US (MI): Winter injury continues to show up in commercial nurseries
The winter of 2014 was excessively cold, and air temperatures in West Michigan were below 32 degrees Fahrenheit for long stretches of time and dropped below -10 F 10 times. Michigan State University Extension educators visited nursery growers in Ottawa County to assess the damage in late May as winter-related damage continued to become evident in commercial nurseries. Twenty-three species of evergreen or deciduous trees and shrubs were reported to have winter or frost damage (see Table 1). Much of the damage observed on nursery stock was in polyhouses over the winter.
In order to understand the winter injury to those plants in hoophouses, MSU Extension examined the MSU Enviro-weather data from a polyhouse in West Olive, Michigan (Fig. 1). This station includes sensors which continually record air temperatures and other variables inside an operational nursery polyhouse. During the winter there were three pronounced temperature cycles where air temperatures in the polyhouses dropped as low as 10 F (Fig. 1). The cycling of temperatures with each outbreak of Artic air caused desiccation and freezing injury on many nursery crops. Some nursery crops that were killed in polyhouses, such as Virginia pine, Western cedar and Doublefile viburnum, had direct freezing damage which killed the plant tissues.
Figure 1. The average maximum and minimum air temperatures in a polyhouse at the West Olive Enviro-weather station.
Nursery growers who experienced winter injury on woody crops should assess the damage of each plant and prune out the dead growth if the plant can still be saleable. For conifers and other trees, corrective pruning and staking may be needed to promote a central leader and restore tree form. In many cases, however, plants with severe die-back will be unsalable and should be discarded.
In order to prevent a similar occurrence next year, one nursery grower reports that they will now be moving fruit trees and other cold sensitive plants into overwintering houses to prevent the cold injury and trunk splitting observed from this winter. If growers experienced significant losses of a species, they should re-analyze the hardiness of plants along with their profit margins on them during a normal growing season.
Dr. Cregg’s work is funded in part by MSU’s AgBioResearch.
This article was published by Michigan State University Extension. For more information, visit www.msue.msu.edu. To contact an expert in your area, visit expert.msue.msu.edu, or call 888-MSUE4MI (888-678-3464).