According to the National Weather Service, here are the definitions of a Frost, Freeze and Hard Freeze.
Frost: Frost occurs when there is a solid deposition of water vapor from the air. Frost will form when solid surfaces are cooled below the dew point. An air temperature range of 33°- 36°, along with very light winds, is usually needed to initiate frost formation. At least minor damage is possible to plants. One must keep in mind that a frost is not guaranteed at these temperatures if moisture in the air is lacking and/or winds are strong.
Freeze: Freeze occurs when the air temperature drops to 32°-or-lower. A freeze will result in significant damage to many unprotected plants, especially if the temperature remains at-or-below the freezing mark for several hours.
Hard Freeze: Hard freeze occurs when the temperature reaches 28°-or-lower for at least a few hours. It usually means that many types of plants and most seasonal vegetation will be destroyed.
Mark Longstroth with Michigan State University Extension posted the following information explaining the differences more clearly than just a definition.
A frost is when we get a visible frost. A freeze is when the air temperature drops below freezing. Sometimes we get frost when the temperatures are above freezing and we often have a freeze without frost. It all has to do with the amount of water in the air.
At the dew point, water vapor in the air condenses on the ground and other surfaces as dew. If the dew point is below freezing, the water vapor condenses as ice, freezing as frost. So the air can be above freezing and the surface of your car (or the roof of your house) can be colder than the air and colder than freezing causing a frost even though the air temperature is above freezing. That is how we get a frost without a freeze. If the dew point is much below freezing then we can get freezing temperatures cold enough to freeze plants without any frost. This is also called a black frost, a freeze without a frost. When frozen plants thaw they have a water-soaked, black appearance as they die.
Sometimes we get a freeze under windy conditions. This is caused by the movement of a cold air mass into the area with subfreezing temperatures. These freezes are called wind freezes.
Idaho Average First Frost Date Map will give you accurate information for your county.
Idaho Average First Frost Date Map will give you accurate information for your county.
Steps to take to help your plants survive as long as they can.
1. Of course cover your plants with a frost fabric or sheets or plastic.
2. Bring what plants you can indoors
3. Water extra with warm water
4. Keep water running at a trickle if possible for a few days. Be careful not to over water.
5. Wrap the trunks of young trees.
6. Wrap pots that cannot be moved indoors.Your plants are counting on you...Protect them!
