The small format, the one for the balcony and the windowsill. About 3.8 litres — just what's needed for a clump of basil, a lettuce plant, chives, parsley, an alpine strawberry, or a compact pepper like Tiny Tim or Pepperoncini. It's also the size used to pot on seedlings that have outgrown their cells but are still too young to go out to the garden.

Why fabric rather than plastic: when a root reaches the wall of a plastic pot it can't get through, so it turns. It keeps turning, again and again, and ends up forming the tight knot you find when you tip the plant out, where the roots strangle one another. In a fabric pot, the root that reaches the wall meets air, dries at its tip and stops lengthening dead. That's air pruning. The plant responds by branching further back, and instead of a few long coiled roots you get a dense mass of fine rootlets — the ones that actually take up water and minerals. At transplanting there's no knot to untangle and virtually no shock.

Two other advantages, less well known: fabric breathes, so the growing medium doesn't cook the way it does in a black pot in full July sun, and excess water passes through the wall instead of stagnating at the bottom. It's almost impossible to drown a plant in a fabric pot.

The trade-off, which you should know about: it dries faster. A fabric pot on an exposed balcony can need daily watering in a heat wave, where a plastic pot would hold two days. A good mulch on the surface largely settles the question.

• Capacity: 1 gallon, about 3.8 litres.
• Made of non-woven fabric. Built-in handles.
• Promotes air pruning and an aerobic root environment.
• Easier, healthier transplanting, with no root knot.
• Folds flat for off-season storage.
• Suited for: herbs, lettuces, alpine strawberries, compact peppers, potting on seedlings.