Frost Warning? Freeze Warning?

Written for the Davis Enterprise, November 24, 2010

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"He comes, - he comes, - the Frost Spirit comes!

On the rushing Northern blast ... " John Greenleaf Whittier

This week marked the first National Weather Service frost warning of the season. Our main frost season typically runs from Thanksgiving through Valentine's Day. There have been frosts after Feb. 14, and occasionally before the last week of November, but those have been infrequent and of little consequence.

What weather conditions lead to a frost warning?

The poem was a bit of hyperbole on Whittier's part: frost doesn't "rush" in, it forms under still, clear skies. But he had a point. Frost can occur after a cold air mass, usually a storm, moves through the region. Cloud cover during the day gives way to clear skies in the early evening. As heat is lost to the sky, the surfaces cool rapidly. When the air temperature gets to the dew point, fog forms. When surface temperatures drop below freezing, frost forms. [Note that the air temperature you measure or see online may still be above freezing; it is the surface temperature that matters.]

When the surface reaches 32 degrees, the layer of water vapor molecules in contact with that surface freezes, and then the layer above that, and so on. Frost doesn't fall from the air, it freezes directly on a surface. Some surfaces cool faster than others. The metal on your car may show frost by midnight; bare soil by dawn, while grass might not even reach freezing that same morning. It all has to do with how efficiently the particular surface conducts heat.

What would prevent the frost?

"The Frost performs its secret ministry,

Unhelped by any wind." - Coleridge

Clouds and fog prevent frost by trapping the outbound heat. Light winds mix the warmer air from above back down to ground level. This is the principle behind the giant fans you see on the edges of vineyards in Napa Valley.

What's the potential for harm in the garden?

Leaves lose heat to the sky! Damage to a plant depends on the structure and contents of those leaves. Soft, succulent plants such as Impatiens turn to mush as the cells rupture. Hardy plants contain sugars and other chemicals that act as natural antifreeze.

A frost is not a freeze.

First, don't panic. If a plant is considered hardy here, damage will be cosmetic. The leaves may look ugly, but the plant will survive. It is a freeze that we are concerned about: a period of colder temperatures when a very cold air mass moves over the region.

Our biggest risk is mid-December to mid-January. Temperatures below the mid-20's are unusual here and spell trouble for many plants. On December 23 1990 we reached 16 degrees. The morning of January 14 2007 got to 20 degrees.

Freeze damage is a matter of how cold we get, for how long. That morning in 1990 was the start of a ten-day episode, with each successive day causing further tissue damage to leaves, shoots, and eventually the woody parts of plants.

Which plants are we going to lose on a frosty night?

Summer annual flowers, such as this showy planting of Impatiens in front of Lyon Real Estate office on 2nd Street, will likely be toast after a night or two of light frost. But there are plenty of winter annual flowers we can plant in their place. Pansies [shown at right] and violas, snapdragons , cyclamen and primroses all give winter and spring color without any concern about cold weather.

Pansy 'Delta Premium' -- no need for protection!

Which plants should we protect from frost?

Some succulents such as jade plant, some aloes, and kalanchoes.
Tender subtropicals such as hibiscus and mandevilla.
Semi-tender subtropicals such as bougainvillea, Guatemalan avocado, Mexican lime, young lemon and citron trees.

Which plants should we protect from a freeze?

Older lemons, other young citrus trees, Mexican avocado, geranium (Pelargonium). Blossoms of Lilac vine (Hardenbergia), which bloom in late winter, can be saved by covering the plants.
As temperatures drop further we may get concerned about some of the hardier citrus. Tangeloes, grapefruit, navel and Valencia oranges can be damaged in the mid-20's.

There are many subtropical plants I don't worry about. Brugmansia, lantana, and passionflower may defoliate, or be partially killed, but will recover. Once you get used to the winter appearance of these semi-tender plants, you'll worry less. Just think of them as deciduous. Leave them alone: wait until new growth begins in spring before you cut off the frost-burnt parts.

Passiflora 'Coral Glow'


We grow many subtropical plants in Northern California. Some are badly harmed by freezing weather, while others (such as the Coral Glow Passionflower shown here) re-sprout readily in spring. Temperatures in the upper 20's will damage the leaves and cause some die-back, but the plants will recover. Lower temperatures may completely kill the tops. Damage can be reduced by taking simple measures to trap or provide heat to the plant.

Some citrus are much hardier than lemons and limes. My mandarins and kumquats have been through major freezes with minimal damage. Fruit of Satsuma mandarins may be harmed at very low temperatures, so you might harvest the fruit if extreme cold is predicted. But citrus fruit won't ever get any sweeter once you pick it, so stripping the tree is a last resort.

There are two main ways to protect plants:

Trap heat.

A plant under an overhang is safe, as the building will keep the heat from escaping to the sky. Frost blankets, which are made of light fabric that allows light in, act to create a mini-greenhouse. Anchored with metal pins, these are the simplest way to provide protection.

Using frost blanket to protect fruit on a mature lemon tree.

Provide heat.

Old-fashioned holiday lights generate enough heat to keep the plant warm. Even a 40-watt bulb on a shop light fixture, attached at the base of the tree (grounded outdoor-rated only!) may be enough for a small tree. Combined with the frost blankets, you can get a few degrees of warmth, enough to keep the local temperature above freezing.


Finally, plants in containers may be damaged by desiccation (cold-induced drying) during freezing weather. Keep your outdoor containers watered during the winter! They won't need it very often: once every week to ten days should be fine if we don't have rain.

Fall Color in the Sacramento Valley!

Why do leaves turn color in the fall?

This is right up there with 'how do they grow seedless watermelons?' among the most-asked questions directed at plant professionals.

The answer is simple! The days get shorter, the nights get colder, and Jack Frost magically paints the leaves in your yard!
Well, actually, the answer is more complicated, involving daylength (actually nightlength), temperature, nutrients, and genetics.

Trees that have adapted to cold winter climates go through several steps as winter approaches. The first part of the process is a reaction to longer nights. The plant forms a layer at the base of the leaves that blocks the movement of carbohydrates (sugars, starches) out of the leaves, and blocks the movement of minerals into the leaves. That 'abscission' layer eventually gets brittle and breaks, so the leaf falls.

October Glory maple

But before that happens, the plant produces less and less chlorophyll because it has less stuff to make it from. That is the pigment which makes leaves green. It is also the most fragile pigment, breaking down in sunlight very quickly. Usually the plant is replenishing it very quickly. But as it breaks down and becomes less abundant, the less common pigments of yellow and orange become visible. So these are the first fall colors that we see. Those colors were already there; we're just seeing them now because the green is gone. This first reaction mostly follows the calendar, although severely drought-stressed plants will begin to go dormant earlier than normal.

Some minerals are mobile within the plant, moving from one place to another as needed. The plant moves phosphorus out of the leaf and into the stem where it is stored during cold weather. The absence of phosphorus changes the chemical reaction in the leaves, and the remaining trapped carbohydrates are now made into certain other pigments, notably the ones that are red and purple. So these are the more spectacular colors that we see next. They weren't there before; the tree is making them now out of what's left in the leaf.

Chinese pistache leaves

Some trees make more of these pigments than others do, so it varies between species. There are also genetic differences of seedlings within a species. Variation in temperature from year to year, and the nutrient status of the specimen, also affects fall color. Within those species that have the pigments to begin with, sunny clear days and cool nights encourage the production of more red and purple pigments. A tree with adequate nutrition will have more basic material to work with to create pigment. Trees that are overwatered have damaged root hairs, so they've been unable to take up nutrients readily during the summer.
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