Astronomically, spring comes to the whole Northern Hemisphere around March 21. The date of the spring equinox is determined by the Earth’s movements through space.
Meteorologically, however, the arrival of spring is related to local climate conditions. Winter can be said to end when the temperature rises above freezing and stays that way. Winter returns when temperatures again drop below 32 degrees Fahrenheit.
The days on which these events are likely to occur are called frost dates. The time between the spring frost date and the fall frost date is the growing season. Because these phenomena were important to farmers throughout history, the average frost dates for each area are well known. In Madison, the growing season runs from about May 20 to September 21. Frost dates for other locations can be looked up here.
Because of climate change, however, estimates of frost dates based on past weather are becoming increasingly unreliable. In general, growing seasons are becoming longer. Because a late frost can kill plants, though, it’s often best to err on the side of caution when beginning the gardening season.