How can I protect my camellia shrubs from very low temperatures?
The website of the International Camellia Society has a discussion of camellias and their cold tolerance, indicating that most spring-blooming Camellia japonica and fall-blooming Camellia sasanqua cultivars will survive a Washington, D.C. winter (zone 7, compared to our zone 8), but there is some concern about extreme cold and drying winds, and sudden drops in temperature (see excerpt below on winter protection, from the University of Georgia Cooperative Extension Service):
Winter Protection
Covering a camellia plant provides frost protection but does little good in a severe freeze. If plants are covered with cloth, plastic or paper, prop up the cover so that it does not touch the buds. Put the cover on after the sun goes down and remove it before mid-morning the next day.
There are additional approaches to providing winter protection against plant damage. Maintain good soil moisture, especially just before freezes. Maintain adequate nutrition by following current fertilizer recommendations. Plant in locations that provide moderate winter shade. Select varieties with good winter hardiness.
The article below from the North Carolina State University Extension has a good general discussion of winter protection for camellias.