Hello all,
In my opinion, it’s better not to use multi cell trays at all. Instead, use soil blocks. With soil blocks, there is no wastage when the cell trays eventually have to be thrown away, no collection, cleaning and storage of trays for re-use the next year, and easier transplanting, because there is no time spent struggling to get the plantlets out of the cells. Transplants in soil blocks get away to a good start because the roots don’t circle around inside a restrictive cell but stop growing at the edge of the soil block, ready to start growing again when transplanted (the fancy name for this is air-pruning).
All you need to get started with soil blocks is a soil-blocking device and a flat surface, such as a gravel tray, on which to place the blocks. There is a UK based manufacture of soil-blocking devices and they sell their products worldwide. They have a nice web-site here:
For small numbers of larger size soil blocks I’ve even used jelly, desert and cake molds from my kitchen. Anything will do to shape the soil/compost mixture into a firm block.
I’ve given away all my multi-cell trays and I now have a lot more space in my allotment shed.