Do I have to buy a greenhouse base?

No, but we do recommend some sort of base. Many people prefer to build their own base using 4" x 6" treated lumber. The optional plastic base is your best option, specially designed for our greenhouses and made of heavy-duty plastic with a honeycomb like design. It is closed on 3 sides. When the base is filled with dirt, it will not move (nor will the greenhouse), and no other stakes etc. are necessary.