Abstract: A support structure for growing plants such as lettuce by hydroponic cultivation includes a buoyant, closed cell, foamed plastic pallet having an array of spaced integral punch-out plugs arranged preferably in staggered rows. Selected patterns of plugs are removed to permit insertion of plants growing in porous seed blocks into the resulting holes, the patterns having different spacings depending on the size of the plants. Blocking members prevent the plants from falling through the holes when the pallets float on liquid nutrient solution in a trough and also space the pallets above the bottom of the trough for access of air to the plant roots when the trough is periodically drained. When plants outgrow one selected spacing pattern, they are readily and precisely transferred to another pallet having a selected pattern of greater spacing. When the plants are ready for harvesting, the pallets with mature plants in them can be carried to a packaging area.