breeding ground
1. Literally, a place where animals congregate for the sake of breeding. The ducks return to their breeding ground in the park every spring. The state helps farmers maintain breeding grounds for livestock.
2. A place, environment, or situation that promotes or provides ample conditions for the development of something, whether tangible or intangible. Offices are breeding grounds for sickness in the wintertime. We hope to create a dynamic research space for our students that will become a breeding ground for innovation and new ideas.
breeding will tell
One's true nature will be revealed. If you're skeptical of someone's motives, just give it time—breeding will tell.
breed up a storm
Of the weather, to become overcast. I wouldn't go outside right now—it looks to be breeding up a storm.