Q: Where do roses sleep?
A: In a flower bed!
When you have a garden with flowers (as opposed to a garden with fruits and vegetables), we can call that area a flower bed. If you prepare the flower bed well, the flowers will grow well. I have never heard anyone call a vegetable garden a vegetable bed.
A bed, without flower, is a piece of furniture where people sleep, but you kew that.
This joke is funny because it plays with the word bed and flower bed. Yes, it makes sense that roses would sleep, if they sleep, in a flower bed. (Thank you, Trevor for the joke!)
Here is a motivational speech titled, Make Your Bed: