How to Fill a Large Planter (Cheaply)

Filling a big pot entirely with bagged potting mix gets expensive fast, and a 24-inch planter can swallow six or more cubic feet of soil. The good news is that most annuals, herbs and vegetables only root in the top 12 to 18 inches, so you can safely take up space at the bottom of a tall planter with lightweight filler. The catch is that the old advice of adding a layer of gravel “for drainage” actually makes drainage worse. This guide explains what works, what doesn’t, how much soil you really need, and when you should skip filler and use soil all the way down.

Part of our Houseplants & Container Gardening guide →

In this guide

1. The Gravel Myth: Why Rocks Hurt Drainage
2. When Filler Is a Good Idea
3. Best Fillers for Large Pots
4. Fillers to Avoid or Use With Care
5. How Much Soil Do You Need?
6. What Soil to Use
7. Step-by-Step: Filling a Large Planter
8. When to Use Soil All the Way Down
9. Weight on Balconies, Decks and Roofs
10. Choosing a Large Planter
11. Refreshing a Large Planter Each Year
12. Frequently Asked Questions

The Gravel Myth: Why Rocks Hurt Drainage

Water does not move easily from fine soil into a coarse layer below it. Instead, it collects in the bottom of the soil until that soil is saturated, forming what is called a perched water table. Adding gravel to the bottom of a pot doesn’t eliminate that saturated layer; it just moves it higher, closer to the roots, and reduces the amount of well-drained soil above it. The result is the opposite of what you intended.

Good drainage comes from two things: drainage holes in the bottom of the planter and a free-draining potting mix throughout. If your planter has no holes, drill them. A cordless drill with a masonry bit works on most ceramic and concrete, and a standard bit works on plastic and resin. To stop soil washing out, cover the holes with a square of window screen, mesh or landscape fabric, not a pile of rocks.

When Filler Is a Good Idea

Filler makes sense when the planter is much deeper than the plants need. A 30-inch-tall planter filled with petunias, herbs or lettuce is a good example; those roots rarely go below 12 inches. Filler saves soil and money, and it makes the planter much lighter, which matters on balconies, roof decks and anywhere you may need to move the pot.

Here’s the important part: the soil above the filler should still be deep enough for the plant’s roots and to buffer moisture. As a general guide:

Plants	Minimum soil depth above filler
Annual flowers, herbs, lettuce, strawberries	10–12 inches
Peppers, bush beans, compact vegetables, ornamental grasses	12–16 inches
Tomatoes, squash, large annuals	16–18 inches or no filler
Shrubs, small trees, long-lived perennials	No filler; use soil all the way down

Best Fillers for Large Pots

• Upside-down plastic nursery pots: probably the best option. They are light, don’t break down, create air space and you probably have a stack already.
• Sealed plastic bottles or jugs: capped water or soda bottles are light and take up a lot of room. They are easy to remove when you replant.
• False-bottom inserts: plastic platforms sold to sit partway down a tall planter. They create a hollow space below the soil and are the tidiest solution.
• A smaller pot inside the big one: set a plastic pot on a riser such as a bucket or bricks inside the decorative planter, so its rim sits just below the outer rim. This is the easiest way to swap seasonal plants.
• Non-biodegradable packing peanuts in a bag: fine if contained in a mesh or plastic bag so they don’t mix into the soil. Avoid the starch-based kind, which dissolve when wet.

Whatever filler you use, place a layer of landscape fabric over it before adding soil, so mix doesn’t sift down into the gaps.

Fillers to Avoid or Use With Care

• Gravel, rocks, broken pottery: add weight and create the perched water problem described above.
• Wood, logs, sticks, pine cones, leaves: these break down over time, which makes the soil sink, and decomposition can temporarily tie up nitrogen. They can work in raised planters you top up each year, but expect settling.
• Soda cans: aluminum cans are light but can be crushed under the weight of wet soil, causing settling.
• Styrofoam pieces loose in the soil: they float up through the mix over time and are messy to remove.

How Much Soil Do You Need?

For a round planter with straight sides, the volume in cubic inches is 3.14 × radius² × height. Divide by 1,728 for cubic feet. Most bagged potting mix is sold in cubic feet or dry quarts; one cubic foot is about 25.7 quarts. Tapered pots hold less than the formula suggests, roughly 70% to 80% of the straight-sided figure.

