Self-Watering Container (aka Subirrigated Planter) 101

[HOMEGROWN.org]

The following 101, on building your own self-watering container (aka subirrigated planter), comes from HOMEGROWN member Anne, an Ontarian who says she doesn’t really know how to garden. We beg to differ! You can get to know Anne in Meet Your Neighbors, and you can follow more of her kitchen and baking adventures on her website, Food Retro. Thanks so much for sharing your planter prowess, Anne, and please keep the good ideas flowing!

 

So you want to garden in an urban environment, eh?

 

I live in southern Ontario and, like lots of people in urban and suburban areas, I don't have much space that's appropriate for gardening. You may think, Hey, Canada is supposed to be cold, but the truth is for a short time it gets very, very hot: upwards of 42C and higher on some days (110+F). This is a major challenge for container gardens and, yes, all of the three plants I planted last year in a regular container garden, as well as most of the lawns in the region, died horrible deaths due to insufficient water and scorching soil temperatures.

 

So, after a bit of researching, I came across the concept of a self-watering container, also referred to as a subirrigated planter, or SIP for short. I found both the EarthBox (idiot proof but costly!) and a hundred how-to guides on making your own cheap SIPs. The problem was, those are usually made with plastics—something I wanted to avoid. And the ones made out of Rubbermaid tubs are even worse, because the tubs aren’t food-grade plastic. To leave those tubs sitting in the sun, leaching chemicals into the soil and water for plants to wick up?  That’s flirting with disaster!

 

Then I found an SIP made out of cedar on The Family Handyman. I ended up tweaking the plan a bit and filling in some of the informational gaps: nontoxic sealants? Soil? Fertilizing?

 

Long story short, it was a bit more expensive, but it’s large, lovely, durable, and best of all, it’s been RIDICULOUSLY successful. It makes even a novice gardener like me look good.

 

IMPORTANT STUFF YOU NEED TO KNOW UP FRONT

1. This planter is very large, approximately 6’x3’. Think full-sized raised bed rather than bucket. It will be heavy even before you add soil and water. Make sure that the area where you intend to put the planter is level and can support its weight. 

 

2. You cannot put dirt in this planter.This planter requires a soilless potting mix, sometimes known as pro-mix or sterile mix. You can read more about the differences between potting soil and potting mix in the Container Gardening 101.

 

3. You’ll need to plan any vertical supports for plants in advance. If there’s one thing that is definitely on my to-do list for next year, it’s figuring out how to support my sprawlies better.  Extremely tall plants and vines with heavy fruit or vegetables need special consideration because soilless mix is not your traditional heavy dirt. A single stake may not be sufficient to aid the root structure in supporting the full weight of a tomato or cucumber vine in high winds or rain. Learn from my fails.

 

4. Consider your choice of plants carefully when planning.Because you are working in a planter that is nearly 2 feet tall, if you use traditional straight-up staking with an indeterminate plant, you will find yourself needing a stepladder to reach your tomatoes—and you’ll probably run out of stake long before you run out of summer.  

 

WHAT YOU’LL NEED

TOOLS

• Tape measure

• Drill with bit for deck screws

• Level

• Square

• Jigsaw

• Hacksaw

• Stapler

• Clamps

• Utility knife or shears

 

MATERIALS

• Six 12' cedar 2x6s (for the sides and ends)

• Three 12' cedar deck boards (for the planter floor)

• Two 10' 2x4s (for the top cap)

• One 8' 2x4 (for the joists)

• Four 8' 2x2s (for the cleats)

• 24' of 4"-diameter perforated drain pipe with sleeve

• 6’x10’ pond liner (Fish-safe EPDM or rubber, not PVC. If it’s not safe for fish, it’s not safe for you!)

• 1 pound of exterior decking screws (2.5”), give or take a handful

• 1 foot of regular-size aquarium tubing (for drainage)

• 1"-diameter copper pipe, approximately 1.5’ to 2’ in length, depending on floor depth (for the fill tube)

• 1 pipe strap large enough for the copper pipe

• 8 to 10 large 1/16” washers

• Landscape barrier cloth

• Soilless potting mix (see “What is Soilless Potting Mix?” below)

 

WOOD CUTTING LIST

Each item in the list below includes quantity, dimensions, and intended use.

