This is a magical time of the year. We awake to crystalline art on our docks, our windows and our plants. We scurry to bring in houseplants and protect those plants that we hope to keep alive, just a little longer. So what is this and how did it happen?
Frost is the solid deposition of water vapor from humid air. Frost forms when the temperature of a surface is below the freezing point of water and below the frost point.
Frost crystals vary in size and are dependent on the time building up or forming and the quantity of water vapor in the atmosphere.
These translucent crystals appear white because they scatter light in many directions. Radiation, window and hoar frost are just some of the forms frost takes. Damage to the plant is not caused directly from the freezing process but rather from the frost crystals that tear the cells of the plant.
At the dew point of the air, when a surface is chilled below freezing and frost begins to form, the spicules of ice or crystals grow based on the amount of water vapor that is available. Upon careful examination sometimes frost arrows may be seen showing the wind direction at the time of formation.
Frost can vary according to elevation and the specific heat of the ground. Lower elevations that are cold and calm may experience frost while at the same time the windy higher elevations do not. Here in Virginia, the waterfront often acts as a buffer, the temperature of the water causing warming while further inland there are frosts. The vineyards of this area know that that warmth causes a longer season and increases the sugar content in the grapes.
Hoar frost, from the old English word meaning old or to show age, refers to the frost that makes shrubs and trees look like white hair. This type of frost actually can be a cause of avalanches as it adds weight to the layers of snow.
Frost can also be formed around commercial freezers or cold storage areas and their nearby rooms.
Window frost, which can have designs looking like ferns or flowers, forms when the glass is exposed to cold temperatures on the outside while warm on the inside. Even with two-paned glass, especially in a room with higher humidity such as a bathroom, window frost can occur.
Other types of frost such as white frost and rime form in locations with much more severe cold than our zone 7.
Frost can be seen along the waterfront etching the ice. The potential damage from frost causes all gardeners to protect their crops and plants.
The National Weather Service has created a hardiness map. This was created from analyzed cold weather patterns over a period of years. This, along with a heat zone map created by the American Horticultural Society, act as planting and harvesting guidelines for gardeners. The zones or areas are based on average annual minimum and maximum temperatures. The frost and heat zones are colored to show their locations throughout the U.S., and are separated by 10 degrees with sub-zones of A and B.
Zone 7 in Virginia refers to the areas of the Northern Neck, Middle Peninsula and surrounding areas. The low temperature is listed as between 0 and 5 degrees. The average last frost date is listed as between April 21st and 30th. The average first frost is indicated as October 30th to November 30th. Most seed packages list suggested planting times consistent with frost dates or the last frost.
Tender plants are defined as those incapable of resisting cold. In zone 7 tomatoes, basil, fuchsias, succulents and citrus are all considered tender. Hardy plants such as radishes, turnips may, with minor mulching, produce all winter long.
Frost damage can be avoided adding days or weeks to harvest or enjoyment. Protection may take many forms. Plants other than succulents may be protected by watering. Wet soil holds heat better than dry. Deep South citrus plants are often protected from frost by alternating sprinklers.
Additional frost protection can be accomplished by covering the plants with bed sheets and drop cloths. Plastic should be elevated and not allowed to come in direct contact with the plants. Lamps of 100 watts designed for outdoor use, mulch, holiday lights and the clustering of plants also may help in this protective process. Most years an additional growing season of weeks can be enjoyed by simply protecting your plant material and keeping an eye on the frost warnings.
Some plants may be killed by frost while others become dormant and emerge in the spring. Hostas are a prime example of this. Their leaves become translucent and wither with the first frost and emerge with healthy foliage in the spring. Other plants are killed dead on the first frost and are headed for the compost pile.
Knowing your plants and their needs is very important. Plants that are tender in this zone can be sheltered with the placement of other more hardy plants around them or placed with the protection of a building or structure between them and the prevailing winter wind. Some plants like Gerber daisy can survive with the addition of stone or pine mulch generously applied before the winter frosts. Gardenias thrive on the south side of buildings. During a cold winter day you can often still thrust your hand down into thick mulch and find the lower parts to be not frozen solid.
Buildings and other structures can create micro climates. A south-facing home, painted a bright sunny yellow, surrounded by heat-absorbing rock will create a possible half zone difference for planting. Sheltering evergreens on the north side will prevent cooling breezes in the summer but protect from the often damaging winter winds. Deciduous trees placed on the south side of a home are ideal. Their shade cools the structure in the summer and in fall they lose their leaves to allow more sunshine to warm in the winter.
When you are choosing plants for your gardens, carefully check on planting zone information. Insist on having clear labels or tags to assist you in your choices. If a plant is listed as borderline in your zone and you are considering an area that cannot provide additional shelter, move on to another! Try not to just fall in love with a plant that will be unsustainable. Research and a good nursery advisor or landscape designer are invaluable in this process.
There is a wealth of information on properly labeled plants. There is also great information on the computer. I always depend on botanical information when the plant is properly labeled with the botanical name. Common names of plants can cause confusion and hence an inability to obtain correct information for planting and care.
Research is invaluable and the time is now while the temperatures are down and you spend more time inside curling up by the fire sipping your hot chocolate. Pull out that horticultural book, take a clear look out those frosted windows and while you are looking enjoy the patterns on the panes. Don’t forget to get out your camera and venture out one early morning while the rising sun is lighting up the frost on leaves and docks.