How to Fill Large Planters | Save Soil, Improve Drainage

Filling a large planter properly means using a lightweight filler in the bottom third, covering it with landscape fabric, and finishing with high-quality potting mix, not garden soil.

Large planters look great but eat up soil—and money—if you fill them the wrong way. The trick is knowing what goes in the bottom, what stays out, and how deep the actual growing zone needs to be. Here is the step-by-step method that saves you dollars and keeps your plants healthy.

Start With Drainage—or Drilling

Before adding anything, check the bottom. If your planter lacks drainage holes, drill three to four ½-inch holes spaced evenly across the base. Without drainage, water pools at the bottom and rots roots. Leave the planter in its final location before you fill it—moving a full planter risks injury and cracks.

Which Filler Belongs in the Bottom Third?

Fill the bottom one-third to one-half of the planter depth with a filler material. The choice depends on whether you need the planter light enough to move or heavy enough to stay anchored in wind.

Lightweight fillers (for mobility): plastic bottles, nursery pots, Styrofoam, packing peanuts, pool noodles, aluminum cans, pine cones, or crumpled cardboard. These take up volume without adding weight. Heavy fillers (for stability): gravel, rocks, broken pottery, bricks, cinder blocks, or sand. These add heft so the planter stays put.

Never let filler exceed one-third of the planter’s height. Above that, roots don’t have enough room to grow. Cover the filler with a permeable barrier such as landscape fabric or burlap. This keeps soil from washing down into the voids while still letting water pass. Never use plastic sheeting or non-permeable materials—they trap water and cause root rot.

Choosing and Calculating the Right Potting Mix

Always use a high-quality potting mix. Never use garden soil, topsoil, or native yard soil. Garden soil compacts in containers, holds too much water, and suffocates roots. A proper potting mix contains about 60–70% peat moss or coir, 20–30% composted pine bark or perlite, and 5–10% screened compost or worm castings. For containers larger than five gallons, mix in up to 15% coarse horticultural grit and a slow-release fertilizer.

To figure out how much soil you need, calculate the planter’s total volume, then subtract the volume taken up by filler. For a rectangular planter: length × width × height. For a round planter: π × radius² × height. Leave 1–2 inches of headspace below the rim so water doesn’t overflow when you irrigate.

Minimum soil depths by plant size:

• Small plants (herbs, lettuce): 6–8 inches
• Medium crops (peppers, tomatoes): 10–12 inches
• Large vegetables (potatoes, squash): 12–18 inches

If your planter’s filler pushes the soil depth below these minimums for your chosen plant, reduce the filler height or skip it entirely.

Step-by-Step Fill Sequence

1. Drill drainage holes if needed.
2. Position the planter in its final spot.
3. Add a coarse drainage layer (gravel or broken pottery) across the very bottom.
4. Add your chosen filler material to one-third of the planter’s depth.
5. Lay landscape fabric or burlap over the filler.
6. Fill the remaining space with potting mix, stopping 1–2 inches below the rim.
7. Plant at the same depth as it was in its nursery pot.
8. Water thoroughly until water runs freely from the drainage holes. If you’re shopping for a new large planter that won’t crack or fade, our tested roundup of extra large plastic planters covers the durable options that skip the weight and worry.

Common Mistakes to Avoid

• Filler exceeding one-third of planter height—starves roots.
• Skipping headspace—water runoff carries soil and nutrients over the rim.
• Using plastic or non-permeable barriers—traps water and rots roots.
• Drilling holes too small or too few for adequate drainage.
• Ignoring the plant’s mature size when choosing planter depth.

Clay pots dry faster than plastic or glazed ceramic, which means they need more perlite in the mix to retain moisture. Deep pots also benefit from extra perlite to keep the bottom layer from staying wet. Refresh the soil every one to two years—old potting mix compacts and loses its ability to drain and breathe.