How to Fill a Large Fiberglass Planter

A large fiberglass planter is a hollow shell. Across the sizes we publish interior depths for, the usable interior runs to a median of 92% of the exterior height, so a 40-inch planter is very nearly 40 inches deep inside. That is the opposite of a molded pot with a shallow well, and it is why the filling question exists at all.

Filling that volume with potting mix is expensive, extremely heavy, and does nothing for the plant. The question is what goes in the space below the roots, and the answer has to satisfy three things at once: the plant, the drainage, and the floor underneath.

What makes a fiberglass planter different?

Four things change the answer compared with terracotta, concrete or a nursery pot.

It is hollow nearly to the base. Median interior depth across our range is 92% of exterior height. There is no molded well limiting how deep you can plant, and no dead space designed in.

The shell weighs almost nothing. Our fiberglass sizes start at 2.2 pounds and the largest cube is 323 pounds packaged, but for most sizes the planter is a small fraction of the final load. In a concrete planter the vessel dominates the weight; in fiberglass, what you put inside is essentially the whole load. That makes filling the main lever you have.

It arrives without drainage holes. A nursery pot comes drilled. A fiberglass planter does not, so the holes are a decision at the point of order rather than something to assume.

The walls are non-porous. Terracotta breathes and loses moisture through the sides. Fiberglass does not, so water leaves through the holes or not at all, and what you pack below the roots affects how the medium above it behaves.

How much soil does a planter actually need?

Less than the planter holds. Jay Scotts publish depths by planting type, and they are shallower than most people assume.

Planting	Soil depth needed
Annuals	4 to 8 inches
Perennials and bulbs	8 inches
Herbs and vegetables	8 to 10 inches minimum
Trees	At least twice the depth of the root ball

Fill to roughly two inches below the rim so watering does not wash over the edge. Everything below the root zone is doing no horticultural work, which is why it is the right place to save weight and money.

8 to 16 inthe working depth for most planting. In a 40-inch planter, that leaves two thirds of the volume free.

What can you put in the bottom?

Two categories, and the choice depends on whether you want the planter lighter or steadier.

Lightweight fillers, where weight is the constraint: styrofoam peanuts, crushed soda cans, capped plastic bottles, bubble wrap. This is the right group for a roof deck, a balcony, or anything you will need to move.

Heavy fillers, where stability matters more than weight: large rocks, gravel, sand, glass pebbles, broken ceramic. These add ballast low in the planter, which lowers the center of gravity in an exposed spot. They also add load, so they are the wrong answer on a structure with a limit.

Whichever you use, two rules hold. Put a layer of landscape fabric between the filler and the potting mix, so the mix cannot wash down into the voids. And keep the drainage holes clear: filler that blocks a hole undoes the reason the hole is there.

Does gravel at the bottom improve drainage?

For Jay Scotts planters, gravel belongs in a separated drainage layer, never mixed into the soil. The Jay Scotts owner's guide method for outdoor and long-term planting is: permeable landscape fabric at the base, a lightweight drainage layer such as rinsed gravel or lava rock, a second layer of fabric, then a well-draining potting mix.

The water leaves through the drainage holes; the layer keeps them clear. The Jay Scotts planting tips make the point directly: holes will not drain if they get plugged, so use pebbles larger than the holes, or mesh or landscape fabric, before adding soil. In a deep planter, any filler sits below the fabric with the drainage layer and the potting mix goes above it. Heavy gravel also adds load, so keep it light on a roof or balcony. Hole size, spacing and placement are in our drainage guide.

What should you never use?

Jay Scotts flag two failure modes. Anything that can leach into the medium is a problem in an edible planting: newspaper and paper cups can leach ink. And containers that trap standing water inside them, plastic bottles among them, can hold water in the planter and harbour bacteria, so cap them and keep them dry.

Their owner's guide also rules out heavy garden soil, fine sand, biodegradable packing peanuts, chemically treated materials and wood chips as a bottom layer. Garden soil and sand are heavy, compact, and neither behaves like a container mix. Wood chips and anything else that breaks down encourage mold and bacteria as they decompose.

