Use a sleeved slab when you grow a row crop like tomatoes, cucumbers or strawberries on gutters, where several plants share one long root zone and you want tight control of dry-back. Use an open-top bag when each plant needs its own container: peppers at wide spacing, blueberries, raspberries, roses and other plants that stay put for more than a season. Loose-filled pots do the same job as open tops but cost you filling labour.
The rest of this page explains why, and where the choice is less obvious. What a grow bag is and how it's made is covered in coco coir grow bags explained. Sizes and plants per bag are in grow bag sizes and plant density. This page is about choosing between the formats.
Three formats, three shapes
Sleeved slab. Compressed coir in a closed plastic sleeve, usually a metre long, laid flat. You cut plant holes in the top, push drippers in and cut drain slits low on the side. Almost all the coir is in the dark. Several plants share the same root zone.
Open-top bag. Coir pressed into a pot-shaped plastic bag with the top left open. You set it where it'll grow, add water from above, and it swells to fill the bag. One plant per bag, sometimes two. It's taller than a slab and has a smaller footprint.
Loose-filled pot. A rigid pot or empty bag you fill yourself from blocks or bulk coir. Same shape as an open top, but someone has to hydrate the coir, fill and firm every pot.
Sizes vary a lot. One retail range of open-top coir bags runs from a 15 cm cube holding 4.6 litres once expanded to a 20 cm cube holding 12.3 litres, each with a centre hole for the plant and two side holes for drippers. Commercial bags for blueberries and cane fruit go much bigger, around 19 to 23 litres per plant on current blueberry farms.
Height, water buffer and dry-back
Coir drains until gravity balances the pull of its pores. That leaves a wet layer at the bottom of any container, and its depth depends on the coir grade, not the container. So height decides how much of the root zone sits above that wet layer.
A flat slab keeps a big share of its coir in the wet zone. It holds plenty of water and less air, and it dries back slowly. That suits a crop that's watered many times a day and steered by small changes in slab water content.
An open-top bag stands taller on a smaller base. More of the coir sits above the wet zone, so there's more air at the top and dry-back is faster. At the same volume, taller containers drain better, which is why strawberry guidance asks for at least 2 litres of substrate per plant and favours depth. The downside: water moves down rather than sideways, so a small shot can run straight past dry coir to the drain. You need either more emitters per bag or shots big enough to wet the full width.
Volume matters less than people expect. In a sweet pepper trial in coir, plants grew in square white bags of 7, 14 and 21 litres (20, 25 and 28 cm across) at 1.4 and 2.4 plants per square metre. Bag volume made no difference to yield. Density did. The smallest bags at the lower density gave the best water use efficiency, with about 13% less crop water use than the largest bags at the higher density.
The open surface: evaporation and algae
This is the real cost of an open top. A slab sleeve covers everything except the plant holes. An open-top bag exposes its whole upper face, and wet coir in light grows algae. The green film then dries into a crust that sheds water and feeds shore flies and fungus gnat larvae. Causes and fixes are in mould, algae and fungus gnats in coco coir.
The open face also loses water straight to the air, and it adds humidity to the house. Growers handle it in a few ways:
- Fold the bag over. In the pepper trial above, the surplus plastic at the top of each bag was folded over the coir to cut evaporation, leaving a gap of about 3 cm around the stem. It's free and it works.
- Mulch the top with a thin layer of coarse husk chips, which dry quickly and don't hold a wet film.
- Let the surface dry between shots. Shots placed off the stem and not too frequent keep the top layer from staying wet all day.
A slab doesn't need any of this. That's a quiet advantage in a humid house or a crop with a long, wet winter.
Irrigation: drippers and drain holes
Slabs. One dripper per plant is the working standard for fruiting vegetables, and the drain slit goes low on the side, away from the drippers. Because several plants share one root zone, you can weigh a slab, read one sensor and steer a whole row.
Open tops. Every bag is its own island. That means at least one emitter per bag, and often more. The pepper trial used one 3 L/h pressure-compensating, anti-drain dripper per bag, split four ways with stakes around the plant base. Blueberry farms on bags or pots typically run two or more emitters per container and aim for about 10% drain. Drain holes are usually pre-cut at the base, so the bag needs to sit on a level surface or a gutter that carries run-off away. Never leave the base standing in its own drain water.
The plus side of separate bags is that a root disease in one plant stays in one bag. On a slab, a problem that starts in one plant has the whole shared root zone to travel through.
Temperature
A slab lies flat on a gutter or floor, with a big surface touching whatever's underneath. That's useful when heating pipes run under the gutter, and a problem when a raised gutter cooks in summer sun. An open-top bag has a smaller footprint and more side wall facing the air. It follows air temperature more closely and catches more direct sun on its sides when plants are small.
Either way, pick white film outside and black inside for any crop that sees summer under glass. The details are in grow bag film: white, black and UV. The pepper trial bags were white too.
Which crops suit which format
| Crop | Usual format | Why |
|---|---|---|
| Tomato, high-wire cucumber | Slab | Row crop at even spacing, many shots a day, slab-weight steering |
| Strawberry (tabletop) | Slab or trough | Many plants per metre at 2 to 3 litres each. Bags in gutters are standard |
| Strawberry (everbearer, wider spacing) | Slab or open top | Open tops work where you want separate plants or a deeper profile |
| Sweet pepper | Slab or open top | Tolerates small root volumes, so 7 litre single-plant bags yield the same as bigger ones |
| Blueberry | Open top or pot | Plants stay in the same substrate for up to five years, and need very open, airy coir |
| Raspberry, blackberry | Open top or pot | Long-cane and multi-year systems grown pot by pot |
| Cut roses | Slab | Long-lived row crop. One coir grow bag example ran eight plants per bag, first harvest at six months, about ten stems per plant |
Blueberries are the clearest case for open tops. The first substrate farms used 10 to 13 gallon pots (roughly 38 to 49 litres). Those were expensive to fill, and the extra root volume didn't improve growth or yield. Farms now use 5 or 6 gallon containers with plenty of drainage, and many have moved from pots to pre-filled bags. More on that in growing blueberries in coco coir.
