Instructions for Building a Sub-Irrigated Raised Bed

Philip Small, Soil Scientist, CPSS

These brief instructions provide a basic description of a sub-irrigated raised bed based on a design demonstrated to work on the author’s site which has prolific sucker sprouting from hard wood trees on neighboring city parcels. The intended audience includes gardeners, horticulturists and small-scale farmers.

Phil is a Certified Professional Soil Scientist and permaculture designer and practitioner. You can reach him through Land Profile, Inc.,

1. Make a garden bed frame 6 – 10 inches deep. Level it.

2. Line the full depth with pond liner or whatever waterproof solution you can muster. This is the water reservoir.

3. Fill the reservoir with fine gravel (2 mm to 1/2 inch) step.

4. Place a soil holding frame on top of the water holding frame. Fill it with soil.

5. Plant it.

During construction I install a vertical 3 or 4” dia. perf drain that reaches to the liner both as a hose fill tube, water level observation point, water chemistry sample point and a siphon point for draining.

The more gravel volume replaced with drain pipe void, the longer the bed can go after watering.

Optional feature: All our sub-irrigated beds have valves pressurized water delivery into the water reservoir at 15 psi. Usually it is 1/2” poly into a horizontal section of 3-4” dia perforated drain closed with geo cloth.

The more gravel volume replaced with drain pipe void, the longer the bed can go after watering.

The capillarity of the fine gravel lifts stored water up to the base of the soil layer. Using gravel vs soil makes that layer less prone to going deeply anaerobic. My night crawlers overwinter in the top of the gravel layer. Seeing that I favor 2-4 mm gravel size for the worms. My personal design preference.

Excess water decants out from between the two frames. Increasing the gap to 1/4” assures air exchange into the top gravel assuring a healthy soil/water table interface, but without too much soil loss out the gap during high (turbulent) flow “events” like when the waterer forgets the water.