We've had our first touch of cold weather, with night temps falling to the upper 30s on October 6thand 7th, but no hard freezes yet. Of course, we're at the end of the gardening season. But if you have tender plants, and you want to keep them growing a while longer, it's important to pay attention to weather forecasts and provide protection if freezing temperatures threaten.
The average first fall frost for Lancaster County is about October 10th- actually between October 6thand 9thto be specific. These dates are determined from over 47 years of weather data, starting in 1949. It’s a good guideline to estimate when frost will occur, but since this is an average first fall frost date, half of all autumn freezes will occur before these dates and half will occur after.
What is Frost
Frost is the formation of small ice crystals on a surface, like leaves, petals or grass. To understand frost formation, you also need to understand dew point. The term “dew point” is the air temperature at which air is fully saturated with water vapor, so water is evaporating at the same rate as it is condensing.
When the temperature of a surface, like a leaf, falls below the current dew point of the air, water begins to condense on the leaf surface. Dew on grass and plant leaves is a common occurrence, but when the leaf surface temperature is also below freezing the water transitions directly to white ice crystals as it condenses on the leaf.
Picture of cabbage withstanding light freeze.Gardeners know to protect plants from frost and freezing temperatures when a cold arctic storm front moves through. But more commonly, the first fall freeze occurs on a clear, calm night when air temperatures simply drop below freezing. No storm or gusty winds. Under these conditions, the garden's soil surface cools quickly since there is no cloud layer to hold warm air close to the ground. The soil's heat is lost to the air above and cold air develops near the ground. This layer of cold air closest to the ground causes damaging frost on your plants. This is called an advective freeze.
If the air is very dry, visible frost may not form even if air temperatures fall below freezing.
Understanding Freeze Damage in the Landscape
Seeing frost on plants in the morning is an indication temperatures have gotten cold enough during the night to cause water to freeze. The frost itself on plant surfaces is less damaging, but freezing of water inside the plant can cause significant damage. Think of what happens when pipes freeze in a house. Cell walls are ruptured as water between plant cells freezes.
The most important factors in plant damage are 1) how cold temperatures got, 2) how long did temperatures stay below freezing and 3) the cold tolerance of the plant. Low temperatures persisting for longer periods of time cause the most damage.
Preventing Frost Damage
Cold air from an advective freeze moves to the lowest areas of the landscape and "pools" there. This makes plants in low areas more susceptible to freeze damage than those in higher areas. When frost is forecast, concentrate your efforts on the warm-season plants or crops you want to save that will be injured by cold temperatures.
Source: unl.edu
Photo Credit: gettyimages-creativenature-nl
Categories: Nebraska, Weather