How do I get the most out of a growbag?

The growbag was designed for commercial glasshouses with automated drip feeding, and it works brilliantly under that regime. In a domestic greenhouse, watered by hand once a day, it is the hardest place there is to grow a tomato. The four arrangements in the diagram are all attempts to fix the same problem: not enough depth.

What you'll need

• A growbag, true
• A shallow tray, gravel trough or growbag tray
• Bottomless ring pots or 25cm plastic pots with the bases cut out
• Canes or a string support system
• Sharp knife

Step by step

1. Shake and knead the bag before you open it.The compost settles hard in transit and in storage. Drop the bag flat a few times and work it with your hands until it is loose and evenly distributed, or the roots meet a compacted block.
2. Cut drainage slits in the lower edges, not the base.Three or four slits along each long side, low down but not on the underside. Slits in the base drain the bag dry; slits in the side let excess water out while leaving a small reserve. Slits in the top only mean no drainage at all.
3. Plant two, not three.The printed cutting guides show three tomato plants. Two gives each plant half as much again of the compost and makes the difference between a crop that keeps going and one that stalls in July. For cucumbers, one or two.
4. Stand the bag in a tray.A proprietary growbag tray, a builder's mixing tray or a gravel-filled trough. Water that drains out is held and taken back up rather than lost onto the greenhouse floor. This is the cheapest single improvement there is.
5. Consider ring culture for tomatoes.Stand a bottomless 25cm pot on the bag and cut through the plastic inside the ring. Plant into the ring, filled with compost. Feed and water go into the ring, plain water goes into the bag or the gravel below. The plant makes feeding roots in the ring and water-seeking roots below, effectively doubling the depth.
6. Or stand the bag on end.Cut a bag in half across the middle, stand each half upright with the cut end up, and you have about 300mm of depth in a small footprint. It suits one vigorous plant on a narrow patio and is far more forgiving of missed watering.
7. Sort the support before you plant.A growbag will not hold a cane — there is nothing under it. Use a growbag frame, a string dropped from an overhead wire, or push canes through the bag into the ground beneath if it is standing on soil.
8. Feed from the first truss, and water little and often.The compost in a bag holds around six weeks of feed at most. Once fruit is setting, a high-potash tomato feed at every watering or every other watering, and enough water to keep the bag evenly moist rather than swinging between wet and dry.

Tips

• Uneven watering in a growbag is what causes split tomatoes and blossom end rot. The tray, the ring, and the upright bag all help by evening out the swings.
• Growbags dry out fastest at the ends. Check both ends of the bag, not just the middle, when you judge whether it needs water.
• In a heatwave, a growbag standing on a hot greenhouse floor or on paving cooks. Stand it on a slatted staging shelf or on a gravel tray to break the contact.

Common mistakes

• Planting three tomatoes because the bag has three marked circles — The bag holds a fixed volume of compost. Three plants is a commercial density that assumes drip irrigation and daily liquid feeding; by hand it produces three stunted plants instead of two good ones.
• Cutting a huge window out of the top of the bag — The plastic top is what stops evaporation from the compost surface. Cut planting holes only slightly bigger than the rootball.
• Using last year's bag as this year's growbag — The compost has lost its structure and its nutrients, and it may carry disease. It is excellent as a mulch or a soil improver, but not as a growing medium for a second crop.

Questions people ask