This article first appeared in the Porterville Recorder newspaper Dec. 7-8, 2013 and was written with mainly the Southern Sierra and Central Valley gardens in mind, but hopefully it’s useful to native plant gardeners across California.
Cold, cold, cold
Frost periods like the one we’ve just had make me glad most of my plants are well-suited to our climate, including hard freezes. I’ve been answering questions all week about preparing plants for hard frosts. I think we ought to name the first really cold spell of the season, the way meteorologists name hurricanes and tropical storms.
Plants that have adapted to frost include winter deciduous species like most of the oak, spice bush (Calycanthus), snowberry (Symphoricarpos), wild grape (Vitis), western mock orange (Philade
lphus lewisii) and serviceberry (Amelanchior). Many California native evergreen shrubs and perennials are also fully adapted to frosty weather. This includes flannelbush (Fremontodendron), coffeeberry (Rhamnus or Frangula californica), toyon (Heteromeles arbutifolia) and many more.
The usual response to extreme weather is to stop growing, but roots can continue to grow as long as the temperature is in the high 50’s, so it is possible that evergreen shrubs and trees are producing roots even during frosty weather, especially if the soil is covered with leaf, bark or rock mulch. Some plants will continue to bloom before and after frost. Among these are Western Redbud (Cercis occidentalis), some species of sage (Salvia), penstemon and California wild lilac (Ceanothus.) Experts say it is day length more than a particular cold or warm period that causes a plant to cease growing for a while or to begin or cease blooming, to set seed, to lose leaves, etc.
Obviously our local native species are frost hardy, although some, like California fuscia (Zauschneria) may die back after a few frosts. These kinds of herbaceous (non-woody) plants are not dead; they just have evolved to sacrifice foliage and keep energy stored in the roots until spring, when they sprout with new foliage. With these types of plants, you can cut them back and cover the crowns with mulch, or wait until spring and trim them back when you see new growth. Don’t leave the fresh-cut crowns of herbaceous plants exposed if you know it is going to freeze soon.
Foliage of evergreen shrubs may show signs of some frost burning, which resembles sun scalding with brown patches on the leaves and stems. This may not show up until warmer weather returns, but usually is not fatal to the plant, and generally greatly-damaged leaves fall off on their own eventually.
For the most part, your established native plant garden requires no extra care before, during or after a hard freeze. .
But what about new fall plantings? Most of the plants should be fine as well as long as the species is frost hardy and the nursery container was at least one gallon sized. Plant species that originate in coastal southern areas may suffer some damage with several nights below freezing if they are left un-mulched in a very cold part of the garden. This list includes species from Baja like California verbena (Verbena lilacina) and fairy duster (Calliandra californica), both of which are frost hardy once established but are more tender than local species. You generally should not plant frost-marginal plants or very small plants (less than one-gallon-sized container) if freezing temperatures are expected within the month. Some experts recommend holding off all planting after the first frost, but I have successfully planted through December and enjoyed plants into maturity. You can decide if you want to take the chance or not.
Any planting done this late in the season must be mulched soon after planting. If you have mulched beds you understand this in action; the top layer of the mulch freezes and the bottom of the mulch and the ground below does not. This protects the roots and the crown of the plant, which are the most vulnerable. If you can’t arrange mulch on short notice, cover the ground around the plant with leaves or extra (temporary) soil or straw or anything you have around, but don’t let plastic or metal touch the plant. If you are really concerned and have only a few plants, you can cover them with frost-protection fabric, but it isn’t necessary if the species is frost-hardy. Just protect the roots by preventing the soil above them from freezing over.
In addition to mulching, you can leave your irrigation system on even if the plants don’t require more water. They use far less water in cold weather, but the water in the soil will also help keep it warm. Don’t overhead water during the afternoon or evening if a freeze is expected that night, though! You may overhead water the morning after a hard freeze. In fact, one of the ways frost stresses plants is by drying the soil out, so sometimes you need to water well following a major freeze event. Again, this is for your new garden. Established plants will have their roots far enough into the soil that what happens with air temperature has less of an effect.
Cold weather kills off soil, insect and some disease pathogens and is a natural process in our area. It’s another way to be glad for that climate-appropriate landscape and another way a native-plant garden can reduce your maintenance chores. Head inside, enjoy a hot tea, watch holiday movies and let the plants take care of themselves, the way their ancestors did for thousands of years.