According to historical data many of the northernmost states may have already experienced their first frost during the last 2 weeks of September. The rest of New England and the mid-atlantic will have their first frost soon; when temperatures dip to 29-32 degrees Fahrenheit overnight a light freeze will occur, killing tender plants, at 25-28 degrees it is termed a moderate freeze and is widely destructive to most vegetation and when temperatures hit 24 degrees and colder plants will incur heavy damage. Sadly, it may be that time of year when we put away the traditional gardening tools, start paying extra attention to the weather forecast and begin to think about protecting our plants for the upcoming winter.
Frost will form first in low-lying areas, denser cold air will settle in lower elevations which may include your landscape and garden. Sloping areas frost less often as it is much more difficult for frost to settle there. As air cools, moisture settles out of it as dew, when plant surfaces reach 32 degrees dew can freeze on them and frost will form. The most opportune times for frost to form is on clear, windless nights; cloudy nights tend to stay warmer as the clouds trap warmth stored during the day.
Annuals and warm season vegetables may be the first to succumb to frost damage, a deep frost will kill them. Frost damage occurs when the moisture contained within the plant cells freezes and damages cell walls. During the spring thaw, damaged cells lose their ability to transport water and nutrients. Be wary though, frost can occur even when the air temperatures are above freezing. Microclimates, elevation, air temperature differentials and other variable factors will contribute to frost developing even in situations where it seems counter-intuitive. Damage extent will be dependant on the types of plants in your garden, their hardiness, maturity, duration and intensity of frost.
To protect your plants from frost damage, cover them; burlap, bed sheets, plastic sheets, milk jugs and inverted flower pots can all be used to cover and protect individual or rows of plants. These materials will preserve stored heat and prevent dew from settling and freezing on plant surfaces. Protective sheeting is most effective when supported by staking or framed over plants. Cover plants prior to nightfall to retain the most heat and remove coverings in the morning to prevent overheating. Extra heat can be collected during the day in your garden by painting jugs or pots black, filling them with water and placing them amongst plants, jugs will heat up and radiate that heat to your garden during the night. Container plants are especially sensitive to temperature changes. If possible bring them inside, if not, try to plant them prior to ground freezing or wrap plant and container in burlap or protective sheeting.
For more information about protecting your plants from frost damage contact your local agricultural extension or your arborist.