only one point of watering for every 2 1/2 square feet. With the exception of the top inch or so between lines I am confident that the entire volume of soil is wetted. The degree of spreading of the downward plume of water does depend upon soil type and application rate. I would attach a diagram below to illustrate this but that forum function does not appear to be working at the moment. In my case I would classify my soil as a silty loam. My single emitter application rate is ~0.25 gph. The OP shows a diagram of soil wetting with very small amounts of water applied daily. I doubt that many people experienced with drip irrigation water in this pattern. Even during periods of highest evapotranspiration during a droughty period I typically would water only twice a week but may apply the equivalent of an inch or more averaged over the entire surface area.
For my situation I can see only downsides to burying drip tape. I do use mechanized tillage which I understand that many/most home gardeners do not. I also do not mulch as I have no need for the moisture retention benefits that it may offer as my supply of water is free and unlimited.

I use the plastic drip tape too because it is easy to use, reasonably durable and best of all, inexpensive. The community garden graduated to a more expensive product called Netifim which is a stout brown plastic hose that is much more resistant to the careless use of garden tools by novice gardeners then the tape. The one bed I think I would like to use it in is my raspberry bed, once I can get the grass out of it.

David Trammel's picture

Kay, let me reply to some of your comments. I've been rushing around here trying to get the garden in and neglected the site.

First, this is going to include some half-formed thoughts, and personal perceptions so I'll just say that I am open to counter-arguments. It wasn't until your comment that I realized that I just don't like mulch.

I know everyone and their brother/sister preach of the wonders and near-religious experience they went through once they started using mulch but something has always felt wrong about it to me. Maybe it's the small-scale of gardening I'm used to. Maybe it's just the excessive amount that gets spread on just about anything that grows out there now. Maybe it's just an unease anytime big business jumps on a trend. We literally have a whole outside wall at Menard's stacked to the roof with bags of mulch, in every conceivable shape, size, and now, color.

Why do we put mulch down?

To keep the soil moist? So we spread an absorbent medium down onto the soil, then soak that 1-2 inches of material so that the water that does get through the mulch and into the soil won't evaporate? So the water we do soak into the mulch will evaporate instead? Water is one of the resources we expect to get harder to access in the Long Descent, can we adopt or promote a method that assumes that we just let a percentage evaporate into the air as a waste?

BTW, do you place your drip irrigation line above the mulch or below it?

To keep out or cut down on weeds? So we provide a decaying organic layer under our plants, with a loose surface that will easily catch and hold seeds dropped by birds or the wind?

To protect plants from insect pests? Again, we provide a decaying organic layer under our plants that insects and pests like slugs can hide in?

To feed our plants? Isn't mulch a pretty one-sided type of organic material? Why not feed our plants with well-made compost instead?

Maybe I just keep getting the feeling that mulch is a solution looking for a problem, And not a good solution as it is.

I feel like having an easily viewed surface under my plants makes it easier to spot pests, that I can amend my soil at the start and end of the season better, and that I can avoid waste of water, by irrigating under a few inches of soil, than under a few inches of a material I have to outsource.

I look forward to you pointing out counterarguments and things I missed or misunderstood.

I use mulch from time to time and for more then just one purpose. True there are problems with mulch and the biggest one for me is that it provides shelter for those insects, sow bugs, earwigs, slugs, that want to eat my seedlings, so I don't usually use it until the plants are big enough to survive a few insect bites. Lately our winters have been on the mild side and we have not used it.

Mulch will conserve soil moisture here in a high desert climate like mine even with a rather thin layer, but I think you have to balance that with the risk of insect damage. I also learned that fresh grass clippings can provide nitrogen to your garden, but I think that might only happen if you get more rainfall then I get. Now days, we use diluted urine for a nitrogen boost. Yes, you have to collect your own for that purpose. I use a bucket.

