increased cold hardiness induced by shorter day length
increased cold hardiness induced by lowering temperatures
dehardening due to abnormally warm temperatures
rapid drop in temperatures with inadequate fall hardening
temperatures lower than plant maximum hardiness
rise and fall of midwinter temperatures
Hot, humid summer nights
Heat tolerance is just as important as cold-hardiness but, because it is not well-understood, it tends to be overlooked. In particular, hot nights prevent many cold-hardy plants from thriving here.
Cool-weather plants also may have increased susceptibility to diseases from our humidity and root rot problems from our poorly drained soils. Spruce, white pine, and lilac often develop diseases in our area because of the stress caused by long, hot summers. Perennials adapted to cooler climates (tulips, delphiniums) are often not persistent here.
A heat zone map has been created by the American Horticulture Society, but it needs refining before growers add the information to plant labels. In particular, the heat zone map references only the daily maximum temperature and ignores night temperature.
Urbanization, particulate pollution, and global warming all cause selectively greater increases in night temperatures. Urbanization is caused by the radiant heat emitted by paved surfaces. This is obvious on heat maps and is seen in Chapel Hill, despite its small population. Global warming and pollution both increase nightime cloudiness, which traps heat.
Improper removal of ice and snow can cause as much damage as the combination of ice, snow, and wind. Frozen, laden limbs are very brittle and snap easily if bent the wrong way. Remove snow by gently sweeping branches upward to lift off snow without further stressing the limbs. Do not attempt to remove ice. Young drooping branches will often recover after thawing.
Pruning tends to decrease hardiness temporarily, so delay removing damaged limbs until late in the winter unless the damage is catastrophic. Plants often come back from the roots even if much of the top growth is destroyed.