A propagation tray assembly comprising a support tray and a plurality of seedling holder inserts locatable in the support tray in side by side relationship, each insert comprising a row of cells, each cell having an open top and an open side to facilitate inspection and removal of the contents thereof when the respective insert is removed from the support tray, wherein the open sides of each cell in a first insert are closed by the adjacent insert when the inserts are located in the support tray in side by side relationship.