Grow bag

A grow bag is one of the simplest ways to grow food without a bed or border. It is a low, rectangular plastic sack, pre-filled with around 30–40 litres of multipurpose compost, that you lay flat and plant directly into through holes cut in the top. Because everything you need comes in the bag, it is a popular first step for people growing on a patio, a balcony, or the floor of a greenhouse.

What grow bags suit

Grow bags work best for warmth-loving crops that you replant each year. In the UK that means tomatoes, cucumbers, peppers, chillies and aubergines, all of which crop happily in a bag against a sheltered, sunny wall or under glass. They are also handy for a quick run of salad leaves or herbs, and for anyone renting who cannot dig up the garden. Two or three plants per standard bag is the usual rule.

The limits

The main drawback is volume. A grow bag holds a fairly shallow, limited amount of compost, so roots have little room and the compost dries out fast, especially in a greenhouse or a warm July. On hot days you may need to water twice. That small reservoir also runs out of nutrients quickly, so plants will need feeding from a few weeks after planting. The shallow shape can make tall plants like tomatoes prone to toppling, and the thin plastic offers no insulation, so they are no use for crops you want to leave in over winter.

Getting the best from them

A few simple habits make a big difference:

• Plump the bag up first. Give it a good shake and a squeeze so the compost is loose and evenly settled before you cut your planting holes.
• Cut drainage slits. Snip a few small holes in the base or lower sides so excess water can escape and roots don't sit waterlogged.
• Water little and often. Check daily in summer. Bag collars, or upturned pots sunk into the compost, help water reach the roots rather than running off the surface.
• Start feeding early. Once the first fruit sets, give a weekly high-potash tomato feed, as the compost's own nutrients are soon used up.
• Support tall plants. Use a grow-bag frame, or push canes into the ground behind the bag, since you cannot anchor a cane firmly in such shallow compost.

Treat the compost as single-use for hungry crops, then empty the spent contents onto a border or the compost heap at the end of the season. For a bigger root run and steadier watering, a deep pot or a raised bed will give you more forgiving results, but for a cheap, cheerful patio tomato crop a grow bag is hard to beat.