Remember when it snowed? Seems like ages ago. Now that the slush is all melted and the temps are back in the 70s, or the 50s, or whatever, I thought I'd offer a few observations on winter gardening in Austin.
Austin's climate sits at the intersection of dry desert and wet tropics, a climactic split personality that produces long summers, mild winters, and exceedingly short springs and falls. If the summer here is an oppressive, one-note slog, winter here is deceptively fickle. Most days are pretty nice. We get long stretches of beautiful weather and then bam! — the temperature plunges. My theory is that the sporadic nature of freezes in Austin makes them seem more seldom than they actually are -- it's easy to forget that last winter, a notoriously warm one, included a dip to 19°F on the morning of January 7.
Dips into the teens are not unusual for us. According to the USDA Plant Hardiness Zone Map (PHZM), Austin’s “average annual extreme minimum temperature” is between 15-20 degrees, putting us in Zone 8b, on par with Seattle, Portland, and Savannah. Of course our climate in general is very unlike those cities; the PHZM only takes into account extreme low temperatures, not highs or precipitation. Still, it’s pretty interesting to examine the chart and compare what a “mild winter” may mean to different people around the country. While my friends in New York City (7a/7b) roll their eyes at our pathetic excuse for snow, another friend who just moved back here from San Francisco (10a/10b) was shocked at the cold last week. And of course Yoopers from sections of Michigan’s Upper Peninsula (4a) will consider 19°F downright balmy for January. It’s all relative.
My point here is that, while Austin’s winters are mild on average, the temperature fluctuations tend toward the extreme. Fifty-degree drops, like we had between Monday and Friday last week, do tend to happen here, which is hard on both gardeners on plants. I find it cognitively very difficult to think about cold-front preparation when it's 75 degrees and sunny, so the night before a freeze is always a last minute scramble in the dark. On Thursday night, when two people texted me to ask whether or not they should bring their potted plants inside in advance of the freeze, I was dashing in and out of the wintry mix doing that very thing.
Dramatic temperature fluctuations are actually harder on plants than sustained cold. Plants’ natural defense mechanisms for surviving winter are disrupted by extreme swings. To simplify it a bit, cold weather signals a plant to stop growing above ground, and to redirect energy into its roots; this is normal winter dormancy. In spring, things warm back up, the plant comes out of dormancy, and top growth resumes. But when it's 85° on Monday, 31° on Friday, and 70° again the following Monday, plants get into a wonky stop-start pattern that increases the likelihood of cold damage: warmth tricks them into producing tender new foliage that will be easily frostbitten. Anecdotally, I observed more frost damage from January 2017’s sudden plummet from the 80s to the teens than I had in the previous winter, which generally cooler but had fewer extreme fluctuations.
Container gardening brings special winter challenges as well. Plants in pots are more susceptible to frost damage than ones in the ground because they lack the natural insulation of the earth. I’ve heard people describe the difference in cold tolerance between container plants and in-ground plants as about ten degrees, or one zone on the PHZM. That’s the difference between north Dallas (8a) and south San Antonio (9a) — nothing to sneeze at.
This is important to remember when choosing plants for containers. Anything the nursery marks as “marginal for Austin” or “pushing it for our zone” means it may not make it here, especially in a pot. Such was the case with this tropical Dwarf Mussaenda (Mussaenda luteola), which I bought from the Natural Gardener. The tag warned me of its Zone 9 status, but I’m a sucker for shade bloomers, and it looked so pretty in their butterfly garden. Here's how it looked in September, with a cute little dragonfly...
...And here's how it fared last week's storm. I covered it on Thursday, and on Friday morning it looked fine, but by Sunday it had succumbed the shock of the freeze.
The plant is still alive, though. (To determine this, I scraped a tiny piece of the stem; the underlying tissue layer is still green, which means the top of the plant is not dead.) I presume my Mussaenda will make a complete recovery if I don’t subject it to any more freezing nights. Ideally I’ll bring it in every time we get a freeze warning. If I forget to bring it inside and it starts looking worse, I’ll cut it back to the base of the plant, put some mulch over it, and hope for the best. Remember that just because the plant appears dead on top, it may still be alive underground. Sometimes the best course of action is just waiting!
So what can you do right now, while we’re having gorgeous weather, to protect your plants from future freezes? First, you can get to know the hardiness ratings of your plants. This will not only help you decide not only what to bring inside and what to cover during the next freeze, but also what not to worry about. I used to expend a lot of effort covering all my outdoor pots with frost cloth every time it got cold, then I got lazy and observed that a lot of plants do just fine without it. Now I group the pots that need covering in a cluster during the winter months, and leave the rest alone.
Another idea is to make a winter home for your most sensitive plants, especially the cute little tender succulents (the aloes, crassulas, kalanchoes, echeverias, sanseverias, etc) that will absolutely perish if left outside. If you don’t have the luxury of a greenhouse, you can improvise. I have a dearth of direct sun in my apartment; my only southern exposure is a frosted glass window in an awkwardly tall stairwell. So I added shelves to the window and stuck my succulents up there. In March, they’ll come back outside to bask in our long, hot summer.
The next time a freeze looms, I’ll post some more detailed suggestions for plant-protection tips. For now I’m going outside to enjoy this bright and gorgeous December day!