• Eight 1-1/2” x 5-1/2” x 33” (ends)

• Eight 1-1/2” x 5-1/2” x 72” (sides)

• Six 1” x 5-1/2” (floor; cut to fit)

• Two 1-1/2” x 3-1/2” x 30” (end cap)

• Two 1-1/2” x 3-1/2” x 73” (side cap)

• Two 1-1/2” x 3-1/2” x 33” (joists)

• Two 1-1/2” x 1-1/2” x 33” (horizontal cleats)

• Ten 1-1/2” x 1-1/2” x 22” (vertical cleats)

 

WHAT TO DO

1. Begin by screwing together the box ends. Use your straightest 2x2s for the corner cleats. Use the washers to space the cedar 2x6s to allow for expanding and contracting.

2. Screw together the box sides. Use the washers to space the cedar 2x. Straighten and hold together with a clamp, if necessary. Use your straightest boards for the top, so that the cap will be straight and tight.

 

3. Form the box. Clamp the sides to the corner cleats of the ends.  Square the box and screw together.

4. Using scrap, mark the depth of the decking joists on the cleats. Determine your floor depth by determining the soil depth required by your plants, adding in another 3/4” for the floor, another 4” for the drainage pipe, and another 1” to keep the soil level below the cap.  The line you mark will be the top surface of the joist, so make sure you put the joist below this line. (How deep does your soil need to be?  Use the soil depth guidelines from a container gardening book as your guide. I found McGee & Stuckey's Bountiful Container to be extremely useful.)

 

5. Screw in the horizontal cleats on either side and the heavier joists in the middle. Lay the floor, notching your deck boards with the jig saw so they fit around the cleats.

6. Carefully insert the pond liner, fitting it to the bottom and the sides.  Cut the excess and staple it around the top perimeter.

7. Center, clamp, and screw on the top cap. Don’t mitre the corners.  Butt joins are not as pretty, but if you’ve never seen the way a miter joint eventually splits on a deck, you don’t want to get firsthand knowledge of it on your beautiful cedar.

 

8. Cut your drainage pipe to fit tightly from end to end of the planter interior, four lengths of approximately 6’. Tuck the sock around the edges as you wedge the pipe into the planter. This will prevent dirt from entering the reservoir.

9. Drill a hole for the aquarium tube in one corner, just above the level of the perforated drainage pipe. Punch a small hole in the top of the drainage pipe and feed the aquarium tubing into the drain pipe then through the hole in the side of the planter. Cut the tubing so that less than 2” protrudes out the side of the planter; a longer tube (like in my picture, below) creates a gravity pump that will drain the reservoir when it overflows.

10. Pack the space between the pipes and sides with your soilless potting mix. Layer the landscaping barrier cloth over the tops of the pipes.

11. Notch or cut your copper tubing at an angle so that the cut end will not sit flush against the bottom of the perforated drain pipe. Punch a hole for the copper fill tube in the drainage pipe in the far corner, opposite the drain hole. Make sure that the copper tube fits snugly in the hole and is near enough to the cap and long enough to be strapped there. You only need the one fill tube; water will flow through the soilless mix (from the pipe with the fill tube to the pipe with the drain hole) and will drain any excess when the water level in the reservoir is higher than the aquarium tube.

 

12. When you’re ready to plant, add more soilless mix to just below the top of the planter then fill the planter with water via the copper fill tube until the water begins to drain from the opposite end. This initial filling will take a while.

 

13. Optional: Treat your cedar with a nice, nontoxic sealant. The easiest sealant to procure would be raw linseed oil, not boiled. Boiled linseed oil has had heavy metals and petroleum solvents added to it to hasten the drying time. Raw linseed oil dries very slowly but is food-grade. In Canada, Home Hardware carries raw linseed oil.

A FEW WORDS ON SOILLESS POTTING MIX

The success of this planter relies on the soil’s ability to wick water upwards from the reservoir to the plants. Real dirt and compost won’t successful wick water from the reservoir; instead, they’ll compact too much and become a sodden, muddy mess that smothers your plants at the roots. 

 

If you go to a garden center and ask for a soilless potting mix, odds are high that, unless you talk to someone with a lot of knowledge, he or she is going to be confused. Don’t get discouraged. Most garden centers employ at least one person who has that key knowledge, and if you ask for promix or sterile mix, that person usually will say, “Oh, yes, we sell that by the bale.” 

