They are pruned once the flowering is over. I would not prune as hard as that though. They do need some old wood from which to shoot.

Is it not possible to plant it out? They want to make a large shrub. They flower on branches grown in the previous year and/or the year before that. If you cut really hard back you will get no flowers at all. By the way it isn't a silly question at all - its always sensible to ask if you want to know.

I agree no such thing as a silly question. often silly answers mind ;o)

As said you need to prune it after flowering next spring.
When i had one in the ground I used to prune mine down to 2ft and the following year it grew to 4ft. year after year.

The only silly questions are the ones never asked. Potted or not, the same rules apply - for me. July 4th is the deadline for pruning Forsythia. Of course we don't live in a perfect world so you have a little wiggle room. How much to prune is up to you but not completely to the ground - it's too harsh and you won't get those gracefully arching branches that look like a fountain. I only prune out the old, broken & weird looking canes to freshen it up and to let sunlight & air in.

There are a couple of dwarf Forsythia (Bronxensis and one called Pauline) which would be ok in a pot. Neither of them grows to much about 30 inches or so.

Wow, I want one!