John, you undoubtedly found a solution some years ago, but this kind of situation will likely surface again for other DIY's. I'd suggest considering using Sonotubes, if they are readily available. Yes, they typically are used for making tall columns (I've used them for building 9 foot high, 1 foot diameter pillars in an earlier backyard of mine), but they also have been used for concrete footings. The only difference in this latter instance is that you are pouring the concrete outside the Sonotube, rather than inside. The end result will be a well-defined, beautiful circle.
Sonotubes can be purchased in short lengths (expect the cost/foot to be higher for small height pieces, rather than for purchasing a full-size Sonotube), as well as different diameters. Ask your local supplier (checking availability at HD Supply or White Cap would be good starting place) to either cut exact dimensions for you, or buy a long enough piece and cut it yourself, marking it first and then cutting it carefully with an electric jig saw, or keyhole saw.
Stake the Sonotube in place along the inside. making certain that you already have in place any irrigation line and elbow (for the riser--that can be inserted following your pour) for the plant going in the circle.
Staking can be with 1/2" diameter rebar, with lateral rebar pieces tied to the verticals, so as to hold the top of the tube in place. This assumes also that you have a firm base for the bottom of the Sonotube, that won't sink when dampening the ground prior to the pour (to extend the curing of the concrete, rather than having its moisture sucked quickly out by dry ground).
Hope this helps out future concrete pad endeavors!