Why are my potted Sunflowers wilting?

I wouldn't pick off smaller blooms. Usually sunflowers use their energy to develope a big bloom. They are not like perennials where if you deadhead it increases the blooms. I am wondering if it is getting too cold for them at night. I am in Wisconsin and have sunflowers. They don't bloom til later in the year. I almost think that you probably bought it from a place where it was nursery raised so it is essentially ahead of normal outdoor growing times. This is what leads me to think it may be getting too cool at night.