Preparing for winter in the nursery
As the days continue to get shorter and we begin to experience freezing temperatures for the first time since spring, there are few sights more satisfying to me than that of the polyhouses covered with plastic and tightly secured for winter, with the plants neatly tucked away inside. Covering the polyhouses is a big job, and I am always impressed by the speed and efficiency with which our experienced crews are able to pull it off. When it comes to having healthy plants in spring, the importance of using the proper overwintering techniques cannot be understated. There are probably no two nurseries that use the same exact methods when it comes to overwintering plants; we all have different infrastructures to work with, and we all employ different techniques which have been passed down to us through the years. But our underlying objectives are ultimately the same.
At our Kenosha nursery, we tip our B&B and Accelerator® material and cover with insulation blankets and white poly once temperatures begin to dip below 25 degrees. This is the temperature below which damage to the root systems becomes more of a threat. We bait under and around all the blankets and apply repellent to the trunks of varieties that are more susceptible to rodent damage. While container material that is overwintered in covered polyhouses can be vulnerable to temperature fluctuations, the B&B and Accelerator® plants are packed tightly together and are somewhat more insulated against large temperature swings.
At our Garden Prairie container nursery, all of our shrubs and perennials are overwintered under 4 mL white poly with no supplemental heat. So when it comes to successfully overwintering our container plants, one of the most important prerequisites is ensuring that plants are adequately rooted. Because we have no minimally heated houses, we need to make sure our spring inventory is planted early enough the previous summer to allow it to root in properly before freezing temperatures set in. We make sure that all of our perennials are planted before the end of August. Slower growing varieties, like Hostas, Sporobolus, and bare root daylilies, are planted in July at the latest. Containerized plants are more susceptible to winter damage than plants in the landscape because they have less of a buffer when it comes to temperature fluctuations, and the smaller the pot, the more sensitive the plant is to these fluctuations. We try to minimize any gaps around baseboards and doorways, and leave a space around the inside perimeters of the polyhouses to act as a buffer to cold air. We pack the pots as close together as possible, cover marginal plants with frost blankets for extra protection, and tip and cover warm season grasses if we have sustained low temperatures for an extended period of time.
Moisture is another critically important factor when it comes to overwintering plants successfully. As temperatures begin to drop and we move into fall, it can become more difficult to manage water needs because pots tend to dry out more inconsistently within the houses. We want to make sure that everything has adequate moisture going into winter, but we also need to be careful not to overwater things as they begin to go dormant, since this can increase the likelihood of root and crown rots. Once we turn off the water and drain our irrigation lines for the last time of the season, we don’t have the opportunity to water again until spring, so we need to make sure that enough water is held within the soil to get the plant material through any warm winter spells we may have.
Since root and crown rots can be one of the biggest threats to the overwintering success of certain plants, we try to protect susceptible varieties like Echinacea, Sedum, and Leucanthemum, by treating them with fungicides in the fall. Botrytis is best controlled by foliar sprays, and we use a drench solution to control Rhizoctonia, Pythium, and Fusarium. Of course, we can’t prevent these types of diseases completely, but by combining responsible water management practices with a targeted fungicide program, we do our best to control them as much as possible.
Living in the Midwest creates a unique set of challenges when it comes to overwintering plant material. We have to be prepared for bitterly cold winter temperatures, early spring thaws, and late spring freezes. Regardless of what part of the industry we find ourselves in, we can all say that our jobs are never easy or boring. And in the end, I think we can all agree that these challenges make us a little more resilient and prepared for whatever new obstacles each new year has in store for us.
