Overwintering plants in an unpredictable climate
Spring: it’s the season of new beginnings, a timeless symbol of growth, hope, and renewal throughout history and across cultures. But those of us in the green industry have seen the other side of spring, and we know that it can also be volatile and unforgiving. Months of planning and preparation lead up to these twelve highly anticipated weeks that set the tone for the rest of the year. Our success, or lack thereof, during these three months can essentially make or break our season. To add to this pressure, so much of what happens in spring is influenced by factors beyond our control, and, as we saw last year, most of those factors are weather related. Unfortunately, the one truly predictable aspect of spring weather is the inevitability that it will continue to become more unpredictable.
For much of the nursery industry’s history, our biggest concern when it came to overwintering was cold protection. The roots of container plants and B&B nursery stock are more exposed to cold winter temperatures since they lack the buffering protection of the soil. An often-cited rule of thumb is that a plant loses at least one to two zones of hardiness in the pot. So we take measures to protect the roots: we consolidate plants pot to pot, we use frost blankets to cover marginal varieties, and we try to ensure that container plants are well-rooted before cold temperatures hit, since poorly rooted and rootbound plants are more vulnerable to frost damage.
But, in recent years, fluctuating temperatures in winter and spring have become a much greater and more frequent threat to the overwintering success of our nursery crops. This January was the sixth warmest in Chicago history and the second warmest in Milwaukee history. But these mild temperatures are much more cause for alarm than below-normal temperatures would be at this time of year. In an ideal world, temperatures would gradually decrease in fall, remain consistently at or below freezing all winter, and then slowly and steadily increase in late winter and early spring. Instead, for the past several winters, we’ve seen an increasing number of days with above-freezing temperatures accompanied by more and more instances of extreme temperature fluctuations. Prolonged periods of warm temperatures in January and February, like we’re seeing now, can put plants at risk for de- acclimating prematurely, and, subsequently, they’ll be more susceptible to damage from late spring freezes.
When it comes to overwintering trees, extreme temperature fluctuations are especially damaging to species with thin bark, like Acer, Malus, Tilia, Liriodendron, and Taxodium, and that’s why we tend to see more instances of sunscald and frost cracking on these varieties. In addition, tree form varieties are more susceptible to frost cracking than multi-stem trees, due to the more exposed nature of the trunk (Cercidiphyllum is a great example of this). Frost cracking is most likely to occur when warm, sunny winter days are followed by sudden, sharp drops in nighttime temperatures, and it’s most often seen on the south- and southwest-facing sides of trees. Damaged areas may callus over, but when the trunk starts growing or when winter comes again, the callused tissue often splits open, resulting in greater susceptibility to insect and disease issues. Frost cracking can be mitigated with the use of tree wraps, but when extreme swings in temperature occur there’s nothing we can do to completely stop it.
Regardless of our politics or individual beliefs around climate change, I think most of us can agree that increasingly unpredictable weather patterns and extreme weather events continue to increase the challenges we face in our industry. While we may not be able to change this, we can put our energy into developing strategies to limit the damage and push for more research that will help our industry cope with the effects of a changing climate. This may mean breeding for varieties that can better tolerate fluctuations in temperature and withstand late frosts, building infrastructure that allows us to mitigate the effects of unstable weather, and further understanding and developing cultural methods that increase overwintering success. There is no short-term or easy solution to these problems, but we’re a resilient industry and we will no doubt find ways to evolve and adapt to meet these challenges for the future.
