Instructions on seed packets will often say "plant after danger of frost." Well, ok, when is that? And how cold does it have to get for that frost to kill my plants.
Well, I looked it up and found that a frost of 29°F to 32°F will kill tender plants, but not damage most other plants. 25°F to 28°F is destructive to most vegetation, and frost 24°F and colder is REALLY BAD.
In Waco March 15th there's a 50% chance of temperatures dropping to 32°F, so if you want to take a risk on an early garden, you can try planting now. Otherwis, wait til April 10th, when there's only a 10% chance of 32° frost.
(The National Climatic Data Center has info on the likihood of 36°F and 28°F frosts too. I went with 32°F because that's the one to watch with most plants).
Picture by Tambako the Jaguar used under Creative Commons.