Frost occurs on clear calm nights when temperatures drop into the low 30s and below. Frost can have a very damaging impact on plants -- and the faster the temperature drops, the lower the temperature drops, and the longer the cold lasts -- the more harmful the outcome can be.
Water expands when it freezes, and when water inside a plant cell freezes, it can expand and bust the cell wall, causing death. The effects of the frost are noticed as the plant thaws. Signs of frost damage and/or death are limp leaves and branches, and black tips on leaves and branches. Some plants that need protection include: avocados, citrus, ferns, tropical plants, cactus and succulents.