Thursday, December 29, 2011

After The Frost: What To Do?

plus: Planting for Winter Color

We have had a lot of frost this year! According to the weather records from the station west of UC Davis, there have been 18 mornings cold enough for frost since Nov. 1. This is a pretty typical La Niña pattern. Low rainfall = low dew points, less fog, and more frost on a clear, still night.

Lowest readings have been the mornings of Dec. 24 and 25 when that weather station recorded 24 degrees. Ordinarily temperatures that low could be expected to do pretty serious harm to citrus and other subtropicals. But damage so far has been minimal.

Microclimates are making a big difference. The areas up against your house on an east-facing or south-facing walls are several degrees warmer due to heat stored during the day, and are warming up quickly when the sun comes out. Cold damage on a plant is a combination of absolute temperature and duration. These frosts have been cold, but short duration.

Dry soil can lead to further damage from cold as plants desiccate. We are way behind on rainfall, with less than half of normal to date. Water newly planted plants, those in containers, and larger citrus and subtropicals. You can simply run your sprinklers through one cycle, or give each plant a quick soaking with a hose.

If your plant is showing damage, it's best to just leave it alone for now. Cutting back frosted leaves and stems opens the plant up to further damage. Those wilted, curled leaves, in some cases, actually provide a small measure of frost protection for parts of the plant further down. I prefer to wait until spring and let the plant begin to grow before pruning. Let the plant tell you how far to cut it back; just cut to the new growth when it occurs.

You've probably already figured out that your summer annuals won't be coming back. Impatiens, coleus, marigolds, and their ilk are dead. Don't rush to rip out begonias, as they often do resprout in spring from the base. Winter annuals are impervious to the cold. Pansies and violas, snapdragons, stock, ornamental cabbage and kale, and paludosum daisy may bend over with frost, but will perk right back up when the sun is upon them. Winter-flowering perennials such as cyclamen are also unaffected. Likewise in your vegetable garden: the salad greens, broccoli, cabbages, onions, and others are all perfectly hardy.

[Science quiz! Why do some plants survive cold and other not? Different chemicals in the leaves and stems. Plants that evolved in colder regions make natural anti-freeze.]

Some common questions:

Should I keep the plants covered?

Only if the material you used allows light to penetrate. Covering a plant with a sheet or blanket has two drawbacks. Each point of contact with the leaves and stems is likely to result in damage as the cold transfers through the cloth. And the exclusion of light for more than a day or so will cause the plant to start dropping leaves.

Should I sprinkle the foliage to protect it?

This common agricultural practice is not practical for homeowners. You need continuously freezing layers of ice forming on the foliage through the freezing period (early morning before dawn). Sprinkling the leaves once doesn't make any difference.

Should I pick all the citrus fruit?

Only if you want a bunch of under-ripe fruit. Citrus don't ripen any further once picked, and most varieties aren't ready yet. Your Satsuma mandarins are ripe, but that also happens to be one of the hardiest citrus varieties. Lemons, navel oranges, and others are still tart. I'd be surprised if a couple of cold mornings have damaged the crop. Stripping the tree was appropriate in 1990 and 1998, when we had epic freeze events. A hard frost of a couple day's duration? I prefer to leave them on the tree.

Do I need to protect my rose bushes?

No. Roses are hardy here.

On a more cheerful note, how about some color in the garden in mid-winter?


We are fortunate to live in a gardening climate where we can have color year-around. The afore-mentioned winter annuals can be planted even in the coldest weather, and will continue to bloom into spring or even early summer. Pansies and violas are most popular due to the color range and profusion of bloom over a long period.

There are a few perennials that bloom mid-winter. Examples include bergenia, gazania, hellebores, and red-hot-poker (Kniphofia hybrids). Take a quick walk through the Ruth Storer garden at the west end of the UC Davis Arboretum to get some ideas; it is an all-season resource. On a recent visit the Kniphofia variety called Christmas Cheer was in full bloom.

Certain shrubs give us reliable winter flowers. Examples include the golden bush daisy (Euryops), laurustinus (Viburnum tinus), Oregon grape (Mahonia), rosemary, and the extra-fragrant sweet Victorian box (Sarcococca ruscifolia). Some winter-bloomers with special requirements include camellias and winter daphne. Well worth the effort, but ask before you buy.

We also enjoy the colorful foliage of a number of evergreen shrubs. Cold weather leads to interesting chemical changes within the leaves of certain plants, leading to bright pigments that are less obvious or missing in warm weather. Just part of the background shrubbery much of the year, these stand out in cold months. Examples include the common heavenly bamboo (Nandina domestica) and the purple hopseed bush (Dodonea viscosa purpurea).
One deciduous shrub with late-season color is Japanese barberry (Berberis thunbergii), whose leaves turn bright red in fall and then hang through December. It grows to five feet tall or so. A dwarf cultivar called Crimson Pygmy only grows to 2 to 3 feet tall and wide.

