At this stage of their development, it’s vital to make sure your tomato plants are getting enough water. Because they are a deep-rooting plant, it’s not effective to spray water at the soil (or worse, at the plant itself, which would encourage blight). The key is to get water right down to the roots.
I find that the best way to do this is by sinking a container in to the soil beside the plant and water in to that. I use upturned 2l milk cartons with the bottoms cut off them and the spout facing down. Watering becomes a job of simply filling the container, confident in the knowledge that the plant is getting 2l of water where it needs it most.
Watering ‘at depth’ like this also means the plant is less vulnerable to drying out in very warm polytunnel days, because the water won’t evaporate as easily as it would from the surface of the soil.
I water the tomato plants in the tunnel this way every other day – as a rough guide you want to give the plants about 10l of water per week.