How do I grow cordon tomatoes?
Barbara Tribe, Honiton, Devon
Stefan says: You’ve always grown bush-type tomatoes successfully outside in pots, but this year you want to try cordon/ indeterminate types as well, also in pots outside. You’d like some guidance about the best size of pots, compost and any other tips. You have also ordered some blight-resistant seedlings to be delivered at the end of March.
It's clear you understand a good deal about tomato growing, but for less experienced readers, I should start by explaining the terms cordon, bush and indeterminate.
Bush or determinate varieties of tomato, which include many outdoor types, produce their fruit on both the main and side branches, do not need any pruning and their fruits all ripen within a short space of time. By contrast, cordon or indeterminate types, which include most greenhouse varieties, must be staked and the sideshoots that arise in the axils of the leaves must be pinched out, so the plant concentrates all its energy on the one main stem on which the fruits are borne.
I don’t think you’ll find much difference in growing cordon varieties, and you’ve made one of the most important decisions in choosing one that is blight-resistant, because blight is probably the biggest enemy of outdoor tomatoes.
Because the pot must support both a tall plant laden with fruit and its supporting cane, you should choose one that is substantial enough; a minimum of 20cm diameter and 25cm if possible.
At one time I would have advised you to use John Innes No 2 potting compost, but the formula has changed and nowadays, I personally find it difficult to use. It’s much more like sieved loam than a balanced blend. I find it better now to use a mixture by volume of half John Innes No 2 and half peat-free multi-purpose. Then start to give a balanced liquid feed weekly from the time the second fruit truss is setting.