I am seeing some good prices on annuals but I don't want to plant them for 3 weeks. Can they be left in the containers until then or do they need to be transplanted into something bigger?
Markey, you've asked for a one-size-fits-all answer with a pretty generic question. Instead of "yes" or "no", one can only answer "maybe".
IMHO, if the plants aren't rootbound...and won't be in the next 3 weeks, they probably don't need to be up-potted. I would give them good light (maybe trips outdoors by day and protected by night) and try to maintain moderately moist soil. I would give little, if any, fertilizer until they have been planted outdoors.
I believe many of us buy early, while the selection is good or during a sale, then hold the plants until planting is appropriate.
I bought Pansies and Geraniams (SP) and planted them, but they are the hardier of the annuals.
I ususally buy my annuals early then keep them in there little pots untill its warm enough for them to be outside 24/7. I watch them carefully though to make sure they are not rootbound or drying out, if they start getting to rootbound I just put them in a bigger peetpot and transplant when I can :)
I live on Vancouver Island, Parksville, it is pretty dry here in summer and we have v. mild winters - zone 8 or 8A or 8B, depending on what book I read. Tender annuals such as impatiens, petunias, heliotripe should be protected until approximately mother's day. Tender annuals planted too soon will suffer from cold nights (could turn 'blue') they may recover but why take a chance. I keep mine on top of boxes and cover them up with T towels at sunset this is done on my patio. Tender annuals should not be kept ON the ground even cement, its just a bit too chilly. Its so tempting to plant early, but wait a bit and you will not be disappointed. Buying now is good (selection) if you have a protected area to hold them.