California cold and native plants
If you are in a climate that is cold and you want to grow plants
that you are uncertain will make it through a winter the following
information should be helpful. Most of this information comes from Halfpenny & Ozanne ( Winter, an Ecological
Handbook) and Sakai (Frost Survival of Plants) Both books are
interesting. Sakai's book was wonderful.
Snow is no big deal in terms of cold. The temperature of snow is
around freezing and snow has excellent insulating properties. It looks
spectacular to have little stems sticking through snow where the
nursery stock is supposed to be. But I'm relieved when we get snow
along with the cold. Sakai says "If snow
falls before the soil is frozen, a covering of 2 M [6 ft.] keeps the
underlying ground at a temperature between 0° and -2° C.[29-32
And "beneath a layer of snow thicker than 20cm [8in], the
temperature seldom falls below -5° C [23° F.] under the winter
conditions of intermediate latitudes." That is warm enough to keep many
types of citrus alive.
Plants we list as being hardy to 0° F will not generally
handle root temperatures below 15 °F.(-8 °C.). Most desert
plants cannot handle root temperatures below 27° F (-3° C).
With a few exceptions (some Oaks, Mahonias, Firs, Iris and Pines)
nothing has survived root temperatures here of 2-4°F (-15° C)
(in containers we get these root temperatures). France et. al. found
that exposure of ectomycorrhizas to temperatures of -10° C (14°
F) for 48 hours delayed fungal growth for 3 to 39+ days.
Treat the plants as unstable for the first few weeks after a bad
freeze. Mulch them, but do not prune until the tissue is dry and woody,
and water only if you cannot avoid it.
At your site if there is snow, how deep is it when the cold
comes? If none, is there a dry winter wind? The worst case is where
there is a strong dry wind with no snow. Plant plants behind rocks or
in wind shadows. Sakai also found what we have found, that mulch
greatly increases the soil temperature and winter survival. A 3-4"
mulch can keep the soil from freezing for 15 hours. Further, even if it
does freeze the effects of the frost are reduced because of the
insulation (as with snow) provided by the mulch and the greater
moisture reserves under the mulch. "Dry soil freezes sooner than wet
soil, and bare ground sooner than covered soil."
" In coarse, dry soil, frost penetrated at a mean speed of 2
cm/day [1 in] . . . In wet humus. . . the speed of frost was only .6
cm/day and only reached [ half the depth].”
One of the biggest problems here is the frozen pot during a warm
sunny day and 20 mile per hour winds. (We've been 18° F(-5° C.)
at night and 95° F (35° C.) during the day more times than I'd
like.) This is the problem in many steppe sites or in some mountain
valleys. We fit the pattern for winter desiccation caused by warm dry
winds on frozen ground. Remember ". . . winter desiccation is an
important factor limiting the natural range of many broadleaf evergreen
trees [and shrubs]."
If you have snow, think about its depth. In most of the higher
mountains the evergreen bushes hide under this snow. You'll find that
deciduous bushes are vertical and evergreen bushes are horizontal. "The
parts of the shoots near the ground . . . remain, on a clear night 2-6
k [10° F.] warmer than the . . . upright parts"
If there are trees in the area make use of them. "The annual
minimum soil temperatures below the forest were 6.4 to 8.4 k [12°
F.] [warmer] than below bare ground." In the nursery here we observed
that a squash plant growing under an open live oak structure didn't
freeze until it got to 20° F(-5° C) in the open.
A rock wall properly
placed so the cold air flowed past it warmed the plants at its base
about 10° F (6° C). On the nursery site, the north side of the
house, where the cold air flows down the mountain and settles against
it is 10° F colder than the front of the house from which the cold
can move away.
If these factors are combined, you can gain a full 15° F
(10° C). Sometimes a spot that is USDA Zone 6a is 20 foot away from
a USDA Zone 8a spot. Prognosticate before the plants are planted.