Posted: 20/05/2015 at 16:42
The main reason (apart from space) is when starting seeds you almost always sow a number of seeds, which don’t all germinate, then you choose one or more from what’s left. A seed tray works a lot better, because instead of putting a few seeds in a number of different pots, you start with all the seeds in one place.
Another possibility is you have some old seeds, and you don’t know if they will germinate anymore.If you take suspect seed and simply broadcast it over an entire seed tray, as thickly as you think is appropriate, and even just a small percentage of it germinates, you can just transplant out the seedlings that do emerge. Even if you get a much higher rate of germination than you expect, it’s not difficult to deal with a lot of seedlings in a tray as long as you don’t wait too long and let them become too established.
Some plants, (tomatoes), become stronger if they are transplanted. Tomatoes have naturally weak root systems, and also have the ability to form roots on any part of the plant that becomes buried. What I do is transplant them from the seed tray, as deeply as possible into a plastic pot. Only the top few leaves should remain above the soil. Then when I transplant again out into the garden, I again bury the plant all the way up to the top few leaves. In this way, the plant is ‘shocked’ into developing stronger roots.