For a greenhouse or polytunnel crop, choose white-outside, black-inside co-extruded film. The white skin reflects sunlight so the root zone runs cooler, and the black inner layer stops light reaching the coir, which keeps algae off the surface and roots in the dark. Plain black film only makes sense where you want the slab warmer, such as an unheated winter planting in a cool climate.
That's the short answer. The rest of this page covers how much cooler white really runs, what "two-year UV" means when your greenhouse isn't in the climate the film was rated for, how thick the sleeve should be, and what happens to the plastic at the end. What a grow bag is, the pith and chip blends and the gauge-to-years table are in coco coir grow bags explained, so they aren't repeated here.
What the two layers do
A grow bag sleeve is a tube of low-density polyethylene. Most are co-extruded: two melts pushed through the die at once, so the film comes out as one sheet with a white face and a black face. It isn't two films glued together, so it can't delaminate.
| Layer | Job | What happens without it |
|---|---|---|
| White outer | Reflects sunlight, lowers peak substrate temperature | Slab absorbs radiation and heats well above air temperature on bright days |
| Black inner | Blocks light reaching the coir and roots | Green algae on wet exposed coir, roots avoid the lit zones |
| Both | Water barrier, holds the slab shape, carries the holes and slits | Slab dries at the edges and loses structure |
White film on its own is not opaque. Hold a single white sheet up to the light and it glows. That's why the black layer is there even in cool climates where heat is no problem. It's the light exclusion, not the colour you see, that stops algae.
White vs black: the temperature numbers
Nobody has published a clean trial of white versus black sleeves on coir slabs under glass, so the data below come from the two closest things: black and white plastic pots filled with soilless substrate, and coloured mulch films on soil. Both are polyethylene heated by sunlight, so the direction and rough size of the effect carry across. The absolute numbers won't, because a slab under glass sees less direct sun than a pot outdoors.
Pots outdoors. In a nursery trial run in Tennessee and Ohio, substrate in a black plastic pot in full sun ran up to 30 °F (about 17 °C) above air temperature, with peaks of 133 °F (56 °C). Roots suffer indirect injury when they sit above roughly 104 °F (40 °C) for five to six hours. Over 14 weeks the loggers recorded how long the root zone stayed above the lower stress line of 100 °F (38 °C):
| Pot colour | Irrigation | Hours above 38 °C in 14 weeks |
|---|---|---|
| Black | Once daily, morning | 332 |
| Black | Three times daily, afternoon | 234 |
| White | Once daily, morning | 152 |
| White | Three times daily, afternoon | 22 |
White pots halved the stress hours on their own, and white plus afternoon irrigation almost removed them. In several species the plants in white pots finished up to twice the size of those in black. Those white pots had an opaque black inner wall, the same construction as a co-extruded grow bag, and cost about 10 to 20 percent more than black.
Mulch film on soil. With 35-micron coloured mulch films on soil in a warm climate, mean root-zone temperatures over a winter and a spring crop came out like this:
| Film | Winter mean RZT | Spring mean RZT |
|---|---|---|
| Black | 16.3 °C | 22.1 °C |
| White on black | 15.4 °C | not reported separately |
| White | 14.4 °C | 20.1 °C |
| Bare soil | 14.3 °C | 20.2 °C |
About 2 °C separated black from white in both seasons. Under weak winter sun white film tracked bare soil almost exactly, and white-on-black sat between. Expect the same pattern from a slab sleeve: black warms the root zone by a couple of degrees in dull weather and by far more in strong light, while white keeps it close to air temperature.
For a fruiting crop under glass the usual aim is to keep the substrate within 5 to 6 °C of the greenhouse air around the plant heads. A slab much colder than the air gives poor root penetration and weak water uptake on bright spring mornings, which shows up as blossom end rot in tomatoes. A slab much hotter stops root growth. White film handles the second problem. Root-zone heating handles the first, and black film is a cheap, partial substitute for it in cool, dull conditions.
When black film is the right call
- Unheated tunnels or glasshouses planted in late winter, where a sun-warmed slab helps early rooting.
- High-latitude sites with low radiation where overheating never happens.
- Bags that sit inside an opaque trough or under mulch, where the outer colour is irrelevant.
