If the birds are eating the seeds, and the seed heads are brown and dried up, it sounds like the seeds are ready.

I used to deadhead the tall Zinnias (State Fair Mix), but last year wanted to collect some seeds, so I left the flowers to go to seed. This worked out okay because the new flowers that grew on the plants would pretty much cover the old flower heads as they dried.

I cut off the oldest dried brown seed heads, and put them into a paper bag to dry for about a month. Then you can remove the seeds from the heads. I left some seed heads on the plants until well after frost, and the Goldfinches went crazy over the seed in the fall. I pulled the plants up once the birds had eaten all the seeds.

This post was edited by terrene on Fri, Aug 30, 13 at 19:52

Interestingly, I learned from a thread on the annuals forum that you can actually collect zinnia seeds when they are still green. As long as you can feel a fullness and the seed has definitely turned green vs still white, you just need to let them dry extra long since the green seed coat is less water permeable. I did this last year and had excellent germination this spring. Anyway, I'm sure yours are fine for collecting now.

Martha