I got an aluminium greenhouse that was advertisd free for uplift. I dismantled it, but the huge hunks of wood it was screwed onto were rotten so I left them. I reassembled it, but it now has no base. There are just right angled, not very strong looking lengths of metal, horizontal, that the uprights of the sides are connected to. I think it is 6 x 8. I see bases advertised on ebay, but have no idea of their quality, and I have concrete, bricks, breeze blocks, wooden sleepers, wooden roof joists, angle iron, hooks driven into the ground and paving slabs all fighting in my mind as I try to work out what I can get off with, without much expense, but still have a solid result. I'd welcome any and all advice with this dilemma I find myself in.

We have a large tent for a garage, which came with a similar metal frame - fairly robust pipes. What we did was: Spread a 4" layer of gravel for the floor, then lay a pressure-treated 6x6 beam dug in across the front end, where there is no metal bar, and attach the front upright corner posts to that. Then drive T-bar fence-posts next to every other upright post on both sides about 3' into the ground (That's not easy!) and bolt them to the uprights. The rear wall of the tent is attached with lumber to the side of a building, to make a continuous structure.
That thing's 30'x20'x14', and it's not going anywhere!
You should be able to secure your little greenhouse quite cheaply. I'd probably go with a complete under-frame of cedar timbers and attach the metal tubing with sturdy brackets or metal straps. If you're careful with the angles, the whole thing will remain four-square - which is important, right?

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