The weather turned sharply colder yesterday for Thanksgiving, and this morning, the temperature nudged the single digits and later just barely made it above freezing. Definitely frosty, and if I needed any convincing about just how cold it was, the Rhododendron leaf thermometer would have conveyed the message. Those hardy broad-leafed evergreens roll their foliage when it drops below 32, and the lower the reading, the tighter the roll. When the mercury reads zero, the leaves will be pencils; these are closer to cigars, indicating that the air's not exceptionally nippy. The rolling is known to botanists as thermonasty, and the thought is that the behavior helps plants avoid frost damage by minimizing surface area. Today, it's merely nasty. Later in the upcoming winter, it will get thermonastier.