3.2.6. Frost and Cold Air Drainage

It’s good to understand how frost and freezes work. Frost is mainly a concern when growing frost sensitive plants like bananas or papayas, or you have fruit trees that bud early in the season and you sometimes have late frosts.

Frost comes from the moisture in the air. When the right conditions exist, that moisture can condense and freeze where it rests on surfaces. Heat loss at night comes from the radiation of heat from the ground to the sky. The ground freezes first, then bushes and trees. Typical frost conditions are still, and cold. When wind blows, it can mix the colder air near the ground and upper air and prevent freezing unless it’s cold enough that all the air is freezing. A sun trap can end up being a frost pocket (a place where cold settles and stays) if it prevents wind circulation.

When moisture freezes on leaves, it damages them, called “freeze burn”. If it gets cold enough, stems and veins can freeze as well; water expands when it freezes, breaking cell walls and thus killing the parts of the plant that froze. 

If you protect vulnerable plants from above on those nights, you can avoid frost damage by keeping heat in, and preventing frost from hitting the leaves. Overstory trees can do this well. Concrete walls can hold heat. If it gets cold enough for long enough, you may also need to insulate the plant to prevent stem damage.

In Central Florida, we do that by using Spanish Moss and blankets. We stuff Spanish moss over the branches where we can on frost sensitive trees like young avocados or starfruit, and then cover it with a blanket. This will prevent all die off during mild freezes, most die off for colder nights, and could preserve some or even most of the stem and leaf in a severe cold of 25F or below when it stays for hours at a time.

It's important to protect the roots of the tree and the graft, if the plant is frost sensitive. Most plants will grow back if killed back by frost, but if the roots freeze, a frost sensitive plant or sometimes young, frost hardy plants can die. You can do that by piling organic material like straw or Spanish Moss around the roots and stem of the tree you’re trying to protect. With avocados, you can cover the graft with soil.

Farmers often protect frost sensitive plants by watering them all night. Water freezes more slowly than air so this can prevent them from freezing. Citrus growers in Florida do this routinely during freezes. This uses a lot of water - to the point where sinkholes have been created by the heavy use (See Water Section). We prefer not to grow crops in volume that regularly need this kind of intervention.

We did protect our two young avocados one year during 3 hard frosts by using the blanket method plus a mini sprinkler tube underneath the blanket. This was enough to keep the flowers alive and we got our first good sized avocado crop as a result.

This handling generally applies to a place with mild winters, but this can also apply to places with long winters, where you may want to protect flowers and new leaves on young fruit trees during a late frost with pine needles or straw or whatever insulation you can muster. This can give them just enough protection to help them to survive. This isn’t practical in every case of course.

The best remedy for frost damage to fruit is to focus on trees that don’t flower until frost season has passed. We have three cultivars of peach trees - two of them flower relatively late, another is early and we sometimes lose the fruit but when it fruits, it's stunningly delicious, so it's worth the gamble.

Another method of protection is to plant in or create microclimates like suntraps, or south facing buildings, near boulders, etc.

As a note, young trees are the most vulnerable to frost. We stagger plantings of vulnerable trees so we have only a few at a time that are young enough to have to protect during freezes. Once they get big enough, they can create their own heat to some degree and largely protect themselves.

One thing to know about cold is that it sinks, just as heat rises. The tops of hills can be warmer than the valleys, even though the valley may seem more protected.

There are ways to protect these low areas if you have sensitive plants there, with frost tolerant foliage or other barriers that capture the cold as it sinks.

Whenever it freezes it’s informative to run out at dawn to see where the frost patterns are. You will learn how much influence buildings and other elements have to keep frost away, and for how far. Frost does tend to end up in the low areas and open areas, with much less or no frost under trees. We’ve found frost pockets in even a small low lying shaded area that stays cold well after everything else has melted.