In Sweden, we pre-cultivate tomatoes indoors. If you want to keep the tomatoes in a greenhouse, it is sufficient to sow the seeds at the beginning of March and outdoors at the beginning of April. The tomato plants should be between 6 to 8 weeks old before they are planted out in a greenhouse or in the open field. Germination temperature should be between 22-26 degrees and the plug or seed soil should be well watered when the seed is sown. Cover the seed with plastic or a transparent cup.

It is extremely important that the small plants get enough light. The lighting can be anything from plant lights to fluorescent tubes. The important thing is to let the plants sleep for 6 to 8 hours per day. A good guideline for how much light to have is 80-100 watts per square meter. With newer LED lighting, you can usually go down a bit in watts per square meter, a little depending on the type of lighting you use. 

When planting in a greenhouse, it should not be below 10 degrees at night. For optimal harvest, you should preferably not go below 15 degrees. Plant directly in the soil or in a large pot. The pot size determines how much you have to water. 
Keep in mind that the tomatoes will be large, so a few different aids are required to prevent the plants from breaking off. Don't forget to wash and disinfect your pots and materials before you plant. It is good to use pots that are 25cm in diameter or larger to avoid having to water several times a day. 

Tomatoes require lots of nutrients to produce fruit and grow. Choose a tomato fertilizer with a lot of potassium and check that there are plenty of micronutrients.

The ratio between potassium and nitrogen (K/N) should be 1.1-1.4 and the ratio between potassium and calcium (K/Ca) should be 1-2. When the tomato plants start to flower, the fertilizer recipe should be changed, so that significantly more potassium is added. The ratios should now be K/N 1.7-2.0 and K/Ca 1.8-2.2. This gives better fruit quality and a longer shelf life for the tomatoes.

When the seed has started to germinate and has two cotyledons, it is time to remove the plastic or the cup. Now it is important to water with lukewarm water and maintain an even temperature around 22 degrees. Extra lighting on the small plants makes them stronger and produces more fruit in the summer.

When the plants are about 3-5cm high and the roots start to grow through the pot or plug, it is time to replant. Here you can put the small plant deep in the soil so that you only see the leaves and no stem under the leaves. Then the plant becomes more stable and gets a larger root system. Plant the seedlings in a well-fertilized soil. Remember that the plants should stand for 4-6 weeks, so choose a larger pot (12-15cm). For the first 2 weeks after replanting, you should keep a temperature around 20 degrees and then you can lower the temperature down to 17 degrees a little, depending on how much light you have.

When the tomato starts to grow, you can start caring for it. Tomatoes get tall so they must have something to climb on or you tie them up. All thieves must be removed, the sooner you remove them the better it is for the plant. Tie up or spin the tomatoes around a string once a week and remove the thieves. Many types of tomatoes can handle 2 tops without being grafted. Most types of hobby tomatoes can handle up to 16 bunches of tomatoes. The leaves below the first ripe tomato can be cut off. Watering must be regular and tomatoes require a lot of nutrition. Carefully read the packaging of the fertilizer you use. Too little produces a weak plant and too much kills the roots and the plants die.