Cosmos are not hardy (I think) but are very very good plants.
Purity have beautiful snow white flowers and keep flowering from July to end
October.
They easily grow in one season from seeds in trays in late April/May.
Christopher Lloyd recommended Cosmos purity to be grown with (have forgotten
name here :-)
ehh Mexican sunflower thing .... Compositae with bright orange flowers;
grows to 4-5 feet/1.5 metres high and also a half hardy annual; sow
in seed trays at same time as Cosmos.
We did them together one year and it was stunning. The following year it
was Arctic and then very windy for may and june and they never got going and
I never did it since.

I've found that a lot of Compositae are very prone to slug damage -
cornflower, sunflower for example.

I've stopped sowing annuals straight into the garden - the young shoots
are too vulnerable. Things which are nibbled as soon as they emerge from
the soil often survive a lot better if they're planted out about 6
inches high.
--
Kay