Did the buds open? If so, you could leave one or two stems and hope they dry out enough for you to harvest the seeds. Some of the buds on mine just rotted. :-((

I cut my Oriental Poppies right down to the ground, leaves and all, and they grow back. In the meantime, I pop a pot of lilies in the space.

If you manage to get seed, no, the plants won't be big enough to flower in the first year.

Thanks SH a few did open. Thanks as always for your advise. I always get stuck when cutting back as to what to fill in it's place. Can you see the actual pot in your border? Do you do the same with aquilegia?

I tie mine up (leaves as well)not too tight, i never cut mine back until the seed pods have dried out enough and are ready to harvest, i have had extra flowering on my plum in previous years.

And don't forget that the chances of getting the same colour as Patty's Plum are small. They do not breed true from seed.
We grew a large number of perennial poppies from seed a long while back and planted them out in rows until they flowered so we could choose the best. Big mistake!. It proved very difficult to get rid of them. I think there is still at least one ordinary red one coming up from the original planting. Even worse every plant was just an ordinary red.

Pdb, sometimes if there are other leaves around the poppy area, I 'hide' the pot amongst them, or else dig a hole to sink the lily pot in. I've given up growing lilies in the beds - the bloomin' lily beetles are so crafty I swear they hear me coming and drop off. Grrrr.....

It's so much easier to check the plants every day if they're in pots. ;-)