This is an amalgamation reposting of much of the information found on my site from last spring, and also part of the SIP 101 that I contributed to Homegrown.org.
- Important Notes
- Materials / Wood Cutting List
- Building Instructions
- Construction Photos
- Soilless Potting Mix
- Initial Fertilization and Starting Up
- Ongoing Feeding
- Pest control
Do you want to garden, but don’t have enough yard? Do you have difficulties getting anything out of container gardening? No problemo. You’re exactly where you need to be.
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 with their small, easy to heat “soil beds” and lack of accessible water. Many container plants will suffer and die horrible deaths despite your best efforts.
There is a thing called a Sub-Irrigated Planter (or SIP for short) or sometimes known as a self-watering container. The concept is simple: you have a large-ish reservoir beneath the surface that feeds water upwards to your plants. And a quick Google will tell you many ways to make or buy SIPs, from the EarthBox, which is simple but costly, to thousands of different types of home-made SIPs from buckets and Rubbermaid tubs.
YOU SHOULD NOT GROW YOUR FOOD IN A — — USED PLASTIC BUCKET OR RUBBERMAID TUB
I cannot say this strongly enough. You are asking for trouble. Not only is there the risk of contamination from whatever might have been used in the bucket before, but food grade plastic is a dubious thing for long-term use, and Rubbermaid tubs are not even made out of food grade plastics. Then you are leaving it in the sun, and heat makes it easier for chemicals to leach… right into the soil and reservoir so conveniently feeding the plants you are going to eat. And you should not use PVC for any food reason at all if you can help it.
I found a plan for a non-toxic SIP made out of cedar wood on The Family Handyman. It’s a good plan. A little vague. I ended up tweaking the plan a bit and filling in some of the informational gaps: nontoxic sealants? Soil? Fertilizing?
Does it really have to be made out of cedar? No. Cedar is meant to survive wet elements just fine. But since only the cap might come in contact with plants/dirt/water, you can build the body from any wooden material, treat or stain it as you wish, and then build the planter cap out of cedar, or ensure that you treat it with a non-toxic substance. Do not use pressure treated wood. Even though the ACQ isn’t arsenic, it is not safe for consumption. Or even touching with your bare skin on a regular basis. It makes me wonder why they switched at all.
You should build this. With a little tweaking, you can make it any size you want. It was a bit more expensive than a Tupperware tub, but it’s large, lovely, durable, non-toxic, and best of all, it’s been RIDICULOUSLY successful.
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 regular dirt in this planter. This planter requires a soilless potting mix, sometimes known as pro-mix or sterile mix. See more info about soilless mix composition below.
3. You’ll need to plan any vertical supports for plants in advance. Extremely tall plants and vines with heavy fruit or vegetables need special consideration because soilless mix is not heavy dirt. A single stake will not be sufficient to aid the root structure in supporting the full weight of a tomato or cucumber vine in high winds or rain. Consider a tripod stake or one that is anchored to a nearby wall.
4. Consider your choice of plants and the layout carefully when planning. Because you are working in a planter that is nearly 2 feet tall, traditional straight-up staking with an indeterminate plant will require a stepladder to reach your tomatoes—and you’ll probably run out of stake long before you run out of summer.
- Tape measure
- Drill with bit for deck screws
- Utility knife or shears
- 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′ 2×4 (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 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)
- 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.
- 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.
- Form the box. Clamp the sides to the corner cleats of the ends. Square the box and screw together.
- 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.)
- 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.
- Carefully insert the pond liner, fitting it to the bottom and the sides. Cut the excess and staple it around the top perimeter.
- 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.
- 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.
- 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.
- Pack the space between the pipes and sides with your soilless potting mix. Layer the landscaping barrier cloth over the tops of the pipes.
- 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.
- 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.
- 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.
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 (Length x Width x Depth = Volume).
If you’re not lucky enough to find promix, take heart. You can make your own. While you can find recipes online, promix is 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.
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. Before planting, blend your soilless mix heavily 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.
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 (cucumbers in particular seem to need special attention in the SIP—everything else I planted did excellent).
Blooming peppers and tomatoes will also benefit from 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. It did no harm to apply it to the peppers and tomatoes, and therefore I recommend it.
The one undeniable truth about about gardening is pests. I was assured by many books that container gardens don’t get them. Or at least, not enough to worry about. Lie.
If they get a foothold, soil-burrowing pests like leaf miners and flea beetles in particular will proliferate and destroy your late summer crops. They can multiply exponentially, because their lifecycle from egg to egg-laying is only a few short weeks, and they can thrive in a soilless mix just fine. There’s no natural predator in your dirt for them.
So take precautions: Netting to keep the flies off your spinach leaves, removing eggs from the leaves or leaves showing miner damage when you find them, a couple trap-crop plants, and you may want to introduce some beneficial nematodes to the soil (to give the soil-dwelling larvae a predator to keep them down). In fact, I highly recommend this last.
If you have a particularly bad infestation, make sure you remove all the mix and replace it in the spring, as these nasty beasties can lie dormant over winter.