 

If you’re lucky enough to find promix, fabulous! For reference, a typical bale is about 3.8 cubic feet, or 107 liters. The packaging is compressed and will approximately double in volume when opened.  Remember that you’ll need to estimate the volume you need based on the planter depth you choose.

 

If you’re not lucky enough to find promix, take heart. You can make your own. While you can find recipes online, promix is typically 75 to 80 percent sphagnum peat moss and 20 to 25 percent perlite (or sometimes 15 percent perlite and 5 to 10 percent vermiculite). You’ll also want to add 1 cup dolomite lime per cubic foot of mix; premade promix usually includes lime. Personally, I have had very good results with this blend, and these materials are all readily available at any garden center. For more on potting soil versus soilless mix, see the Container Gardening 101.

 

INITIAL FERTILIZING AND WATERING

The bad news is that using a soilless mix means your plants will depend on you for all of their nutrition. The good news is that, because you’re using a SIP, fertilizing will be more economical since it won’t get washed away. Before planting, blend your soilless mix with an organic slow-release fertilizer (SRF) and use (more or less) as directed for the initial fertilizing. I started with an organic container plant food (7-2-5) and mixed in about one-third of the 700g container into the top 2 to 3 inches of my soilless mix.

 

Depending on what you plant, you may opt for some additional nutrients upfront. I kept it simple and only hedged against blossom rot by planting my peppers and tomatoes with a bit of organic, free-range eggshell, in addition to the dolomite lime already in my mix. 

 

You should be able to feel the dampness from water wicking up from the reservoirs when you insert a finger into the soil. Initially, the top layer will be prone to dry out because peat moss acts like a dry sponge and can hold up to 20 times its weight in water. It will gradually release this trapped water to the plants, but until it is thoroughly wetted and the plants send some taproots down and begin to provide shade, the soilless mix will require some supplementary top-down watering. 

 

My transplants did poorly until I realized I hadn’t watered the top nearly enough to get the peat saturated. The dry peat was competing with my plants and sponging up all the water. It took a few days of moderate watering from the top down three times a day in warm, sunny weather to get the peat soaked enough that it wasn’t robbing the plants. After that, most of the plants did perfectly well, though the lettuce transplants were sensitive to low moisture for a week or so after. I also continued to water the areas that I had seeded directly until they broke ground, just to ensure there was sufficient moisture for germination.

ONGOING FERTILIZATION

There’s a lot of confusion about how to properly fertilize an SIP. Some advocate for dispensing it via the water. Others say this can cause a buildup of mineral salts in the reservoirs. Well, you can hardly go wrong with putting fertilizer where the roots are, and fortunately this planter design provides easy access to the soil.

 

It’s a good idea to supplement the SRF with a liquid fertilizer. I had excellent results on lettuce and spinach greens through the spring and early summer by applying a 4-1-1 organic fish emulsion diluted to root-application strength biweekly according to package instructions. Be careful in your application, though, as even a low-dose fertilizer, such as the fish emulsion, can cause nitrogen damage to your leaves if you pour it over your plants at root strength. Look for a long-snouted watering can that can reach under the plant.

 

Once the weather begins to heat up and your fruiting plants begin to blossom, you’ll need a more intensive feeding regimen high in phosphorus, potassium, and other trace elements. You should reapply the SRF at the surface if you haven’t already and you may opt for a small, weekly feeding with a balanced liquid fertilizer. It doesn’t take much; a 1-liter watering can is sufficient to give all the plants in your planter a drink every week, though be careful to monitor the health of your plants to ensure you’re meeting their needs and not burning them. Blooming peppers and tomatoes will also benefit from—or at least won’t be harmed by—a foliar spray made with Epsom salts, 1 tablespoon of salts per gallon of water. Regular dirt in North America has lots of Magnesium, but peat moss is known to absorb it when used to condition soil. 

 

Good luck with your gardening endeavors!

MORE FROM HOMEGROWN

• To make a smaller, more portable self-watering container (disclosure: it does involve a plastic tote), download this HOMEGROWN 101 card: front and back. (Find more cards to steal, print, and share.)

• Check out HOMEGROWN member Donna’s 101 on making your own ollas for irrigation.

 

• You might also be interested in the Garden Planning 101, Container Gardening 101, Raised Beds 101, Companion Planting 101, and Drip Irrigation 101.

SPEAK UP!

