How do you fix poor drainage in an existing raised bed?
Written by
Paul Reynolds
Reviewed by
Prof. Samuel Fitzgerald, Ph.D.In existing raised beds, poor drainage results in the suffocation of roots and nutrient loss. Soggy soil conditions create an anaerobic atmosphere, which kills beneficial living organisms and rots roots. I have brought beds back into use by retrofitting solutions, rather than rebuilding them. The key is making a diagnosis of your particular drainage disaster through trench testing before deciding upon a repair scheme.
Supplemental Drainage Holes
- Drill ½-inch holes every 12 inches along base
- Angle downward at 15° to prevent soil washout
- Use diamond-tip bits for metal/plastic beds
- Install mesh screens behind new holes
Soil Amendment Process
- Mix coarse sand into clay soil at 30% volume
- Add perlite for aeration in all soil types
- Work amendments 12 inches deep using garden fork
- Retest drainage with trench method after 48 hours
French Drain Installation
- Dig 8-inch deep trench around bed perimeter
- Lay perforated pipe wrapped in landscape fabric
- Cover with gravel and slope toward drainage area
- Connect to rain barrel for water recycling
Slope Adjustment Technique
- Jack up one end of bed 2-4 inches
- Insert shims or patio stones as permanent supports
- Check slope with 4-foot level (0.5-inch drop)
- Reinforce bed joints after elevation
Select treatments according to your type of drainage failure. For surface pooling, changing the slope is the fastest way to correct the condition. French drains are needed for subsurface drainage. Soil amendments are called for in slow percolation. I usually combine hole boring with soil aerating for all conditions. Be sure to retest the drainage after the improvements are implemented.
Implement seasonal timing for best results. Fix the drainage in early spring, before planting, and in late fall, after harvest. Avoid fixing drainage when the soil is saturated during rainy periods. The wooden beds should have wood preservative applied at the time of retrofitting for longevity. Timely intervention has produced 5-plus-year recoveries.
Results are monitored through the plant's responses. Strong new growth occurs in a period of 2-3 weeks following modifications. Watering should then be reduced by about 30 percent when drainage improves. The drainage speed of rainfall should always be noted for the effectiveness of the long-term changes. These refurbishments will save the beds that otherwise would have necessitated complete replacement.
Read the full article: 10 Essential Raised Bed Drainage Fixes
