Raised planter boxes with a 3-inch-deep reservoir of water beneath 12 inches of soil (also known as sub-irrigated planters) make it easy to grow an abundance of vegetables in a small space, with minimal water and minimal effort. Once plants are established, they often only need watering once a month. With the main water source below the soil, plants grow deep roots, becoming more resistant to pests, and very unlikely to dry out if you forget to water for a few days or even weeks. The top inch of soil remains bone dry, so virtually no weeds germinate.
Step 1: Build or Purchase Box
with an interior width that is a multiple of 4-3/4” (the outside diameter of irrigation pipe) plus a little more (1/4 - 3/4”). The inside width of this box is 24”, and nicely fits five pipes—4.75 x 5 = 23.75.
Step 2: Line box with rubber
(not toxic PVC) pond liner. If your ‘box’ is a plastic or other water-tight tub, no liner is necessary.
Step 3: Cut perforated irrigation pipes
to fit. The only purposes of these pipes are to create a ceiling for the water reservoir, and to allow for air movement beneath the soil.
Step 4: Cut drain pipe openings
Copper is best; any pipe will do. Drill hole in box slightly (1/8-1/4”) larger than pipe, so that the bottom of drain pipe is 3” above bottom of box. This forms the height/depth of water reservoir. Cut hole in rubber liner about half the diameter of the pipe. Cut hole in nearest irrigation pipe to fit drain pipe.
Step 5: Install drain pipe*
Cut long enough to extend 3” into planter. Drill a few 1/4” holes near end (see pic). Temporarily cover end of drain pipe with duct tape. Apply thick layer of silicone caulk in hole in box. Push taped drain pipe through holes in box and liner. Fill any remaining gap in box hole with silicone. Remove duct tape, cover holes in drain pipe with screen or shade cloth to prevent clogging. Fit into nearest irrigation pipe.
Step 6: Install fill pipe
3” ABS will accommodate adjustable sprayers; 1-½” ABS is fine for a standard garden hose/nozzle. Should stick up about 1/2”-1” above sides of box. Cover with cup or tape when adding soil.
Step 7: Cover pipes with landscape cloth
to prevent roots from growing into reservoir. Wrap cloth around pipes and secure underneath with duct tape. Or, even better, tape perimeter of cloth to liner—this prevents any soil from settling into reservoir. Use lengths of thin pipe, rebar or the like to hold cloth down between irrigation pipes while taping.
Step 8: Fill reservoir
(till water pours out of drain pipe). Check for leaks. Optional: catch drainpipe overflow in a bucket or watering can. Pour in fill pipe later.
Step 9: Fill planter to the rim with nutrient-rich, well drained soil
1/3 organic compost, 1/3 perlite (or vermiculite), 1/3 coir (a sustainable alternative to peat moss, made from coconut husks). Fully saturate soil with water (this may require some stirring, by hand or with a small spade), let settle for a couple days before planting.
Step 10: Plant seedlings and/or seeds
Gently top water daily until plants stand up strong (a couple of weeks for most veggies). Add ½”-1” layer of compost to reduce evaporation from soil.
Seven weeks after planting
*Plastic tubs need no drain pipe; just drill a hole or two. Boxes longer than about eight feet, or in areas with heavy rainfall, may need two or three drain pipes/holes.
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