For anyone wanting to know how to grow tomatoes from seed you need to start by planning your sowing time to be around 6 weeks before you want to plant them out. Ideally when you get your plants to a point where they need to be put in their final position for fruiting you will have seen the last of the frosts and the temperature during the time the seeds will be germinating will be in the range of 15ºC to 25ºC (60ºF to 77ºF) which for most people will mean either keeping them indoors or at least in a heated greenhouse.
The best way to get your seeds started is in a multi-cell seed tray containing a good quality potting compost. All you need to do is fill the cells with the potting compost and then dampen slightly with a watering can using a fine rose head. When that is done drop 2 or 3 seeds into each cell and then push the seed into the compost with a pencil or similar to a depth of around an 1/8″ to a 1/4″.
If you have bought your seeds from a supplier don’t forget to read the instructions on the packet for a source of additional information for the variety you have selected. Sometimes germination might require that the seed trays are placed in a dark cupboard until the plants start to show. Mostly though it is good enough to slip your seed trays into a polythene bag or put a plastic dome over them while they germinate.
As the tiny plants come through you will need to become the grim reaper and select the strongest plant from the ones in each cell. Choose the plant you think is the healthiest and then simply snip the other one (or two) off at the surface of the compost. Don’t pull them out as this is not good for the root ball of the remaining plant.
Once the plants have appeared they should be placed somewhere that has plenty of light, a window sill or a conservatory are good options. The plants will benefit most if they can get access to around 9 hours of sunshine a day. They will also require regular watering to keep them moist, note I said moist not soaked but also don’t let them dry out or you will be dead in the water before you start; excuse the pun.
To make the stems strong and flexible you can brush them lightly with your hand once or twice a day to flex them as they grow. This stops the stems becoming brittle. When they are big enough to handle they should be transplanted to a larger pot of around 3″ diameter and then left for another 4 weeks or so until they are ready to be moved to their final spot. At this point if you are using pots they will need to be around 12″ diameter to give the plant plenty of room and sufficient nutrients, a general purpose compost is good for the last planting stage.
Once they are in the 3″ pots you treat them pretty much as you would through the rest of their life with respect to watering and general care, but before you put them outside permanently it is a good idea to pop them out during the day and take them in at night for a few days to harden them off. So you would normally do this in the last week just before transplanting to the 12″ pots.