Is 40 degrees too cold to put annuals outside?
Is 40 Degrees Too Cold for Annuals
Certified GKH Gardening Expert
It is for many, but not all, annuals. Petunias are usually pretty hardy down to that temperature. A good rule of thumb for leaving annuals unprotected outside is to wait until the nighttime temperatures consistently stay around 50 degrees, but you'll want to follow the directions on your seed packets or plants or consult your local nursery.
There's also a list of cool-temperature annuals in this article: https://www.gardeningknowhow.com/ornamental/flowers/fgen/plants-fall-garden.htm