How to Plant a Large Planter | Fill, Plant, and Water Right

Planting a large planter correctly means starting with drainage holes, adding lightweight filler in the bottom third, using quality potting mix with slow-release fertilizer, and arranging plants using the thriller-filler-spiller method for a full, healthy display all season.

Big planters look impressive, but they come with their own set of challenges — heavy weight, high soil cost, and watering that’s easy to get wrong. The good news is that a few simple techniques solve all three. Whether you’re planting flowers, vegetables, herbs, or even a small tree, the same core steps apply. Here’s how to get it right the first time.

Drainage and Filler: What Goes in the Bottom

Drainage is non-negotiable. If your planter doesn’t have holes, drill several ½-inch openings evenly spaced across the bottom. Never skip this step — without drainage, roots rot and plants die quickly.

Fill the bottom one-third to one-half of the planter with lightweight material. Upside-down plastic nursery pots, empty water bottles, or clean styrofoam all work well. This layer saves money on soil and keeps the overall weight manageable — important if you ever need to move the planter or if it sits on a deck or balcony. Cover the filler with a sheet of landscape fabric to keep soil from washing down into the gaps, then add your potting mix on top.

Use a high-quality, peat-based potting mix — never garden soil, which compacts in containers and suffocates roots. Leave 1 to 2 inches of space between the soil surface and the rim of the pot so water doesn’t wash over the sides when you irrigate.

Fertilizer and Plant Spacing for Big Containers

Mix a slow-release granular fertilizer (look for an NPK ratio between 7-7-7 and 14-14-14) into the top few inches of potting soil. For a large planter, about 3 tablespoons of a 6-month feed formula is a solid starting point. You can supplement later with a water-soluble fertilizer like 20-20-20 for quick nutrient boosts during the growing season.

Follow these general plant counts by container width:
• 10–12 inch planter: 3–4 plants
• 14–16 inch planter: 4–6 plants
• 16–20 inch planter: 6–8 plants

Space plants about one-third closer than you would in the ground — containers let you pack them in, but don’t overcrowd to the point where roots have no room. If you’re selecting a container, browse our picks for the best 15-inch planters for options that hit the sweet spot of size and manageability.

For trees or very large shrubs, you’ll need 2 to 3 feet of soil depth minimum. Match plants with the same sun and water needs in each container — most vegetables and herbs require 6 to 8 hours of direct sun daily. Grouping thirsty plants with drought-tolerant ones is a recipe for failure.

The Thriller, Filler, Spiller Arrangement

This three-part design method works every time. Place your tallest, most dramatic plant — the “thriller” — slightly toward the back if the planter sits against a wall, or in the center if the planter will be viewed from all sides. Add “filler” plants (medium-height, bushy varieties) around the base, keeping root balls close together and securing them with soil as you go. Finally, install “spiller” plants — trailing varieties that will cascade over the container’s edge as they grow. Water slowly and deeply until moisture runs out the drainage holes.

A few practical tips for long-term success:
• Water when the top inch of soil is dry — large containers may need daily watering in hot summer weather.
• Avoid dark-colored or metal planters in direct sun; they can overheat roots. Move to partial shade or choose lighter colors.
• Elevate the planter on pot feet or bricks to keep drainage holes clear.
• If you’re placing matching planters flanking an entryway, set them 12 to 24 inches from the door frame for good proportion and clearance.

Common mistakes to avoid: no drainage is the fastest way to kill a container garden (use a cachepot with a nursery pot inside or add holes). Never pack soil too tightly — roots need airflow. And don’t use garden soil in any container; it turns into brick-like mud.