Hi
@gardengateplants and welcome to the forum
My preferred way to anchor a small greenhouse would be to buy 4 pressure treated 4 x 4's and 8 1-1/2 to 2 foot 1/2" rebar. If you live in a windy area, go with the 2 foot rebar. Cut wood to 6x8. Lay out frame and temporarily use scrap wood to keep corners square. Drill 1/2" holes, one near each end of each 4x4 (8 holes) at an angle pointing into the greenhouse. Drive rebar into ground securing frame. Mount greenhouse on foundation and secure with screws.
Let us know what you wind up doing, and as always, we just love pictures!
Jim