Planter (diameter × depth)	Approx. volume if filled	With bottom third filled with filler
12 × 12 in	0.8 cu ft	0.5 cu ft
16 × 16 in	1.9 cu ft	1.2 cu ft
20 × 20 in	3.6 cu ft	2.4 cu ft
24 × 24 in	6.3 cu ft	4.2 cu ft
30 × 30 in	12.3 cu ft	8.2 cu ft
Rectangular 36 × 12 × 12 in	3.0 cu ft	2.0 cu ft

Buy about 10% extra because fresh potting mix settles after the first few waterings. For more help matching container size to plants, see how to choose the right pot size.

What Soil to Use

Use a quality potting mix, never straight garden soil, which compacts in containers and drains poorly. To stretch your budget in large outdoor planters, you can blend potting mix with compost, roughly three parts mix to one part compost, and add a few handfuls of pine bark fines or perlite for structure. For long-term planters holding shrubs, many gardeners use a heavier container blend with more bark and compost, which breaks down more slowly. Our roundup of the best potting mix compares options.

Step-by-Step: Filling a Large Planter

1. Put the planter in its final spot first. A filled, watered 24-inch planter can weigh a few hundred pounds. Set it on pot feet or a caddy if it sits on a deck or patio, so water can drain and you can move it later.
2. Check drainage holes and cover them with mesh or screen.
3. Add filler to the depth you planned, leaving enough room for the soil your plants need.
4. Cover filler with landscape fabric, tucking it against the sides.
5. Add potting mix in layers, moistening each layer as you go. Dry mix is hard to rewet in a large pot.
6. Plant so the top of each root ball sits about 1 to 2 inches below the rim, leaving room for watering.
7. Water thoroughly until it drains from the bottom, then top up any settled areas.
8. Mulch the surface with an inch of bark or compost to reduce evaporation.

When to Use Soil All the Way Down

Skip filler for trees, shrubs, roses, Japanese maples, citrus, bamboo, and perennials or grasses you plan to leave in the planter for several years. These plants need the full root volume, and more soil provides better insulation for roots in winter, which matters a lot in colder zones where container roots are much less hardy than the same plants in the ground. More soil also holds more water, so you’ll water less often during summer heat.

Weight on Balconies, Decks and Roofs

Weight is the other big reason to use filler. Potting mix is light when dry but heavy once saturated, and mineral-heavy blends with sand or topsoil weigh considerably more. Several large planters lined up along a balcony railing can add up to a load the structure was never designed for. If you’re gardening on an elevated surface, a few precautions help:

• Check with your building manager or HOA about load limits before installing big planters, especially on older balconies and rooftops.
• Use lightweight planters made from fiberglass, resin or plastic rather than concrete or stone.
• Use a lightweight potting mix with plenty of perlite or bark, and avoid adding garden soil or sand.
• Fill the bottom of tall planters with light filler where the plants allow.
• Place heavy planters near load-bearing walls and supports rather than in the middle of a span.
• Use saucers or drip trays so excess water doesn’t stain or rot decking, and raise pots slightly so the surface underneath can dry.

Self-watering planters are another option for large containers. They have a built-in reservoir in the bottom, which also acts as a filler, and a wicking system that keeps the soil evenly moist. They reduce watering frequency in summer, which is especially helpful for large pots in full sun.

Choosing a Large Planter

Material affects weight and how fast soil dries. Fiberglass, resin and plastic are light and hold moisture well. Terracotta and unglazed concrete are heavy and dry out faster. Glazed ceramic sits between. In freezing climates, choose frost-resistant planters or empty and store ceramic and terracotta for winter. Compare options in our roundups of the best large outdoor planters and best large indoor planters. For more container ideas, visit the houseplants and containers hub.

Refreshing a Large Planter Each Year

You don’t need to empty a large planter every season. For annuals, remove old root balls in spring, loosen the top 6 to 8 inches, mix in fresh compost and a slow-release fertilizer, and top up with new potting mix. Every two to three years, replace the top half of the soil entirely. For long-term shrubs, scratch fresh compost into the surface each spring and repot or root-prune every few years when roots fill the container.