The internal shelf

Many Jay Scotts models offer an optional internal shelf, and it is the cleanest version of this whole idea. The shelf sits below the rim so a nursery pot rests on it, leaving the volume underneath empty rather than filled with anything at all. Each shelf weighs about 3 to 5 pounds and supports 75 to 100 pounds.

It also makes the planting removable. A seasonal display can be lifted out and swapped without excavating the planter, which is the usual reason a commercial installation looks tired by August.

How much weight does this save?

Enough to change what the structure has to carry, and on a roof or a balcony that is the whole question. The planter itself is a small share of the final load: our fiberglass sizes are published from 2.2 pounds up, so almost everything you are carrying is medium and water.

Soil weight depends entirely on the mix, so use your supplier's stated figure rather than a generic one. What you control is volume, and filling a tall planter properly can cut the medium by half or more without the plant noticing. Our rooftop guide works through load, access and wind where that matters most.

Filling by planter shape

Shape decides how much of this applies.

Tall columns and cubes benefit most: the root zone is a small fraction of the interior, so most of the volume is fillable. These are also the shapes that concentrate weight over a small footprint, so the saving matters twice. See tall planters and extra large planters.

Low bowls barely benefit. The interior depth is already close to the root depth, and a filler layer would leave too little medium. Fill these with mix.

Long troughs and boxes sit in between, and the win is usually weight rather than cost, because a long planter box planted to the base is difficult to move once it is wet.

The number that settles it is interior depth, which we publish per size on the product page rather than leaving you to guess from the outside dimensions.

Shop the collection Large outdoor planters

Fiberglass from Jay Scotts, with the interior opening, weight and delivery method published on every size.

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Before you fill

• Check the interior depth for your exact size, not the exterior height.
• Work out the root depth your planting needs, then fill below that line.
• Choose light filler for roofs, balconies and anything that moves; heavy filler for exposed, windy ground.
• Lay landscape fabric between filler and mix.
• Keep drainage holes clear, and make sure the planter has them. Drilling is free on request, and a drilled planter is final sale.
• Fill the mix to about two inches below the rim.
• Cap anything hollow so it cannot fill with water.

Frequently asked questions

What do you put in the bottom of a large fiberglass planter?

A permeable filler, chosen for weight. Styrofoam peanuts, crushed cans, capped plastic bottles and bubble wrap are the lightweight options that do not break down, and rocks, gravel, sand, glass pebbles and broken ceramic add ballast instead. Avoid wood chips and anything else that breaks down, which encourages mold and bacteria. Separate the filler from the potting mix with landscape fabric.

Should I put rocks or gravel in the bottom for drainage?

For Jay Scotts planters, as a separated layer, yes. Their owner's guide puts a lightweight drainage layer, such as rinsed gravel or lava rock, between two layers of permeable landscape fabric under the potting mix. The water drains through the holes, and the layer keeps them clear. Heavy gravel also adds load, so keep it light on roofs and balconies.

How much soil does a large fiberglass planter need?

Annuals need 4 to 8 inches, perennials and bulbs about 8, herbs and vegetables 8 to 10, and a tree at least twice the depth of its root ball. Fill to roughly two inches below the rim. Anything deeper than the root zone is doing no work.

Can I use styrofoam or plastic bottles?

Yes, both are on Jay Scotts' lightweight list. Cap bottles so they cannot fill with water, since standing water inside a filler can harbour bacteria. Avoid newspaper and paper cups in an edible planting, because ink can leach.

Does filling a planter this way harm the plant?

No, provided the remaining medium is deep enough for the roots and the drainage holes stay clear. The roots never reach the bottom of a large planter, so the volume down there is storage, not growing space.

What is the internal shelf for?

It holds a nursery pot near the rim so the volume beneath stays empty. On Jay Scotts models the shelf weighs about 3 to 5 pounds and supports 75 to 100 pounds, and it makes seasonal planting swappable without emptying the planter.

Do I still need drainage holes if I use a filler?

Yes. Filler changes how much medium sits in the planter; it does not give water a way out. Drilling is free on request, cut to Jay Scotts' published standard of 1 to 2 inch holes every 12 inches, and a drilled planter is final sale.

• Read nextPlanter Drainage: Holes, Sizes and Setup
• Read nextHow to Choose a Tall Outdoor Planter