Got a question for Anne? Or an SIP tip to share? Post it below and keep the conversation rolling. You can always find more things to plant, grow, cook, preserve, make, craft, and hammer in the HOMEGROWN 101 library.

ALL PHOTOS: ANNE RADCLIFFE

[Ron Lohr]

I really like this concept,not sure why there are two different

methods for the drain tubing.One says cut so that 2" comes out the side,

but the photo shows a longer piece(gravity pump)so would you use both?

[Anne Radcliffe]

Hi Ron!  I'm sorry for the confusion, and apologies for the delay in responding.  There is only one drain.

When I took the photo I hadn't cut the piece of aquarium tubing... I thought I would leave it long originally.  I realized afterward that was bad to do! Once I had filled the reservoir to the point of draining, instead of stopping the overflow once it had reached the correct "full" point, gravity took over and continued to suck the water through the overflow tube, emptying the reservoir.  

Long story short, it kind of defeated the purpose of having an SIP!  Cut the piece of aquarium tube short... where it only sticks out past the cedar wall about 2 inches. :)   Perhaps I will see if I can get Jennifer to swap the picture with one of my drain as I have it now.

Hope this helps.

[Ron Lohr]

Thanks Anne, I had a feeling that might be the case,need to put one together now.

Really like the idea!

Anne Radcliffe said:

Hi Ron!  I'm sorry for the confusion, and apologies for the delay in responding.  There is only one drain.

When I took the photo I hadn't cut the piece of aquarium tubing... I thought I would leave it long originally.  I realized afterward that was bad to do! Once I had filled the reservoir to the point of draining, instead of stopping the overflow once it had reached the correct "full" point, gravity took over and continued to suck the water through the overflow tube, emptying the reservoir.  

Long story short, it kind of defeated the purpose of having an SIP!  Cut the piece of aquarium tube short... where it only sticks out past the cedar wall about 2 inches. :)   Perhaps I will see if I can get Jennifer to swap the picture with one of my drain as I have it now.

Hope this helps.

[Jean McKeen]

Ann said:  Once I had filled the reservoir to the point of draining, instead of stopping the overflow once it had reached the correct "full" point, gravity took over and continued to suck the water through the overflow tube, emptying the reservoir.

=================

I'm pretty sure that as long as the tube inside the box is above where you want the water to stop, it won't pull water no matter how long the tube is outside the box. 

The only way it could drain the reservoir is if it was at the bottom instead of the top of the reservoir in the box.

I recently made one and tested the overflow/drain.  I filled the reservoir.  At the top it starts draining.  When the water is below the overflow/drain it's in the air.  It can't pull any more water from the reservoir. 

I  think the length of the tube would depend on where you want the overflow water to go.

Oh, I think I just figured it out.  I'm guessing you didn't make sure the tube inside the box would be above the water line...so you have to make sure the tube outside the box is above the water line.  That's why the shorter tube is necessary. 

I'm glad there's an edit feature.

Jean

[Noah Davis]

Time to revive this discussion.  I built one of these last year and I'm building another this year.  Had a couple of quesitons

1. I found at the end of the season I didn't have a good way to drain the remaining water, which gives me some concerns around anaerobic activity with the saturated pro-mix in the bottom.  What do you think about using lava rock, pea-gravel, or clay pellets for the bottom fill around the drain pipes?
2. Since we lay down a fish safe pond liner covering the entire interior do you you think cedar is absolutely necessary?  You can lower the construction cost 50% with pressure treated lumber.
3. What are your thoughts on adding a worm tube?

My box I built last year performed phenomenally, but I was looking for slight improvement this year.  

[Spencer Bailey]

Thanks for the great information!

A question: For the SIPs  (and specifically the designs where soil "legs" are suspended in water) is there a certain ratio we should observe when considering how much of the soil is suspended in the reservoir? For a system using the infamous plastic tote and 2.5" hydroponic baskets (materials I had access to), how many baskets would be required for that volume of soil. The containers are typically 18gallon/68L, 25"x17"x15" (64cm x 44cm x 39cm). My first experiment will be: 4" supports to hold the lid or mesh, etc, which will be the base of the soil; 3.25" water reservoir; 2" of soil from each basket resting in reservoir; .75" air space between lid/soil base and water. I plan on evenly spacing 6 baskets per plastic container. Any thoughts or advice would be greatly appreciated since this is the first time in the garden for me. Thank you.