This wonderful collection is for the gardeners who have to keep their plants in pots a bit longer and need deeper seed trays. Starting with the 40 cell tray, which gives seedlings plenty of room to grow while creating a healthy root ball. The 28 cell tray is a biggie!! So many people love this tray. It is good for peas and other veggies with long roots. The 15l is good for saving space. It's like you glued 15 small pots together! This efficient design saves space and is eco friendly since you won't be throwing away any plastic pots.