Forecast lows for Monday night/Tuesday morning (source: Washington Post)
A few days ago I wrote about a major cold wave pouring south from Canada. The weather forecasters are starting to talk in apocalyptic terms, telling us that this event is likely to give us our lowest temperatures in 20 or maybe even 30 years. A story in yesterday's Washington Post states, "Some of the coldest air in years, if not decades, is poised to pour into the U.S., with mind-boggling low temperatures.... it’s possible all of the I-95 cities from D.C. to New York City drop below zero [= -17.8 C] for the first time in nearly 20 years."
The last time Washington, DC had temperatures like that was February 1994 when we had -4 F (-20 C). Many windmill palms (Trachycarpus fortunei)--including my own--have been planted in the Washington, DC area since 1994. Monday through Wednesday will be a major test for them and for many other plants that people have convinced themselves are perfectly hardy here! The coldest my own garden has seen since 2000 (when I purchased the property) is 6 degrees F (-14.4 C) in January 2004, and I think it may get colder than that on Monday night. I did cover my Schefflera delavayi because it's a long shot in my area, and also because I planted it rather late in the growing season. My other concern is Cycas panzhihuaensis, which has been in the ground for several years and has survived single digits on more than one occasion, but has been declining gradually since I transplanted it about 4 years ago.
Cycas panzhihuaensis in January 2013
Causes for concern are that (a) we we are forecast to have two very cold nights in a row, with the daytime high staying well below freezing during that period; and (b) this cold is coming early in the winter, so any damaged plants will have to wait several weeks before they can recover. Another cause for concern is that I have a cold microclimate within the city; while I do benefit from the urban heat island compared to locations outside the city, low temperatures in my garden are often several degrees lower than the official low at DCA (Washington National Airport) several miles away. The good news is that temperatures are supposed to be well above freezing both immediately before and immediately after this event, so the soil shouldn't freeze very deeply. That should make a big difference because when roots freeze, all bets are off!