Frost and low temperatures
The polytunnel should provide enough protection to keep frost off plants for most of the year. In an exceptionally cold winter, however, plants might freeze inside the polytunnel. To reduce damage, pour cool water over the frozen plants before the sun hits and temperatures start to rise. A sudden thaw will do the most damage.
Frosted tomato and pepper plants will usually wilt. Leaves may start to brown after a light frost, or turn black after a hard one.
Low temperatures can damage tender plants in the early months of the year. Newly planted tomato plants will survive temperatures as low as 3˚C/37˚F, but the leaves can become almost transparent and silvery, and the plants will never go on to crop to their full potential. Cover vulnerable plants with extra layers of fleece if temperatures are low.
Split fruit
More tomatoes split as the season progresses. Skins tend to be thinner in autumn fruit. Poor watering can also cause fruit to split, especially if you give a lot of water after having previously allowed the soil to dry out. In order to get sound fruit, water regularly and avoid extremes.
Dry set
The first truss of tomatoes may not set fruit. The same is true of peppers and aubergines. The answer is to lightly dampen the pollen by misting plants overhead with water. Spray in the morning and don’t soak the plants. This only needs to be done for the first flowers; later ones seem to have fewer problems setting fruit.
Blossom end rot
The flower end of tomato fruit sometimes turns black, hard and dry. Applying lime to the soil is one solution to this problem, but lack of water can be another cause. Adjust watering regimes so that plants never go short.