Colorful winter fruit is a bonus of citrus, and the hardiest one is also the most ornamental: the kumquat. Not everyone likes to eat the tart/sweet fruit. The peel is sweet and edible, the flesh is tart. So to eat a kumquat, you pop it in your mouth whole and chew it up. But even if you don't favor the flavor, you can appreciate the abundant bright golden fruit out in the yard. Kumquats are cold-hardy well into the teens, have fragrant flowers in the summer, and the plant has dense foliage and grows upright.
Many ornamental shrubs have colorful winter fruit, including natives such as Toyon (Heteromeles arbutifolia) and Coffeeberry (Rhamnus californica). There are great examples in the Arboretum near the road which goes to Mrak Hall. The non-native heavenly bamboo, mentioned above, also bears red winter fruit with the colorful leaves.

Is it ok to plant these selections now? Sure, if your soil is workable. Digging in muddy soil is bad for the soil structure. But one silver lining to our dry season is that soil can still be turned and proper planting holes can be dug. If not, just hold the plants in the pot until spring. Don't forget to water them if we don't get rain!

For statewide weather data, go to the UC Davis IPM web site and look for the "Weather, models, and degree days" link here. I use the Davis.A station.

Brugmansia in the UC Davis Arboretum after a frost.

Angel's trumpet

Taken from the building side of the bush, this photo shows the effect of microclimate. The outer, exposed branches have been damaged, with leaves and flowers killed. Branches closer to the warm building survived. What to do? Just leave it alone. Damaged leaves will fall, and new growth will emerge somewhere along the branch in spring. Then you can cut it back safely.


Who doesn't like pansies and violas? The familiar blooms come from October through May in a wide range of colors. Violas (shown here) are smaller than pansies and hold their heads up better in rainy weather.

Bergenia crassifolia Winter-blooming bergenia

Shade-tolerant Winter-blooming bergenia (Bergenia crassifolia) carries its pink blooms above the foliage from January through early March. The large leaves are attractive year-around, mixing well with ferns, hellebores, Japanese maples, and other shade lovers. Will tolerate some drought.



'Cherry Blossom'
Once upon a time, hellebores were expensive and hard to find. Then growers figured out the trick to mass propagation, and plant breeders had fun creating new colorful hybrids. Different species bloom at different times, but all during winter or spring. This two-tone cultivar is called Cherry Blossom. Hellebores love shade, regular watering.


'Christmas Cheer'
Red hot poker
One of those perennials your grandmother grew, which then fell out of favor. But now gardeners are rediscovering the hardiness, drought-tolerance, and long bloom season. These plants are in full sun, with little water, in the UC Davis Arboretum.

Euryops pectinatus
Golden bush daisy

A reliable winter and spring flowering shrub. Sun-loving, drought-tolerant, and unaffected by frost. They may begin blooming as early as fall, and continue usually until hot weather arrives in May. There are green-leaf and grey-leaf forms.

Rosmarinus officinalis

'Miss Jessups'

Common garden rosemary is a winter bloomer! There are upright as well as spreading varieties, and all have blossoms in shades of blue. They are tough and drought-tolerant, and all types can be used in cooking.

Berberis thunbergii 'Atropurpurea'
Redleaf Japanese barberry

A deciduous shrub, but in our climate it loses its leaves very late in the season. First they turn bright red, then they hang on the branches for several weeks. Shown here in mid-December. Barberries make formidable barrier plants, bristling with spines. Best with regular watering.

Nandina domestica
Heavenly bamboo

Winter leaf color of Nandina (Heavenly bamboo) is one of its great garden features. Lush green leaves turn red with the first cold nights. Berries are a nice bonus.

Dodonea viscosa

Purple hopseed bush

A tough, drought-tolerant evergreen shrub that grows quickly to 8 to 10 feet tall, half as broad. It has purplish-green leaves that turn intense burgundy when cold weather comes. Excellent with California natives, as it can live with little or no summer watering once established.

© 2011 Don Shor, Redwood Barn Nursery, Inc., 1607 Fifth Street, Davis, Ca 95616

Feel free to copy and distribute this article with attribution to this author.

Wednesday, December 14, 2011

Friday, December 9, 2011

Tree Associates Blog

Happy to see that John Lichter's Tree Associates has gone to a blog format. John and his associates are very knowledgeable about trees. This should be one to bookmark!

Tree Associates

Thursday, December 8, 2011

Do you have an Android?

sorry for the unsolicited mail but I noticed an article on your blog today about frost and wanted to see if there was any chance you would be willing to mention an app I have written for android mobile phones. I realise it's a bit cheeky asking for free advertising but as you'll see, I put the app on for only 99p and will never make a fortune from it...but would like to see it become a little more popular given the number of hours I've put into it ! I originally developed it purely for myself, after losing a lot of stuff to a late frost in the UK, then realised others might find it useful so published it on the android market to see what's been quite interesting to see who downloads it (I have about 120 users now and many aren't gardeners at all !)

It's the only app on android that will notify you based on the weather forecast, if the temperature is due to drop below the temperature you set (I didn't need yet another weather app...I just wanted something that warned me when there was going to be a low temperature...rather than me have to remember to check the forecast every day).

Many thanks for taking the time to read this, and any feedback or suggestions you might have are most welcome.

many thanks,
Antony Cook

ColdSnap! Frost Alarm

Monday, December 5, 2011

Frost warning? Freeze Warning?

Written for the Davis Enterprise, November 24, 2010

Click on any image for a larger version

"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.


The Frost Spirit, by John Greenleaf Whittier

Frost At Midnight, by Samuel Coleridge

(Yes, Dr. Hayden, I still read poetry!)