Drainage for Planting Beds Around a Flagstone Patio

Raising the Bed

• Raising the planting bed alone can greatly improve drainage while also providing a unique landscape interest. The bed can be raised using walls made of timbers, bricks or stones that match the existing patio. Excavate some soil, particularly around the edges of the bed where the wall material must be partially buried, and stack the materials in walls as high as desired or whatever is necessary to ensure adequate drainage. Consider adding a subsurface drain, especially if the flagstone patio could potentially drain into or below the bed. If the bed is deep enough, crushed gravel can be placed at the bottom followed by soil. Using a good soil mixture will greatly improve drainage.

Amending the Soil

• The best soil type for many plants is a sandy clay loam. Many poorly drained soils have a high clay content. Amending the soil or using a mix so that the soil in the bed has adequate organic matter will permit better drainage. Organic materials like compost or peat moss improve the structure and aeration of clay soil. Work organic matter into the soil thoroughly. Consider having the soil tested to determine if any additional amendments or fertilizers are warranted.

Subsurface Drains

• A variety of traditional or makeshift subsurface drains could be implemented to improve drainage with or without raising the bed. The difficulty of this type of project would depend on the existing setup. Subsurface drains are most practical if they can drain externally into a ditch, basin or away from any structures without having to pull up the flagstone to place pipe or trench. Septic line tubing or drain tiles can be placed in the ground below the plants for the length of the bed. Another option is to dig a trench with a slight slope in the direction of the desired drainage, put about 4 inches of coarse gravel in the trench, lay tiles or perforated tubing atop the gravel, cover it with more gravel and then top it off with soil. Similarly, a homeowner could install a sloped trench simply filled with coarse gravel and covered with vegetation cloth to prevent fill-in with soil.

Plants for Poorly Drained Sites

• If raising the bed or implementing subsurface drainage does not fully alleviate drainage issues, consider simply planting shrubs and herbaceous plants known to perform well in wet soils. Extension services are often able to provide recommendations most suitable for a given area, but some common plants include shrubs like certain dogwoods, rose of Sharon, species of Ilex, wax myrtle, some azaleas, Virginia rose and elderberry. Water-tolerant herbaceous plants include astilbe, sedges, rushes, cardinal flower, rose mallow and certain species of iris.