How do I grow tomatoes in a greenhouse on strings?

Canes work in a greenhouse but they limit you to the height of the cane, they get in the way, and they do nothing about airflow. The string system solves all three, costs almost nothing, and lets a single plant carry ten or more trusses in a season. It only applies to cordon — indeterminate — varieties, not to bush types.

What you'll need

• Galvanised or plastic-coated wire and eye bolts, true
• Soft jute or polypropylene twine, true
• Cordon tomato plants, true
• High-potash tomato feed
• Sharp scissors or a knife

Step by step

1. Fit a wire along the greenhouse just below the eaves.Galvanised or plastic-coated wire between eye bolts at each gable end, tensioned firmly. It carries the full weight of every plant on it when they are laden, so fix into the frame properly rather than to a glazing clip.
2. Set the strings at planting time.Cut a length of soft twine long enough to reach the wire with a metre spare. Lay the bottom end in the planting hole and plant the tomato on top of it, so the plant's own rootball anchors the string. Failing that, tie a loose non-slip loop round the base of the stem.
3. Wind, do not tie.As the plant grows, wind the stem gently round the string a half turn at a time, always in the same direction, leaving the growing tip free. Do it when the stem is soft and flexible, in the afternoon rather than first thing when it is brittle.
4. Pinch side shoots out while they are small.Cordon tomatoes produce a shoot in every leaf axil. Take them out at 20 to 30mm with finger and thumb — a small wound heals fast, a large one is an entry point for disease. Check every plant twice a week in high summer.
5. Strip the leaves below a ripening truss.Once the fruit on a truss is starting to colour, remove the leaves below it, a few at a time. It improves airflow and light around the fruit, reduces botrytis, and makes picking easier. Never strip above the ripening truss — the plant needs those leaves.
6. Feed from the first set fruit.Switch to a high-potash tomato feed once the first truss has set, and apply at every or every other watering. Water evenly and thoroughly — swinging between dry and soaked is what causes splitting and blossom end rot.
7. Help pollination.Under glass there is less wind and fewer insects. Tap the wire or the string sharply around midday on sunny days to shake pollen loose, and damp down to keep humidity in the range where pollen releases well.
8. Layer the plant when it reaches the wire.Leave surplus string coiled at the top when you set up. When the head reaches the wire, release some string and lower the whole plant so the bare lower stem lies along the ground or along the border, keeping the head at working height. This is how a commercial crop runs all season.
9. Stop the plant at the right point.In an unheated British greenhouse, pinch the growing tip two leaves above the top truss around late July or early August. Anything set after that will not ripen, and stopping directs everything into the fruit already on the plant.

Tips

• Airflow is what the whole system is really for. A greenhouse of crowded, unstripped tomatoes with the vents shut is where botrytis and mildew come from.
• Jute twine rots down and can be composted with the plant at the end of the season, which is far tidier than unwinding polypropylene.
• Space plants at roughly 45cm along the row. Closer looks efficient in May and produces a still, humid, disease-prone thicket by July.

Common mistakes

• Tying the stem tightly to the string — The stem thickens substantially over the season and a tight tie strangles it. Winding accommodates the growth automatically.
• Letting side shoots grow large before removing them — You lose the energy the plant put into them, you leave a large wound, and on a badly neglected plant you end up with a bush you cannot untangle.
• Stripping all the leaves at once — The leaves are the plant's factory. Remove them progressively from the bottom, only below trusses that are already ripening.
• Winding in alternate directions — The stem unwinds itself and the plant falls away from the string. Pick a direction at the start and keep to it for the whole season.

Questions people ask