Losing track of where the garden ends on the canvas

Halfway through a big tulip bed last spring I looked up and realized I had no idea where I wanted the flowers to stop and the background to begin. The whole right side was just... uncommitted paint. I had been so happy laying in pinks and purples that I forgot to decide where the edge of the composition actually lived.

I ended up cutting a viewfinder out of cardboard and holding it over the canvas, which helped, but it felt like something I should have sorted before I started. This happens to me more than I'd like to admit, especially with garden scenes where everything wants to keep blooming off the edge.

Do you use any kind of framing trick or compositional tool to lock down your boundaries before you get deep into a piece? Or do you just let it resolve itself as you go? I make too much art to keep losing an hour to "where does this flower bed end" every time.