I don't use mulch to suppress weed growth as I plant everything with close enough spacing (John Jeavons method) to provide shade mulch as the vegetable plants get bigger and shade out the weeds. Shade mulch also conserves moisture as well. Naturally I have to do hand weeding until there is enough shade but by mid to late summer, I only have to weed the paths which don't get water and are pretty hard packed so are not much of a problem.

The one thing that we don't get from using mulch the way we do is a break down of organic matter for soil health. Our community garden guidelines urge that at the end of the year we cut all of our spent veggie plants in to hand size pieces to return matter to the soil over the winter. This has not worked well for us at all. Our mild winters present us with a pile of tough, tried out plant material that we now have to rake out of the way to plant anything at all. I can promise you that it won't break down during the summer either and is more of a nuisance then a useful mulch as by hand, we just can't chop it fine enough. We are just too dry here and chopping things fine enough without a husky garden shredder isn't practical. Also, if you want to make your own compost here, you have to water it.

What works pretty well in terms of reusing garden debris for mulch is to truck it to the one municipal dump that does a really great job of composting and come back with a load of really fine compost to work into our garden beds.

I have looked into ollas, or buried unglazed clay jars, but they are expensive and have the problem of damage if you are digging around them. I have found several hacks to keep the cost of these jars down. I haven't tried this, but since you already use something similar for your potted plants, it might work well in your smaller beds.

I think there is no one way to mulch, irrigate,or garden regardless of how a particular method is promoted as the best thing since sliced bread. Every space is different and every gardener does it differently. It seems to me that gardening is an art form and it is great to see what other people are doing and how they are doing it as it just might give you an new idea or provide and answer to a problem you are having.

Happy gardening this spring.

mountainmoma's picture

When I mulch I do not need to use compost because the mulch composts in place. By leaving it straight ont eh beds or under the trees, I get alot more bang for the buck so to speak. It is like I have thin compost piles all over. And, we all know, the best soil is directly under the compost pile. Look up sheet composting or lasagne composting. But, you do not need to be so formal about it. It does not need to be so many layers. Sometimes it is just store bought alfalfa if you do not have any mulch material built up yet, think Ruth Stout gardening method, the lazy gardener. So, the mulch composts in place and feeds the soil. One of the ways it does this is earth worms, if you feed the earthworms, they poop, and earthworm manure is very very good for your vegetables. Besides leaving earthworm manure, the worms dig and turn over and loosen the soil for you. I find this very convenient and time saving. I also sprinkle on wood ash and bits of charcoal from wood fires too. If my mulch materials are not alfalfa, which it usually is not, I also add watered down human urine, this adds nitrogen to help break down mulch materials and feed plants. Not alot. When I made separate compost piles, I would have to work to keep it watered and large enough to break down and at teh end there would not be as much material to use over as large of a garden as I have. I still have abit of a compost pile that I ignore and it eventually degrades. But, usually it is better if I tuck that stuff under the mulch by a fruit tree.

Downsides of mulch is slugs and pill bugs. So you have to pull mulch away before direct seeding or from young transplants. Sometimes it helps hides seedlings from the birds, in that case, I get extra dry mulch materials like stems to lightly tent over the seedling to disguise them from the birds watchful eyes.

SO, look up ruth stout for teh queen of using nothing but straw mulch in the garden. I have seen videos of someone who only used woodchip mulch, but I think that might take longer for the first good year.

I would never buy bagged mulch from a big box store, that is not what I call mulch, I believe that is shredded bark and isnt it dyed ? . The weeds you pull should be laid sideways on top of the soil as mulch. The leaves you rake up from teh front yard in teh fall should be put on top of all garden beds, take your neighbors leaves too.

lathechuck's picture

Regarding moisture management, I think that a good mulch is not dense enough to wick moisture up out of the soil, nor to block rainfall from tumbling through it down to the soil layer. It prevents solar heat from baking water out of the soil, too. A mulch layer should prevent particles of soil from splashing up on the vegetables, which means less washing of the greens, and less chance of soil diseases attacking leaves and stems (e.g., of tomatoes).