Does a Planter Box Need Drainage?

Drainage Concerns

• Moisture builds up in the soil if it has no way to drain from the planter, resulting in soggy conditions that drown and suffocate the roots. As the roots suffer, they become more prone to rot and fungal problems, which eventually results in the plant's death. Large planter boxes without drainage may feel dry near the soil surface, but the moisture may still collect in the lower portion of the soil and cause rot problems. Plants suffering from poor drainage may develop yellow foliage, drop leaves, grow or flower poorly, or develop squishy roots and stems.

Drainage Holes

• Drainage holes provide the best method for supplying even drainage in a planter box. Drill holes in the bottom of wood or plastic boxes and troughs using a 1/4-inch drill bit. The number of holes necessary depends on the size of the box. Spacing holes four to six inches apart in rows spaced at the same distance is usually sufficient. If the box sits directly on the ground, place it on top of stones or bricks so the drained water doesn't become trapped beneath the box. Line the box with fine-mesh screening to prevent soil from falling through or clogging the holes.

Gravel Method Concerns

• A common method of adding drainage requires lining the planter with gravel, then filling it with soil. Unfortunately, this doesn't help drainage and still results in wet soil and stressed plants. Water tends to accumulate in the less coarse soil layer, immediately above the gravel, instead of draining into the coarser gravel. Instead of putting gravel in the same planter you are potting, use a double potting method where the gravel is outside of the pot that contains the soil.

Double Potting

• Concrete and clay planter boxes also need drainage, but these aren't simple to drill. A better method to ensure drainage is to line the interior of the box with a two- to three-inch layer of pea gravel. Plant flowers and other plants inside smaller pots that do have drainage holes and set them inside the box, on top of the gravel layer. The excess moisture will drain from the bottom of the pots, and the gravel will elevate the pots enough so that they don't reabsorb moisture into the soil. Use pots that sit lower than the sides of the big planter box so you can't see them peeking above the rim. As an added benefit, you can quickly change out the flowers in your planter by swapping out the pots.