Raised beds generally improve the drainage for your garden plantings simply by the fact that they are elevated above the surrounding grade. However, if the subsoil is dense and impermeable--such as hard-packed builder's fill, or heavy clay--and the raised beds are tightly built, they may turn into muddy pools under heavy rainfall or spring snowmelt, unless they have proper drainage. Install drainage channels in the underlying soil before you fill and plant your raised beds.
Dig a trench 10 inches wide and sloping from 8 inches deep to 12 inches deep from one end of the raised bed location to the other, using a trenching shovel. Dig a round hole, 12 inches deep by 2 feet in diameter, at the low end of the trench.
Place 4 inches of coarse gravel in the bottom of the trench, maintaining the slope.
Cut nylon window screen with scissors into roughly 10-inch circles. Secure the circles of window screening over the ends of the PVC pipe using large rubber bands. Lay a single length of 4-inch-diameter perforated PVC drainpipe in the trench.
Cover the pipe with gravel to grade level, and fill the hole at the end of the trench with gravel to grade level.
Cut 12-inch-wide strips of nylon window screening with scissors. Lay these strips over the gravel-filled trench, tucking the side edges down along the outer edge of the gravel to help keep it in place.
Fill raised bed with desired planting mix, wholly covering the drainage trench, and plant the bed as desired.
Things You Will Need
- Trenching shovel
- Coarse gravel
- Nylon window screen netting
- 4-inch PVC drainpipe, perforated
- 2 large rubber bands
- Planting mix
- Use a spading fork to loosen the soil under the raised bed area before beginning raised bed construction to help improve substrate drainage.
- If you are planning multiple raised beds, run all the drainpipes to a central pathway. Dig the bottom of this pathway down 12 inches, sloping toward the desired drainage location, then fill it wholly with crushed stone or coarse gravel.
- Use loosely textured loam, not heavy soil, in your raised beds to ensure good drainage, and try not to step on the raised bed soil so that it does not get compacted.
- A raised bed without proper drainage can fill in with heavy rain, causing its side to burst and water and soil to flow out, destroying your garden plants and possibly damaging other property. If your garden bed looks over-saturated, dig a trench around the outside of the raised bed framing, then a straight-line trench away from the bed to a lower elevation.