Anywhere with a hot summer, or any long-season crop that will see midsummer under glass, gets white outside. If you're not sure, white. A cold slab is easy to fix with heating pipes. A cooked one isn't.
Film thickness: what the trade uses
Sleeve film is quoted in microns or gauge, and one micron is four gauge. A mil, used in North America, is 25.4 microns. Slab sleeves sit in the 75 to 125 micron range. Heavier co-extruded sheeting around 150 microns is sold for slab covers and ground sheets, and greenhouse roof film runs 150 to 250 microns because it's stretched over a frame and flexes in the wind. A sleeve lying flat on a gutter doesn't need that.
Thickness buys puncture resistance when you push drippers through it, tear resistance when you lift a wet 15-litre bag by its ends, and more stabiliser per square metre. Bag weights are on grow bag sizes and plant density.
What "UV-rated for two seasons" means
Polyethylene has no natural resistance to sunlight. Ultraviolet breaks the polymer chains, the film chalks, and it goes brittle and splits along the folds. Film makers slow this with a stabiliser package, and the rating they give it is a cumulative radiation dose, not a calendar time. The unit is the kilolangley, where one langley is one calorie of solar energy per square centimetre.
The "two seasons" on a spec sheet is that dose converted to time for a reference climate, normally a temperate one with moderate sunshine. In practice:
| Situation | Effect on film life |
|---|---|
| High-radiation site (Mediterranean, Middle East, tropics, high altitude) | The same dose arrives in far less time. A two-year rating can become one season. |
| Roof film that absorbs UV, or whitewashed in summer | Dose arrives slower, so the sleeve outlasts its rating. |
| Bags stored outdoors before planting | The clock is already running. A pallet left in the sun for a month has spent a month of its rating. |
| Sulphur burning or chlorine-based sanitisers | These deactivate the stabiliser and shorten life regardless of dose. |
| Contact with hot galvanised steel or PVC | Local degradation where the film touches the metal, or where plasticiser migrates from the PVC. |
Ask your bag supplier how many years the film is rated for and in which climate. If they can't answer the second, assume a mild climate and derate for yours. Coir bag suppliers offer stronger UV stabilisation on custom orders, so a bag meant for a second strawberry crop in southern Spain should be specified that way, not bought off the shelf.
Pre-cut holes and slits
Holes are cut at the factory in whatever pattern you order: plant holes on the top face, small dripper holes beside them, and drainage slits either pre-cut or left for you after wetting up. The wetting and cutting routine is in the grow bags article. The film-specific points:
- Cutting holes yourself. A sharp blade and a template beat scissors. Ragged edges tear further when roots and stems push against them.
- Hole edges and algae. The ring of coir exposed through a plant hole is the one place light reaches the substrate. Keep it small, let it dry between irrigations or cover it with the propagation block, and algae stays off.
- Slits go low on the sides, not the base. After planting, the film's job is to hold a water buffer in the bottom of the slab. Slits in the base defeat it.
- Pre-cut slits on a stacked pallet let rain in during storage as readily as they let drainage out during the crop. Keep them under cover.
Open-top bags and pots: what changes
An open-top bag is coir compressed into a pot shape inside a plastic bag with no top face. The film is black inside and white or black outside, same as a slab sleeve, but three things differ:
- The coir surface is exposed. Algae on the open top is normal and mostly harmless. A mulch disc or a layer of husk chips covers it if it bothers you.
- The side walls face the sun. A slab lies flat and shows the sky a narrow strip of film. An open-top stands 14 to 24 cm tall with its walls in direct light, so the white outer layer matters more and the UV dose per square metre is higher.
- Drainage holes are punched, not slit, usually a ring a little above the base so they don't seal against a wet floor.
Sizes and crops are on open-top bags.
Handling and storing bagged slabs
Compressed slabs in film store well because the film keeps moisture out and the coir is dry. What they don't tolerate is sun and heat on the pallet.
- Keep pallets under a roof or an opaque cover. Sun before planting comes straight off the film's rated life, and the outside of a pallet gets a full dose while the centre gets none, so bags from one pallet age unevenly.
- Clear stretch wrap is not a sunshade. It transmits UV.
- Hot storage turns a sealed bag into a condensation trap.
- Handle bags by the body, not the sealed ends. The heat seal is the weakest line.
