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, from either brand, 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: coconut fiber, wood chips, sphagnum moss, pinecones, 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, wood logs. 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?

No, and this is the most repeated piece of bad advice in container gardening. Water does not move freely from a fine potting mix into a coarse layer beneath it. It sits in the mix until the mix is saturated, which raises the wet zone into the roots rather than draining it away.

David Trinklein at the University of Missouri sets out the mechanism: “a layer of gravel causes the roots of a plant to encounter saturated growing media that is poorly drained”, because “by adding a non-capillary layer of material to the bottom of a container, the depth of the water column in the container is shortened and drainage is reduced”.

The distinction worth holding onto: gravel low in a planter is ballast, not drainage. If you want it for weight in a windy spot, use it, keep it below the fabric, and keep the holes clear. If you want the planter to drain, that is a question about holes, and our drainage guide covers hole size, spacing and placement.

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 is separately explicit that heavy garden soil and fine sand should not be used in the planter at all. Both are heavy, both compact, and neither behaves like a container mix.

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.

Which planters already have a false bottom?

Six sizes in our range do not need filling at all, because the false bottom is molded in. They are the Cosapots Lofto Tall Column, a slim column 30 to 43 inches tall in six stone finishes, and the Reglo Modular Rectangle, a low modular trough meant to be run in a line. In both, the interior stops well above the base.

The Lofto shows it clearly in the published numbers: at 30 inches tall the interior depth is 14 inches, and at 36 inches it is 18. That is 45 to 49% of the exterior height, against a median of 92% across the rest of the range. The volume below is sealed, not empty and waiting to be packed.

• Lofto Tall ColumnCosapots · 30 to 43 in slim column · false bottom
• Reglo Modular RectangleCosapots · low modular trough · false bottom

For everything else, check the interior depth published on the product page against the exterior height. Where the two are close, the planter is hollow to the base and the fill below the root zone is yours to choose.

Does it differ between Jay Scotts and Cosapots?

The principle is the same for both, but two practical details are not.

The optional internal shelf is a Jay Scotts feature, offered on many of their models. Where a model has one, it is the tidiest way to fill a deep planter, because nothing goes underneath at all. Where a model does not, or on a Cosapots planter, you are filling the void yourself with a light material and a fabric layer above it.

Drainage differs at the point of order rather than in the method. Cosapots ship sealed, so an outdoor planting needs the holes requested when you buy. Jay Scotts drill to their own published standard. Both drill free on request, and in both cases a drilled planter is final sale, so it is a decision to make before checkout rather than after.

Interior depth is published for both, and for Cosapots it comes off the manufacturer's own dimension drawings. That is the figure to plan the fill around: a Cosapots Reglito Cube and a low Cavo bowl of similar footprint want completely different amounts of medium.

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 and Cosapots, 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 from both brands, 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. Jay Scotts list coconut fiber, wood chips, sphagnum moss, pinecones, styrofoam peanuts, crushed cans, capped plastic bottles and bubble wrap as lightweight options, and rocks, gravel, sand, glass pebbles, broken ceramic and logs where you want ballast instead. Separate the filler from the potting mix with landscape fabric.

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

Not for drainage. A coarse layer under a fine mix raises the saturated zone into the roots rather than draining it, as the University of Missouri explains. Gravel low in a planter is useful as ballast in a windy spot, but drainage comes from holes, not from a layer of stone.

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 any fiberglass planters come with a false bottom?

A few. The Cosapots Lofto Tall Column and Reglo Modular Rectangle have one molded in, so the interior stops well above the base: a 30-inch Lofto has 14 inches of interior depth. Most fiberglass planters are hollow nearly to the floor, with a median interior depth of 92% of exterior height across our range.

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 from both brands, 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