After the frost: what to do?
plus: planting for winter color.
Written for the Davis Enterprise, December 29, 2011
Click on any image for a larger version
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.]
Brugmansia in the UC Davis Arboretum after a frost.
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.
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.
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.
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.
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.
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.
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.
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.
© 2011 Don Shor, Redwood Barn Nursery, Inc., 1607 Fifth Street, Davis, Ca 95616
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