Yes, you should wait until the warm season to paint. Roses are easy to move when not leafy, maybe grow them (in the ground) someplace else this summer and plant them next to the shed next winter.

Save yourself some headaches and use good quality exterior paint. My can of Sherwin Williams Duration says it can be applied when temps are as low as 35 F. Metal siding needs to be 50 F.

For more info Google 'Duration paint'. Consumer Reports also rates paint.

The best suggestion is to plant the roses in large pots for now, and not put them in the ground until the shed is painted. To some extent you could work around them if you planted them first, IF you're doing the work yourself, but best is to not put them in harm's way. It's not going to be reliably warm and dry enough to paint anytime soon and if it is, the roses are going to start growing.

I've applied latex paint in what is February weather here. Those conditions are summer weather where I was (Alaska), so there wasn't much choice. It is not a problem except the paint will take longer to dry.

Bonus unsolicited advice: Always use the best paint you can afford. Cheap paint isn't really a bargain. :-)