The unusually cold weather of a few weeks ago may be gone, but its effects are still with us. I’m referring to the dead leaves on garden plants. Around my neighborhood, the tender leaves of the non-native ground cover have turned brown. The outer leaves of the neighbor’s rubber tree are bleached yellow.
Freezing temperatures kill leaves by freezing the water inside. The ice crystals disrupt the cells, and the structure collapses as the ice melts. Needles of evergreen trees like pines and firs contain natural compounds that act like antifreeze, so they survive.
Interestingly, caterpillars that feed on the needles pick up the same freeze-resistance. Cold air is heavier than warm air, so it settles in low spots, like canyon bottoms. The winter is colder there than elsewhere; trees and shrubs growing along streams lose their leaves in winter. In my hillside neighborhood, there are more killed plants at the bottom of the hill.
Freezing weather also damages some native plants. You may have noticed a lot of brown shrubs recently on the drive along Laguna Canyon Road. They are mostly Laurel Sumac, a dark green shrub with large, thin, taco-shaped leaves and red stems.
The plant stems grow through the winter; this tender growth is susceptible to frost damage. The severity of the winter can be judged by how far up the hillside you see winter-killed Laurel Sumac. But the plant is not dead. In spring, the sumac re-sprouts vigorously from underground structures known as root crowns. It’s the same way they recover from a fire.
Early settlers in southern California looking for the right place to grow citrus noticed the frost-sensitive behavior of Laurel Sumac, and used the shrub as an indicator of a mild local climate. It’s a rule of thumb that citrus will thrive where Laurel Sumac grows.
Citrus growers can usually get their trees through a freeze by keeping the air moving through the groves, preventing the frost from settling.
Sometimes, they sacrifice the fruit but keep the trees alive by spraying water on them. The resulting cover of ice insulates the trees from falling below freezing temperature. Water actually gives off heat as it freezes, and this may also help.
Native plants have no one to help them. If a young plant of a frost-sensitive species has sprouted too far down into the canyon, the next cold winter will eliminate it.
A cold winter (or a very wet one, or a very dry one) is a crunch point for native plants. Nature is an unforgiving gardener. Ultimately, plants growing in the “wrong” place do not survive.