How to Drain Your Garden

Grassed Waterway

• 1

  Dig a broad, shallow trench, also known as a swale, that connects the low-lying points in the garden area. A swale 3 to 4 inches deep and 3 to 4 feet wide with gently tapering sides is generally sufficient. If necessary, create a network of smaller swales to carry water from various points around the garden to a central swale that leads from the garden area. A swale's slope should drop at least 1 1/4 inches every 10 feet but not more than 5 inches every 10 feet.

  Prepare to measure a swale's slope by cutting a 10-foot section of string and attaching a line level to the string's midpoint. Hold the string to the ground on the top of the swale's slope, and stretch the string downhill. Raise the string up or down as needed to make it level with the part of the string at the swale slope's top, and measure the distance between the string and the ground at the downhill side to see if it the slope's drop is within the acceptable range. On steep terrain, a swale can be routed laterally across the terrain's slope to keep the swale at a moderate slope.

• 2

  Line the swale, or each swale, with rip-rap rock at its outlet to diffuse the force of the water at its exit point.

• 3

  Spread grass seed over each swale's bare soil to establish a layer of turf that will keep the swale from eroding over time. Follow the instructions on the bag of grass seed for the appropriate application rate for the grass species. Spread a light layer of straw over the grass seed to aid in germination and prevent birds from eating the seed.

Underground Piping

• 4

  Dig trenches 2 feet deep and 6 inches wide wherever space permits in the garden. The main requirement is for the bottom of each trench to slope continuously downhill at a minimum 1 percent slope to a point where it can come to the surface outside the garden area. No maximum slope exists for an underground piping drain.

• 5

  Line the bottom and sides of each trench with porous landscape fabric. The material commonly sold as weed fabric or drainage cloth in garden centers and landscape supply yards can be used as well. All of those products allow water to seep through them but act as barriers, preventing soil and roots from entering the drain.

• 6

  Lay 4-inch-diameter perforated, flexible drainpipe in the bottom of each trench. Add enough gravel on top of the drainpipe so the top of the gravel layer meets the original soil level.

• 7

  Make a trapezoidal bed of rip-rap rock starting at the outlet of the drainpipe and fanning outward to 3 feet wide and 4 feet long. The arrangement will dissipate the force of the water from the drainpipe and prevent erosion.