- Wet bags within a day or two of laying them out. A dry bag in a hot greenhouse takes its UV dose at the highest temperature it will ever see.
Recycling and disposal of used film
The sleeve is LDPE, resin code 4. It is not biodegradable and cannot be composted. It is one of the most recyclable plastics there is, provided it reaches a recycler clean and sorted.
| Problem | What to do |
|---|---|
| Coir and roots stuck to the film | Shake out and brush. Recyclers tolerate some substrate but reject bundles that are mostly coir. Grower take-back schemes in some countries separate film from substrate for you. |
| UV-degraded, brittle film | Still recyclable, but worth less. Shorter polymer chains make lower-grade pellets. Don't leave spent bags out for a season after the crop. |
| Black film | Optical sorters at general recyclers can't detect carbon-black plastic, so white/black film may be refused at kerbside and accepted at an agricultural scheme. Ask. |
| Mixed with drip tape, clips and twine | Sort it. Mixed-polymer bundles get landfilled. |
Agricultural film schemes typically want film folded into flat bundles to a pallet footprint, under a weight one person can lift, free of clips and string. Some accept only clear and white film. Where there's no scheme, the film goes to general waste and the spent coir goes back to the soil, which is covered in what to do with spent coco coir.
Compostable sleeves have been trialled and dropped for slabs, because they break down in moisture, which is exactly where a grow bag lives for a year. Non-compostable bio-based films still have to be separated and recycled like LDPE. Three-layer film with a recycled middle layer between virgin skins is further along and is the likelier near-term change.
Specifying the sleeve
When you order grow bags, give the film lines a sentence each: colour, thickness in microns, UV rating in years with the reference climate named, the hole and slit pattern, and opaque pallet wrap if the bags will ever stand outdoors.
CVM Fiber and Industries in Kanyakumari, Tamil Nadu, India, presses grow bags and open-top slabs from coir pith and pith-chip blends, washed with water to reduce EC and not buffered. To talk through film colour and hole patterns for your crop and climate, get in touch.
FAQ
Should grow bag film be white or black?
White outside with a black inner layer for almost every greenhouse crop. White reflects sunlight and keeps the slab close to air temperature, and the black layer excludes light so algae and roots stay in the dark. Choose all-black only where you want the slab warmer, such as a late-winter planting in an unheated house in a cool climate.
How much cooler does white film keep the root zone?
In nursery pots outdoors, white roughly halved the hours the substrate spent above 38 °C compared with black, and black pots peaked at 56 °C on hot days. Under coloured mulch films on soil the gap between black and white was about 2 °C in mean root-zone temperature. A slab under glass lands somewhere between those cases depending on how much direct sun reaches it.
What does a two-year UV rating mean?
The stabiliser package is designed to absorb roughly two years' worth of solar radiation in the climate the maker used as a reference, usually a mild one. In a high-radiation climate the same dose arrives sooner and the film can fail in one season. Sulphur, chlorine sanitisers and time on an outdoor pallet before planting all shorten it further.
How thick is grow bag plastic?
Slab sleeves are generally 75 to 125 microns, which is 300 to 500 gauge. Heavier co-extruded sheeting around 150 microns is sold for slab covers, and greenhouse roof film runs 150 to 250 microns. Thicker film resists punctures from drippers and tearing when a wet bag is lifted, and carries more stabiliser.
Can I store grow bags outside before planting?
Under an opaque cover, yes. In open sun, no. The film's UV life starts counting when sunlight hits it, so a pallet left outside for a month has used a month of its rating before the crop starts. Clear stretch wrap doesn't help because it transmits UV, and heat on the pallet drives condensation inside the bag.
Why does algae grow on coir around the plant holes?
Because that ring of coir is the only place light and moisture meet. The black inner layer keeps the rest of the slab dark, but the exposed coir at the hole edge is wet after every irrigation and lit all day. Keep the exposed area small, let it dry between irrigations, or cover it with the propagation block or a mulch disc.
Can used grow bag film be recycled?
Yes. It is LDPE, resin code 4, which is widely recycled when clean and sorted. Shake out coir and roots, keep it separate from drip tape and clips, and fold it into flat bundles. Black-backed film can be refused by kerbside sorting because optical sorters can't detect carbon black, so use an agricultural film scheme or a grower take-back where one exists.