How to plant tomato seeds in the greenhouse
What type of soil?
Any good soilless potting mix will do. These mixes are typically made predominately with peat moss, with a small amount of perlite and vermiculite. You can certainly use the same soil as you use in the garden, but the negative to this is that you can import pests to the greenhouse that would otherwise be controlled outside by weather or predators. I do use some mushroom compost from my garden for some of my larger pots to save on cost, but these get pulled out in the warmer weather anyway.
What type of pot?
I like to use a ¾ of a gallon to a gallon sized pot for my transplants. This size allows for plenty of space, and it lessens the transplant shock. I also like to plant 3 tomatoes in each pot as I will remove 2 of the seedlings once the most vigorous seedling becomes obvious. Seed is cheap and plentiful, but your garden space and time is not, so selecting only the very best seedlings to care for all the way to harvest is a good idea.
What type of tomato seed?
There are around 7500 different varieties of tomato seed to choose from. I like open pollinated seed, because I like to save my seed from my best tomato plants from year to year. I have heard from others living in harsher climates that only hybrid seeds will grow a good tomato undamaged by rot. It’s fun to try a variety that you could never find in the grocery store.
A good seed planting rule of thumb that works is for you to plant the seed at a depth 3 times the thickness of the seed. Tomato seed is pretty thin, so a ¼” depth works pretty well.
In the greenhouse, I make indentations in the soil with my finger to the approximate depth I need. I then place the seeds in the indentations. I then brush the surrounding soil into the indents to cover the seed. Finally, I give them their first watering. It is important to use a mist or a light spray when watering before they are established, as the higher velocity sprays can move the soil.
***Update 5-2-2012 See below tomato seedlings almost ready to be transplanted out