Some people use a flattened spoon glued to the back of the plate with the handle bent, and insert the handle into some metal conduit pipe or something similar. I only use GE ll silicon caulk made for outdoor use.
If you search for the plate flowers there's many that show the backs.
Some people also drill and bolt the pieces together.

Another tutorial
Just Google plate flowers diy or tutorial
Search here on this forum at the very bottom of page 1 using the search box.

Here is a link that might be useful: Plate flowers

Or you can glue a small bottle on the back of the plate and insert the rod into it. Small flat sided bottles work well like salt and pepper shakers.
Get creative with it.

I just made 3 this spring and used e6000 adhesive to glue a hex coupling to the back of the plate, and the rebar threads right into it. I followed instructions on a blog and it worked out fine. Time will tell how well the adhesive holds up, but I like how they turned out with a little glass paint, some Goodwill candleholders/ plates and a bit of rebar. I think I will be making more soon! It is easy to get carried away and make gorgeous ones using antique glassware, etc. but I was still pretty pleased with my thrifted/painted versions, especially when the sun shines through and illuminates the colors.