We used to use them in tobacco plants. I have seen several people here use them in there greenhouses. One tray like that would just about do one garden.

They are called Speedling Plug Flats, come in all sorts of cell sizes, and manufactured by Speedling Inc. several horticultural suppliers offer them for sale. But the primary purpose for them is germination only, not for growing to transplant size. And while they do happen to float that isn't why they are used - unless growing hydroponically. Soil grown seedlings aren't left to float. They are watered and then immediately removed from the water. And that is one of the big disadvantages to them - finding a tray big enough to hold the water that they will fit in.

The main advantages to them are the shape of the cells that encourage deep root growth, the insulating aspect of the foam, and the "easy-to-pop-out" plugs they produce. But all the same advantages can be obtained by using plastic plug flats and they fit into a standard 1020 tray for watering.

<One tray like that would just about do one garden.>

Only if they were all the same plant. You should never mix plants with different germination times in the same flat.

Dave

Park Seed offers a seed starting "Bio Dome" that is basically floating styroform tray with plugs you place the seeds in, and it comes in several sizes.