How do I mount metal wagon wheels in the garden?

You could pound a piece of metal conduit in the ground and then use tie wraps/cable ties to attach the wheels to the conduit. Make sure you get the cable ties that are weather resistant for outside. You can pull the cable ties enough so the wheels don't slide on the conduit.

I would lean them into a wall or drive rod-iron down into the ground before tying it onto it.

Hi Lisa, short T-posts would work and stand up to the weather. Attaching the wheels to them I would use steel wire of a pretty high grade so it lasts through the weather.

A piece of rebar driven into the ground about a foot with a sledge and cut to the desired height you want the wheel to be displayed. Wire the wheel to it. Just leaning against a tree looks cool as well.

If you want them free standing, I would say mounting onto a post would be the way to go. A 4x4 that is pressure treated and then bolted onto the post through the lug nut holes should be sufficient.

If you’ve a truck, go down to the river or a creek and find a distinctive large piece of driftwood- one that has some big branches.

Then you’ll have 3 elements to your design.

Lean the wheels against the driftwood and use some fine stainless wire to safety wire them to the driftwood.

Where would you like to mount them? On a wall? On a fence? In the ground?

I've seen them partially buried in the ground. Supported on posts. Held upright by a few boulders in rock gardens. Leaned against trees. Bolted to front porch walls.

I would partially bury them and support it with a post or landscaping large rock . They would also look good against a free and put a garden around the tree !