Sitting by the warmth of my wood stove, in January, is an all too real reminder that Wisconsin winters are long and often brutally cold. As a species, humans are incapable of surviving a Wisconsin winter without some form of protection or retreat. During winter, trees do not have the luxury of retreat and are subjected to the full force of our harshest winters. To survive, they have developed remarkable protection mechanisms. Most notable of these protection features is the ability to stop growing before winter sets in. This break from growing is called dormancy. In Wisconsin, above ground tree growth stops in late-July or August. For most trees, growth cessation is a response to shorter days; however, in some fruit trees (pears and apples), growth stops with colder temperatures, regardless of the length of day. Coinciding with this is a reduction in the growth-promoting hormone auxin and an increase in the growth-inhibiting hormone abscisic acid. In Deciduous trees, the increase in abscisic acid is accompanied by the activation of a layer of cells at the base of the leaf petiole that breaks the connection between the leaf and the tree. Leaf separation and subsequent shedding is a protection feature that reduces water loss during the dry winter months. Conifers do not shed their leaves each year; however, the leaves have physical adaptations that reduce water loss during winter. At this point, trees have entered a state of true dormancy. The duration of dormancy varies; however, most native trees require a period of sustained cold temperatures (chilling phase) before growth can resume. This cold requirement is necessary to break down the growthinhibiting effect of abscisic acid. Trees native to northern climates have a longer chilling requirement than trees native to southern climates. Under ideal conditions, the inhibitory effect of abscisic acid has been degraded by the time spring arrives and growth can resume. The dormant season is an optimal time to prune trees. On deciduous trees, the lack of leaves reveals the true structure of the tree. As a result, conflicting branches and structural defects are easily recognized. Additionally, the potential to transmit or attract insects and/or disease through pruning is greatly diminished. Trees such as oaks and elms should only be pruned during the dormant season. There are a number of things that can be done to ensure that your landscape is winterized.
- Select plants that have a hardiness zone rating of 3, 4, or potentially 5. Plants suited to southern climates are susceptible to frost damage in fall and spring.
- Maintain the health of your trees. Eliminate prolonged periods of drought through irrigation, and manage insect and disease problems.
- Avoid excessive pruning of live branches late in the growing season. Pruning stimulates new growth that may not mature before cold temperatures set in.
- Avoid applying excessive, high nitrogen fertilizers. Fertilization can stimulate late-season growth.
- Create an organic/mulch layer over the root system. The organic layer retains soil water and provides insulation against temperature extremes.