Every year when I am thinking about my last frost date (which averages April 25) I don’t look at the calendar: I look at the oak trees.
There’s an old saying that when the oak leaves are the size of little mouse’s ears, it’s safe to plant.
You may think that this sounds like an old wives’ tale. Call it what you want. I think it’s a farmer’s saying. And in the years before sophisticated technology and weather satellites, what did the farmers use? Phenology–the study of signs in nature.
Internet searches yield lots of different ways to use this for planting guidelines. There are all sorts of regional planting guidelines cued to the flowering of various local trees and shrubs–search for “farming guides and phenology.” To make it local, add your state or region.
As for me, I am not sure where I first heard the “little mouses’ ears” advice. But I have plenty of oaks and I watch them carefully. And, so far, the advice has been great–no frost after the oaks leaf out.
But of course, by the look of these oaks, I won’t be planting anything tender anytime soon!
Gardendaze 
I tell you what, there is still a lot of information that can not be found online. Some of the simplest but most useful information such as this is lacking.
There’s a lot of wisdom in what I call the “old farmer’s sayings.” Here, I can make a great fungicide from milk and water–another old farmer’s thing. I am not sure if it would hold up well in really hot climates (although last summer for us was brutally humid–off the charts)–and we do occasionally have those torch like summers where it’s in the 100s. Haven’t had one since I worked in retail gardening, though, and I don’t mind.
Karla