Handling and set-up labour
A slab is the fastest to set up per plant: lay it, water it in, cut holes, plant several plants. Open tops take a little longer per plant because each bag gets placed, watered and planted on its own. Loose-filled pots take the longest by far. Blocks have to be hydrated, broken up, filled into pots and firmed, and filling varies from pot to pot unless you're careful. Pre-filled bags cut the labour needed to start a field, which is why blueberry growers adopted them.
Compressed open tops and slabs also ship small and dry, so storage before planting is easy. Keep them dry and out of the sun until you need them.
Reuse and disposal
Slabs often get a second season, sometimes a third, in tomatoes and cucumbers if the roots came out clean. Open tops on perennial crops simply stay in place for years, so reuse isn't really the question. Loose coir from pots is the easiest to reuse because there's no sleeve to work around. Reusing coco coir covers steaming, flushing and when not to bother.
At the end, the coir and the plastic have to be separated. A slab is one long sleeve that slits open and peels off. Open tops leave one small bag per plant, which is more handling at clear-out. Either way the coir can go to compost or soil, and clean polyethylene film can go to film recycling where it's collected.
Comparison table
| Sleeved slab | Open-top bag | Loose-filled pot | |
|---|---|---|---|
| Plants per unit | Several | Usually one | Usually one |
| Profile | Low and flat | Taller, small footprint | Taller, small footprint |
| Water buffer | Larger share of coir wet | More air at the top | Same as open top |
| Dry-back | Slower, easy to steer by weight | Faster | Faster |
| Exposed coir | Plant holes only | Whole top surface | Whole top surface |
| Algae risk | Low | Higher unless covered | Higher unless covered |
| Drippers | One per plant along the slab | One or more per bag | One or more per pot |
| Disease spread | Shared root zone | Contained per bag | Contained per pot |
| Set-up labour | Lowest per plant | Low | Highest |
| Clear-out | One sleeve to strip | One bag per plant | Tip out, pot reused |
| Typical crops | Tomato, cucumber, pepper, strawberry, rose | Blueberry, cane fruit, pepper, everbearer strawberry | Blueberry, nursery stock, trials |
Choose this if
Choose a sleeved slab if you grow a row crop on gutters, you irrigate many times a day and steer by slab weight or sensors, and you want the least exposed coir and the lowest set-up time per plant.
Choose an open-top bag if each plant should have its own root zone, the crop stays in place for several seasons, you want disease contained to one plant, or your spacing is too wide for a shared slab.
Choose a loose-filled pot if you already own rigid pots, you need a custom mix pot by pot, or you're running a small trial and labour isn't the constraint.
What we supply
We make grow bags and open-top bags in Kanyakumari, Tamil Nadu, India, from coco peat that's washed with water to reduce EC, with no calcium nitrate or other chemicals. It isn't buffered. If you're weighing slabs against open tops for a specific crop, send us your crop, spacing and set-up and we'll talk through the format.
FAQ
What is an open-top coco bag?
It's coir pressed into a pot-shaped plastic bag with the top left open. You place it, water it from above, and the coir expands to fill the bag. It's planted like a container, usually with one plant per bag.
Are open-top bags better than slabs for peppers?
Neither wins on yield. Peppers tolerate small root volumes, and in one trial 7, 14 and 21 litre single-plant bags gave the same yield. Slabs suit dense row planting with slab-weight steering. Open tops suit wider spacing and growers who want each plant separate.
Can I grow strawberries in open-top bags instead of grow bags?
Yes, as long as each plant gets at least 2 litres of coir and the bag drains well. Tabletop growers mostly stay with slabs because many plants share one bag at close spacing. Open tops make more sense for wider spacing or where you want a deeper profile per plant.
Why does algae grow on open-top bags and not on slabs?
Algae need light, water and nutrients. A slab sleeve keeps the coir dark except at the plant holes, while an open top exposes the whole wet surface. Folding the spare bag plastic over the top, or a thin layer of husk chips, cuts both the light and the evaporation.
How many drippers does an open-top bag need?
At least one per bag, and two or more for larger bags or long-lived crops like blueberries. Because water moves straight down in a tall bag, spreading the water around the plant wets the full width better than one emitter at the stem.
Do open-top bags dry out faster than slabs?
Usually, yes. The taller profile puts more coir above the wet layer at the base, and the open surface loses water to the air. That gives more air at the roots, but it also means shorter gaps between irrigations in hot weather.
Is a coir slab or an open-top pot better for blueberries?
Open tops or pots. Blueberries stay in the same substrate for up to five years and need a very airy, well-drained root zone, which suits individual containers of around 19 to 23 litres. Pre-filled bags save the labour of filling pots.
Can open-top bags be reused?
On perennial crops they stay in place for years, so reuse isn't usually the question. For annual crops the coir can often be reused after the old roots are removed and the coir is flushed, but each bag has to be emptied by hand, which takes longer than stripping one slab sleeve.