Does a raised bed need drainage?
Written by
Paul Reynolds
Reviewed by
Prof. Samuel Fitzgerald, Ph.D.Poor drainage leads to root rot in raised beds, suffocating the plant roots. Waterlogged soil creates anaerobic conditions that kill beneficial microbes. This problem is aggravated in heavy clay soils or during wet seasons. I have rescued beds by punching holes for drainage as soon as I saw yellowed foliage. Plants need a supply of oxygen to their roots as much as water or food.
Hole Engineering
- Drill ½-inch holes every 12 inches for clay soils
- Space holes 18 inches apart in sandy compositions
- Angle holes downward at 15 degrees to prevent soil loss
- Install mesh screens behind holes to contain soil
Slope Calculation
- Create 0.5-inch drop for beds under 4 feet long
- Longer beds need 1-2 inches of slope per 4 feet
- Use laser levels for precision over 8 feet
- Crown slopes work best for wide vegetable beds
Soil Amendments
- Mix 40% coarse sand into heavy clay soils
- Add coconut coir to sandy soils for moisture retention
- Include perlite for aeration in all soil types
- Balance with 30% compost for nutrient availability
Trench Test - A trench test gives you a good indication of whether you have drainage problems before you design your beds. Dig a hole about 10 inches deep and fill it with water. Time the water for drainage. A good bed should drain within 4 to 10 hours. If this is too lengthy, amendments are warranted. I fixed a client's bed by using sand as an amendment after finding that it failed the test because it did not drain within 24 hours.
Certain materials need particular drainage methods. Wood blocks utilize natural grain channels but require protection against rot. Metal must be drilled for holes, as about 40 percent of the pores will be blocked by corrosion. Plastic, because of its difference in expansion and contraction, must have extra holes. Stone blocks must have mortar gaps every third course of stone.
Prevent wood rotting by ensuring that your drainage and elevation are correct. Providing holes for drainage allows constant moisture to be kept away from the wood. Have your beds elevated by an inch above the ground using either stones or plastic feet. Use rot-resistant woods such as cedar. I prolong the life of the wood in my beds by applying linseed oil to all interior surfaces of untreated wood on an annual basis.
Different plants require different soil depths. Shallow-rooted herbs need 6 to 8 inches, while tomatoes require a depth of 18 inches. Deep-rooted vegetables require a soil depth of at least 24 inches. When planting, consider the depth requirements of your bed. Deep beds are well-drained, but they need more material for amendments. Test the drainage system yearly to ensure the proper surroundings are maintained.
Read the full article: 10 Essential Raised Bed Drainage Fixes
