Frost occurs between temperatures of 28-32°F, depending on moisture factors like dew & humidity. In the in Citrus Belt (USDA Zone 9), a Hard Frost is when temperatures drop below 28°F for four hours or more. Many plants won’t flower in Spring or even spring back without annual frost, while others cannot tolerate temperatures less than as much as 70°F.
Methods of Plant Damage:
Many plants, such as blueberries, have liquid ‘antifreeze’ defensive mechanisms that lowers the temperatures at which the water inside the plant freezes. Yet the frost crystals become like a thousands optical prisms on each leaf surface. So when the early morning sun first rises and hits the prisms, before the air temp can melt them, it can literally fry the deeper tissues of the leaves & stems. Furthering the effects, the rapid change of internal temperature extremes can be so taxing on the plants vitality that the plant itself may never recover.
PROTECTION: Cover plants completely with anything such as a sheet. With some plants its safe to leave exposed, and then wash the frost away by spraying water just before sunrise.
Some sensitive tropicals can be severely damaged just by dessication alone. Freezing temps tend to cause dessication, as evidenced by “freezer burn” (leave a block of ice in the freezer and it’ll eventually disappear). The typical refrigerator temperature is somewhere between 35-38°F, and most fruits and other plant portions we ever put inside a fridge store quite perfectly at these temps.
PROTECTION: A humid microclimate around the plants of concern, made via various methods such as irrigation or tubs of water in strategic locations, could be all that is needed in some circumstances.
Water expands when it freezes and inside a plant cell the freezing goes beyond the plant tolerances it expands and ruptures the cell walls. Frozen solid, the plants will maintain their form, but upon thawing they’ll get soft and lose their natural color & UV resistances which allows the sun cook do those portions.
PROTECTION: Completely shield from cold air, especially wind chill with plastics or similar materials. Treatments with plant biostimulants such as seaweed products strengthen the cell walls helping prevent rupture.
Protection & Overwintering Methods
Covering plants gives them a 2-5°F temperature boost. Note that sealed greenhouse-like chambers (even just plastic sheeting) can quickly overheat after sunrise. A 2′x20′ driveway greenhouse (made for $13 with used electrical conduit pipe) on the grounds would heat up to 115°F by 10AM. Stems can also be wrapped to result in insulating artificial bark. Covering the tips of cactus plants with Styrofoam cups is effective. Don’t prune or remove frost damaged bits of in-ground plants until all threats of frost are over, as this dead matter provides insulation from subsequent frost bites during future nights.
Post Harvest Ripening:
Tomatoes and other fruits can be picked green and under proper storage may ripen after many weeks. In terms of plant growth regulators, cytokinins can delay senescence (yellowing & decay), and zeolite that strips ethylene gas effectively prevent fruits from decaying (i.e. “Green Bags”).
Not only does a humid micro-climate prevent possible plant dessication damage, but moist soil can hold up to four times the heat as dry. For example, it can easily be 5 warmer just above moist soil. Another method is radiant heat, such as allowing 55 gallon drums full of water to heat up all day in the sun to then slowly dissipate the warmth at night.
Perennials such as peppers can be heavily pruned, mulched and brought indoors. Plants in a dormant state don’t require much water, and over watering cant ruin them. Some even report habits of leaving inside cold garages for several months, and during months Root Cellars are used for storing dormant root crop plants. Dessication is still an issue and plants require being checked on although not very often.
Cold Frames & Greenhouses:
Greenhouse are the ideal choice where budgets allow, or profits & losses are a major concern. Small scale Cold Frames are more affordable for the casual grower, and with proper planning collapsible cold frames can be integrated into existing raised bed fixtures.
Fire, Smudge Pots:
Contained fire is a sure-fire method of localized ambient air heating, and low-tech “Smudge Pots” can yield high-tech results.
Christmas lights can give trunks and stems a heat shield. The more densely the specimens are wrapped the warmer they will remain.
The Enthalpy of Fusion method, commonly used in citrus, exploits a property of water known as Latent Heat of Fusion. Existing overhead sprinklers may be used to deliver water at a low average precipitation rate ranging from 2.5 to 5ml/Hour for frosts down to -5deg. C. As the water is deposited on the foliage it freezes, giving off its latent heat and preventing the temperature of the foliage from falling below zero.
Selective Inverted Sinks (SIS):
The selective inverted sinks prevent frost by drawing cold air from the ground and blowing it up through a chimney.
Wind can mix ascending warm air back down into the descending cold air to stabilize temperature. Huge fans mounted on 30′ towers can be used to create wind currents which physically mix the warm & cold air layers.
Typical Frost Plant Guidelines
Completely devastated by any frost:
Amaranth, Cashew, Kiwano Horned Melon (all except the fruits), Peanuts, Sweet Potato (above ground portions) & Turmeric (above ground portions).
Typically damaged by light frost:
Beans, Carica Papaya (fan leaves & fruits drop), Citron (Citrus medica), Coffee, Corn / Maize, Cucumbers, Eggplants, Jaboticaba, Lemon (C. limon), Lime (C. aurantifolia), Lychee, Miracle Fruit, Mountain Papaya (Vasconcellea pubescens) [fan leaves & fruits drop], Muskmelon, New Zealand Spinach, Okra, Peppers, Pigeon Pea shrubs (50/50 rate), Psychotria Viridis (drops leaves), Pumpkins, Summer Squash, Tomatoes, Watermelon & Winter Squash.
Withstands light frost:
Artichokes, Arugula, Beets, Bok Choy, Calea Dream Herb, Carrots, Cauliflower, Celery, Chayote Squash (Sechium edule), Chinese Cabbage, Citrus, Endive, Escarole, Grapefruit, Green Cardamom, Lettuce, Mache, Orange (Citrus sinensis), Parsnips, Passion Flower / Fruit, Snap Peas, Peruvian Apple Cactus (Cereus repandus), Potato, Radicchio, Salsify & Swiss Chard.
Withstands hard frost:
Broccoli, Brussels Sprouts, Cabbage, Collards, Kale, Kiwi, Kohlrabi, Kumquat, Loquat, Mustard, Onions, Parsley, Peas, Radishes, Satsuma Mandarin, Spinach, Turnips, Leeks & Sorrel.
Requires frost to fruit:
Apple, Cherry, Nectarine, Peach & Plum.
USDA Zone 9B Season Guidelines
Spring & Fall Crops (2 seasons each year):
Beans (most Phaseolis vulgaris) & Tomatoes.
Beans (Lablab purpeus, Lima & Vigna sp.) & Okra.
Fall Through Winter Cool Season Crops:
Broccoli family, Cabbage family, Carrots, Mustard, Radishes & Turnips & Water Cress.
Year Round Crops:
African Eggplant, Chives, Mints, Green Onions, Peppers (protect from dessication & frost), Leeks, Musk Okra, Stevia & Sweet Potatoes.
Best Planted Just Before Early Winter:
Bulb Onions (when by seeds), Potatoes & Rhubarb.
Dormant During Winter:
Miracle Fruit, Turmeric & Water Chestnut.