January 1, 2000
Question: This is the time of year we often get cold fronts from the north that can be very damaging to plants. I understand the danger of freezing temperatures and cold-damaged plants. However, what’s the connection between air temperatures and frost? Can we still get a killing frost even if air temperature is above freezing?
- Lewis, LeHigh Acres
Answer: Dew is the condensation of water on cool objects including the surface of leaves. Frost is frozen dew formed at temperatures at or below freezing. Whenever dew becomes frost, tropicals and other cold sensitive plants are at high risk of becoming cold-damaged. As night approaches, and especially on clear nights, leaves loose heat through a natural process, and, in fact, can cool well below air temperatures. When leaf surfaces become cooler than air, they collect dew from condensation. This can be a vital source of water to plants in arid areas. However, in our area, frozen dew can be deadly to tender plants. Low temperatures associated with frost expand and then rupture cells of plants. Consequently, plants may suffer damage even when air temperatures around them are well above freezing.
For gardeners with this knowledge, the Weather Service dew point forecast takes on added significance. The dew point is the temperature at which water condenses to form dew. This information is critical with respect to cold protection of plants. For example, while air temperature may remain in the 40s, well above freezing, the dew point (temperature) may simultaneously dip below freezing. The former condition may not hurt your tender plants, but the latter may mean an unexpected disaster.
- Stephen Brown is a horticulture agent with the Lee County Extension. To submit questions call the horticulture desk at 461- 7504 between 9 a.m. and 4 p.m. or via e-mail at shb@gnv.ifas.ufl.edu. Listen to the ‘Garden Show" at 8 a.m. on 1200-AM
WINK/WNOG.