Every region has its own growing season, ranging from just a few days in the far North to all year long in the tropics. Understanding frost and freeze dates helps you plan your garden projects.
The Growing Season
Frost-free days are your gardening window of opportunity. During the growing season, plants have time to mature, blooms to form and fruit or vegetables to ripen. Most vegetables purchased, whether seed or seedlings, will have a "days to maturity" listed on the tag or package.
Frost dates are based on historical data compiled by the United States Department of Agriculture, and should be used as a reference. The chance of an early or late surprise frost is always a possibility. The probability of frost or freeze is greatly affected by elevation, the direction of sunlight (north, south, east or west), prevailing winds and other factors. Cold air "drains" or flows down slopes to create frost pockets at the base of the slope. Valleys are more susceptible to frost. Areas near lakes or ponds are less likely. Spots sheltered by buildings get additional protection. A light breeze can reduce risk of frost, a cold wind can turn a mild frost into a killing freeze.
Frost or Freeze
Frost occurs when water vapor freezes on a surface when the temperature reaches 32 degrees F or below. It usually occurs on a clear night when heat radiates up from the ground. Tiny ice crystals form when water vapor condenses. Similar to dew, the temperature at ground level is the key. The temperature there is often colder than the air temperature just a few feet higher. Tender plants need to be covered for protection from frost.
Freeze involves a 32 degree F surface temperature that lasts for a significant length of time. Frost is not necessarily present. Vegetation damage is usually a result. Hard freeze is usually defined as 25 degrees F or below. The term killing freeze or frost depends upon the hardiness of the plant and the level of exposure.
Freeze warnings are issued in spring and fall as the first frost and freeze conditions begin to appear. For a quick reference use the chart below:
|USDA Zone||Last Frost Date||First Frost Date||Minimum Number of |
Zone 11 has no frost
Protecting your Plants from Frost
Tender, herbaceous plants are more likely to suffer from frost than hardy woody plants. When sustained temperatures are at freezing or below, long-term protection is usually not feasible. But in the event of an early fall or late spring cold snap, you can provide temporary protection.
Methods of safeguarding plants vary. You can cover plants with specially designed row covers or common household items such as cloth, paper or plastic. In extreme cold, try to prevent the cover (especially plastic) from contacting foliage. The contact will conduct the cold and can damage the leaves.
A simple wooden frame can be draped with the cloth or plastic sheets. The radiated heat will be kept inside.
A layer of mulch will also help retain the heat from the soil to protect from cold.
You can keep a garden journal and record your own garden weather data. For advanced warning of cold weather listen to the weather on television or radio.