9 Coir Grow Bag Sizes and Specifications That Shape Your Whole Season

Most substrate complaints are really specification complaints. A grower calls to say the crop is uneven, and after twenty minutes of questions it turns out the slab volume was wrong for the plant density, or the drainage cuts sat in the wrong place, or the bag was fifteen centimetres too long for the gutter. None of that is a quality issue. It is a numbers issue, and coir grow bag sizes and specifications are the numbers that decide whether your season runs smoothly or fights you from week three.

Let me walk through what actually matters on that spec sheet, because the brand name on the pallet tells you far less than the dimensions printed beside it.

Why the Spec Sheet Beats the Sales Pitch

Two slabs can look identical on a warehouse floor and behave nothing alike. One holds eighteen litres of expanded substrate, the other holds twelve. One has a fine pith blend that holds water tightly. The other carries thirty percent chips and drains like a sieve.

Your gutter system does not care about branding. It cares about length, width and height. Your irrigation controller cares about volume per plant. Your labour team cares about whether a bag can be carried one-handed at the start of a long day.

Get those three constituencies happy and the rest tends to look after itself.

Start With Volume, Not Length

Almost everybody starts by asking for a length. Wrong end of the problem.

Volume per plant is the number that drives root development, your safety margin against irrigation faults and how forgiving your system is when a valve sticks. Once you know the volume each plant needs, the external dimensions fall out of that decision naturally.

Rough working figures used across commercial protected cropping:

• High-wire tomato: four to six litres per plant
• Cucumber: three to five litres per plant
• Capsicum and bell pepper: four to six litres per plant
• Melon: five to seven litres per plant
• Strawberry on table-top: one point five to three litres per plant
• Leafy greens in slabs: under one litre per plant, though most run plugs instead

Multiply by plants per bag, and there is your target volume. Everything else is arithmetic.

The Nine Coir Grow Bag Sizes and Specifications Worth Committing to Memory

Here are the formats that cover the vast majority of commercial greenhouse work. Dimensions are given as length by width by height in centimetres, with expanded volume in litres.

Those are working formats rather than fixed law. Most manufacturers will adjust any dimension by a few centimetres without much fuss, which matters more than growers expect.

Open Top or Fully Wrapped? Not Just Preference

A fully wrapped slab arrives sealed in film. You cut planting holes and drainage slits yourself. It stays compressed until you hydrate it, which makes storage tidy and handling quick.

An open top bag ships with the upper surface exposed or with a pre-cut opening. Evaporation from the surface is higher, which some growers want and others fight all summer.

The choice usually comes down to climate. Hot, dry houses in the Middle East or southern Spain tend toward wrapped slabs to hold moisture. Cooler northern houses running high humidity sometimes prefer open tops because the extra evaporation helps. Blueberry and cane fruit growers almost always use open top or planter formats, since those crops sit in the substrate for years and need surface access for topping up.

Neither is better. They are answers to different questions.

Drainage Cuts: Small Slits, Large Consequences

This is the specification growers most often get wrong, and it costs them yield quietly rather than dramatically.

Drainage slits are usually cut into the base of the slab along the lower edge, not on the underside. Cutting the underside sounds logical and drains too aggressively, leaving the base dry. Cutting along the lower side wall holds a shallow reservoir in the bottom of the bag, which is exactly what you want when a pump fails on a hot afternoon.

Typical practice: two to four slits per slab, each two to four centimetres long, positioned about one centimetre up from the base. Some growers cut them at planting. Others prefer them pre-cut at the factory for consistency across a large house, which removes a variable from a job that gets done by different people on different days.

Honestly, if you have ten staff cutting slits by eye across forty thousand bags, you do not have a specification. You have a range.

Film Colour, Thickness and the Bits Nobody Reads

The plastic sleeve is not just packaging. It is part of the growing system.

White outer film reflects light and keeps root zone temperature down, which matters in high-light regions. Black inner layers block light and stop algae growing on the substrate surface. Most commercial slabs use white outer with black inner for exactly this reason.

