The trouble with most tall annuals or perennials for that matter, is that they have to attain the height before they bloom.
Try Lavateria 2 to 3'--a beautiful bushy plant with large flowers. To me it always looks like a shrubby petunia with a more open flower. Another favorite is Zebrina mallow. In fact there are quite a few mallows that I've tried.

Try securing a lattice to your fence and planting vines. Lots of beautiful choices there--Sweet pea, nasturtium, morning Glories, or perennial Clematis and many more. Another option is to try a combination of plants and vines or get some shepherd hooks and put hanging baskets there. There are a couple of ladies here who use grow bags hung by hooks in the fence. They are spaced all along the fence--They look wonderful

Lots of options for you --you just have to find the one that works for you

Take a look at "Kiss Me Over the Garden Gate". I have been growing it in containers for the last few years with good results. It would work well against your fence and grows quite tall and flowers throughout the summer into the early fall.

The seeds do need to be cold stratified (2-3 weeks in the refrigerator - in a damp coffee filter, etc.). Still plenty of time to get them started.

Good luck

Cleome and cosmos.

Have you considered vines?

corkscrew
morning glories
hyacinth bean vine

Or datura

What height are you looking for?

Zinnias can get 3-4 feet and the are lovely.

Keriann~