This will be a more in-depth version of a post I put on Grow Your Own forum.
Advice for early tomato/cucumber growers
The last thing I want to do here is to teach people to 'suck eggs', but I have grown tomatoes commercially in all sorts of conditions. So I would just like to pass on some of my 'limited' knowledge, and in no way do I profess to be an expert. Also, we all have different conditions to work with, so I am generalising.
I apologise in advance for the long post.
Tomatoes stop growing, and are susceptible to chilling injury, at temperatures between 0 and 10°C (32-50°F). Chilling injury can show up after short periods of the lower temperatures or long periods of the higher temperatures and can cause:
wilting, surface pitting or necrosis of foliage
increased susceptibility to disease
Low soil temperatures also stunt plant growth and prevent root development.
In addition, plants that are not actively growing would be expected to have a tougher time metabolizing herbicides. Cool soil temperatures mean that slow root growth also limit the plant's access to soil nutrients, such as phosphorus. Using a starter fertilizer with high phosphorus and a bit of nitrogen can help get the plants off to a better start in these conditions.
The combination of cool weather impacts can result in some poor looking plants, but the crop should recover when warm temperatures return. Plants that have been severely stressed, however, may be slow to resume growth and may have lost some yield potential.
Not all cool-temperature effects show up right away. Low temperatures experienced by the plant 4-5 weeks (!!) before flower buds are visible, can affect flowering and fruit set.
In tomato, freezing damage occurs at -1 to -2°C (28-30°F). It may be difficult, initially, to determine whether the growing point has been killed and damage may become more evident on the day after the frost.
Another reason for scorch is a poor root system, so the plant can't draw enough water on hot days. So a light water spray will help cool the leaves.
Over watering can damage root systems, so watering correctly is critical at this time of the year, a little and often is the best advice to keep the peat constantly moist.
Roots need air as well as water and if the conditions are too wet for too long, this will suffocate the roots and kill them, too dry and the plant will overheat on hot days.
Another good test is to lift the pot, if it is really light, then obviously it is too dry and vice versa, you will soon learn to judge this.
To give the best buffer, make sure your plants are in big enough pots, the simple rule of thumb is to not let the leaves grow wider than the pot and make sure they are spaced well enough, so they are not competing for light.
Your seedling compost will have a small amount of fertiliser in it to help the plant until it is transplanted, if not left too long. The same goes for potting compost. However, this can be washed out if over watered, so if your plants are looking a bit pale, a half-strength tomato feed should perk them up.
For really leggy plants, a high nitrogen feed (flower feed) will help give good foliar growth, if your plants set tomatoes too early with not enough foliage they will be struggling all season. Commercially, we used a high nitrogen feed always at the start of the season, then went on to high potash feed for the rest of the year, unless we found that our plants needed a foliar boost if over laddened with fruit.
Well, without a heated greenhouse we are all struggling here, too hot in the day, and too cold in the night; perfect conditions would be 15 degrees at night and 25 degrees in the day with plenty of light.
So the dilemma is, in the greenhouse, it can be too cold at night, but good light, or on a window cill, too strong sunlight on hot days, and not enough light on dull days.
Eventually you will run out of space, so will have no choice, but I would say that a greenhouse is better once your plants have been repotted, as long as the temperature doesn't go below 1 degree at night, but then you could always cover them with a fleece.
If you do have a warm conservatory and you can stake the plant in a large pot, then that would be the best option while the nights are still cold, but unfortunately many of us do not have that option.
These seem to have plenty of light, but are far too close together, so they are all competing for the light. The easiest way to avoid this is to transplant them early enough into larger pots.