Thickness usually runs somewhere between forty and eighty microns. Thinner film is cheaper and tears when a forklift breathes on it. Thicker film survives handling and a second season if you plan to reuse. For a single short crop, thin is fine. For a twelve-month tomato run followed by a reuse cycle, spend the extra.

UV stabilisation matters if any part of the bag sees direct sun through the glass for months on end. Ask. It is rarely listed unless you request it.

Compression Ratio and What Actually Arrives

Here is a practical detail that catches importers off guard. Coir ships compressed, and the compression ratio decides how much substrate fits in a container.

A five to one ratio means the material expands to five times its shipped volume once hydrated. Ratios in commercial slabs commonly run between four to one and eight to one depending on the press and the grade. Higher compression means lower freight cost per litre of finished substrate, which on a full container is a serious number.

But higher compression also means more thorough hydration is needed before planting. Rush that step and you get a slab that looks full while the core is still dense and dry. The coconut husk material behind all of this, described in detail on the coir reference page, is naturally fibrous and takes water at its own pace. Give it time.

Check the moisture content on loading too. It affects landed weight, and freight is priced by weight as often as volume.

Matching the Format to the Crop

Some quick pairings that hold up across most regions.

Tomato. Wide or long slabs, eighteen to twenty-four litres, three to four plants. High-wire crops run long, so volume buys you safety margin.

Cucumber. Standard slabs work well. Cucumbers are thirsty but the crop is shorter, so extreme volume is wasted money.

Capsicum and bell pepper. Similar to tomato, though peppers appreciate the slightly deeper formats because root mass builds steadily rather than fast.

Melon. Deep or extended slabs. Melons put on enormous vegetative growth in a short window and punish any water stress at fruit set.

Strawberry. Trough bags or compact slabs on table-top systems. Volume per plant is low, plant count per bag is high.

Blueberry and cane fruit. Planter bags or open top formats, since these crops stay put for years. Many growers here also blend in coco peat blocks when they want to build a custom mix with a coarser fraction.

When Standard Sizes Simply Do Not Fit

Gutter systems vary more than catalogue photographs suggest. A house built in 2009 may have gutters that suit nothing currently sitting in a distributor’s warehouse.

This is where custom manufacturing quietly earns its keep. Send your internal gutter width, your plant spacing and your target volume per plant, and a production run can be set up around those numbers rather than the other way round. The formats listed on our coco peat grow bags page cover most standard requirements, and adjustments outside those dimensions are routine when the specification arrives early enough to schedule.

I have used this approach with a capsicum operation running unusually narrow gutters. Rather than force a standard slab and lose contact area, we shifted width down by three centimetres and added length to keep the volume identical. Our customers are really happy when a bag simply sits flat without anyone kicking it into place, and the grower said afterwards: “The planting crew finished a day early and nobody mentioned the bags once, which is how I knew we got it right.”

That is the whole point. Good specifications are invisible.

Frequently Asked Questions

What is the most common coir slab size in commercial greenhouses?

The 100 x 20 x 10 centimetre slab holding around eighteen to twenty litres is the workhorse format across tomato and pepper production worldwide. It suits three plants comfortably and fits most standard gutter widths without modification.

How many drainage slits should a coir slab have?

Two to four slits per slab is standard, each around two to four centimetres long, cut into the lower side wall rather than the underside. This retains a small reserve of solution at the base while still preventing waterlogging.

Can coir grow bags be made in custom dimensions?

Yes, and it is far more common than growers assume. Length, width, height, film specification and drainage cuts can all be adjusted to a specific gutter system, provided the order is planned with enough lead time for a dedicated production run.

Does slab height matter more than length?

Height drives the water column and how sharply moisture varies from top to base. A taller slab holds a wetter bottom layer and a drier top, which suits crops with deep root activity. Length mainly affects plant spacing and handling.

How much does compression affect what I receive?

Considerably. A slab compressed at six to one occupies a sixth of its final volume during shipping. Always confirm the ratio and the moisture content on loading, since these determine both your freight cost and how many pallets your store needs to hold.