I need help to put a flooring in my greenhouse

Hello Patricia, Your flooring should be something that you can hose down if necessary. If I had a greenhouse, I would use something like 16" x 16" patio blocks. You could even use them for a walkway too. I would LOVE to have a greenhouse. I wish you well in yours.

Will a thick layer of gravel do? If not, how about a pallet floor?

When I worked in a greenhouse we used woodchips, but in a small greenhouse you could put pallets down. You want good water drainage.

Most just put a walkway between the beds, you could make them out of pallets. Cut in half lengthwise, put a board the width of the end piece in each half near the edge. Then cut up additional pallets to fill in the missing boards leaving a small gap for drainage of dirt and water. If the flooris dirt, I would put plastic underneath to prevent rot.

Check these out. I think they would be perfect for a greenhouse. http://www.homedepot.com/b/Outdoors-Garden-Center-Landscaping-Pavers-Step-Stones/Rubber/N-5yc1vZbx4bZ1z0ugz3

concrete will help