To harvest winter crops in your greenhouse, starting seeds soon is a must. How do you choose the best pot in which to start seeds? Do you use a seed flat? A seed flat with an insert? A square pot? Round pots? Jiffy pots? There is a huge choice of pots and pot materials available to the greenhouse grower so I decided to start seeds in as many pots as I could find. I made paper pots, bought jiffy pots, seed pellets, clay pots, coir and peat pots, cow and compressed cardboard pots, added a couple of coffee cups, a cut down juice container, and adapted several other containers to grow seedlings in.
I planted the same seed (early girl tomatoes) in each pot and noted the germination times and growth under fluorescent lights in my basement germination chamber and got some interesting results. Each pot was filled with potting soil (Pro-Mix BX – Pro-Mix is a peat moss based medium) and started with two seeds. I used two seeds in case one didn’t germinate and thinned them to one plantlet after germination. Each pot was covered with kitchen wrap plastic until the seedlings germinated. The plants were placed under fluorescent light fixtures (two to three inches below the fixture) with a cool white and a warm white tube in each fixture.
The 11″ x 22″ seed trays were the first to germinate, followed closely by the 4″x4″ square plastic pots. The slowest to germinate were the coir, clay and Jiffy pots. I think this is due to the porosity of the natural materials in the pots which allows moisture to wick out through the sides and bottom of the pots, keeping them drier. The plastic pots did not let moisture wick away and consequently kept the seedlings a little wetter giving them earlier germination. The difference between the seed flat and square pot is, I believe, due to the square pots being slightly nearer the lights and maybe a little hotter.
After germination the seedlings in the plastic pots grew faster than did the seedlings in coir, Jiffy, paper, and compressed natural material pots. I believe this is due to moisture evaporating faster out of the pots with natural materials. The plantlets in the 72 plug insert also grew slowly. I think this is due to the insert being so small that it dries out fastest under lights. These inserts needed watering twice daily to keep the seedlings growing fast.
In conclusion, the square 4″ black plastic pots seemed to work best for several reasons; They were black, which kept the seedlings warmer, they held more potting soil and stayed moist longer than the pots made of natural materials, being square they are also easier to water with less spillage than round pots. The plants in the seed trays needed to be transplanted as soon as they got two true leaves and before their roots could get tangled.