A grow bag is a soft-sided bag filled with soil, coco coir or another medium and used instead of a rigid pot. It can be made of fabric or plastic. Among cannabis growers the word usually means a fabric bag of nonwoven, felt-like material, the same idea as a fabric pot: air and excess water pass through the walls, and roots that reach the edge are air pruned instead of circling. In British gardening a growbag usually means something different, a sealed plastic bag of compost that you lay flat and plant straight into. Grow bags are cheap and store flat, but fabric ones dry out faster, so plan on more watering in hot weather.
Two meanings of grow bag
| Fabric grow bag (fabric pot) | British plastic growbag | |
|---|---|---|
| Material | Breathable nonwoven fabric | Plastic bag, sold full of compost |
| How it is used | Filled with your own mix, stands upright like a pot | Laid flat; you cut a panel in the top and plant into it |
| Typical crops | Cannabis, vegetables, herbs | Tomatoes, peppers, salad crops in greenhouses |
| Air pruning | Yes, through the walls | Little, the plastic does not breathe |
| Drainage | Through the whole fabric | Only where holes are made in the plastic |
A university extension article describes fabric grow bags as bags made of nonwoven felt-type fabric from natural and synthetic fibres. The Royal Horticultural Society describes the other kind: lay the bag down, cut out a long panel along the top, and sow or plant into the compost. New British-style bags offer a clean, sterile medium, which is why they are popular in greenhouses. The rest of this article is about the fabric kind, because that is what cannabis growers mean in most cases.
Fabric vs plastic pots
| Feature | Fabric grow bag | Rigid plastic pot |
|---|---|---|
| Root shape | Air pruned, dense and branched | Roots circle the wall over time |
| Drainage | Excellent, hard to over-water | Only through holes in the base |
| Root temperature in sun | Stays cooler than plastic or metal | Dark plastic heats up in sun |
| Watering frequency | Higher, medium dries faster | Lower |
| Feeding | More frequent feeding needed | Standard |
| Lifespan | Several seasons, then replace | Many years |
| Storage | Folds flat | Stacks, takes more space |
How air pruning works
In a plastic pot, a root that hits the wall turns and follows it. Weeks later the plant is root bound. In a fabric bag, the root tip meets air and drier medium at the edge and stops growing in that direction. This is air pruning. The plant responds by growing new roots behind the pruned tip. The result is a robust root system with many small feeder roots that take in water and nutrients.
An organic cannabis grower writing about living soil rates fabric pots very highly for the same reason: air reaches both the roots and the soil mix. Tree nurseries use air-pruning containers to avoid circling roots too.
Watering in hot climates
The biggest change when you switch to fabric is water. Because the bag drains and breathes, the medium dries out faster and needs watering more often. It is hard to over-water a grow bag, but easy to let it dry out.
This matters a lot in Southeast Asia. One cannabis grower advises that in hot grow rooms, hot dry greenhouses, or outdoors in hot dry weather, you should use larger fabric pots than normal unless you are happy to water at least once a day. He reports that 1 m tall flowering plants in 11.5 litre fabric pots sometimes needed water up to three times a day.
Ways to manage it:
- Choose a bigger bag than you would choose in a plastic pot.
- Water slowly until the whole bag is wet, then let excess drain.
- Consider drip irrigation on a timer for outdoor bags in the dry season.
- In the rainy season, the fast drainage becomes an advantage, since the bag sheds excess water.
- Do not let peat-based mixes dry out completely. Dry peat can be hard to wet again.
- Feed more often than you would in a bed, because nutrients leave with the drainage water.
Saucers need care. They protect floors, but water left standing in a tray for more than 30 minutes can make the bottom of the root zone airless.
Sizes
There is no single right size. It depends on how big the plant will grow and how long it stays in the bag. Extension guidance written for vegetables gives useful starting points:
- At least 5 gallons, about 19 litres, for one plant.
- 10 to 15 gallons, about 38 to 57 litres, for most annual edible plants.
- Larger bags give more room but become heavy and hard to move.
For comparison, one indoor organic grower finishes plants of about 60 to 90 cm in 7.5 to 9.5 litre pots, while the same grower warns that 11.5 litre fabric pots can need watering several times a day. Larger outdoor plants in full tropical sun need far more medium. When in doubt, go bigger, because a larger volume dries more slowly.
Filling a grow bag
Use a potting mix, not garden soil. Garden soil compacts in a container. A good mix contains peat or coir plus perlite or vermiculite to hold moisture and still drain well. One organic grower names vermiculite as his first choice for fabric pots, with pumice second. Fill the bag right to the top so you use all the space.
Cleaning and reuse
Fabric bags usually last several seasons before they start to wear out.
- After harvest, empty the old medium into compost or a recycling bin.
- Brush off dried roots and soil from the fabric.
- Wash the bag. For disinfecting containers, one extension guide uses one part bleach to nine parts water.
- Rinse well and dry fully before storing.
Starting each season with fresh medium lowers the risk of built-up soil diseases. Note that some bags contain polypropylene or polyethylene fibres, so they are not biodegradable.
Common mistakes
- Watering on a plastic-pot schedule. Fabric bags need water more often.
- Choosing a bag that is too small for the climate. In tropical heat, extra volume buys time between waterings.
- Leaving bags in full saucers. Empty trays after watering.
- Filling with garden soil. It compacts and drains poorly.
- Letting the mix dry out completely. Rewetting dry peat is slow and uneven.
- Under-feeding. Frequent watering means frequent feeding.
- Storing bags damp. Dry them fully to prevent mould in humid weather.