Perennials Section: Overwintering Checklist

Compiled by Bill Calkins

• Timing: Many perennials will benefit from sizing up or bulking before they’re overwintered. Most perennials have relatively quick bulking periods (six to eight weeks) allowing them enough time to establish good roots before winter. Some (like Helleborus and hosta) have longer bulking periods and should be planted in the late spring or early summer the year prior to expected sell dates. Long day perennials such as echinacea, asclepias and Phlox paniculata need to be fully rooted and mature by Week 39 (12-hour photoperiod).
• Understand cold hardiness: Check tags and remember to add one hardiness zone to the listed zone (a Zone-5 plant in the landscape is a Zone-6 plant in a container). Following this tip—some crops will require more cold protection than expected.
• Moisture control: Dormant perennials won’t take up much water, but keeping the substrate moist (NOT overly wet) will help protect roots from freezing. Aim for moist bottoms and dry tops.
• Feeding perennials before overwintering: Although perennial plants won’t take up nutrients when dormant, fertility shouldn’t be at zero. Plants with some feed before overwintering tend to look better in the spring.
• Trimming or removing foliage in the fall:  Perennials can be trimmed as necessary, but do not trim too late into the fall. Allow at least four weeks of growth before dormancy. As perennials go dormant, don’t remove foliage until it’s completely died back.
• Fall fungicide drenches: If you notice unhealthy (brown, mushy) roots before preparing perennials for overwintering, consider a broad-spectrum fungicide drench before the weather gets cold to allow heathy, new roots to develop. A broad-spectrum drench is a best practice to prevent crown rots for all perennials as they’re going dormant.
• Managing temps: Actively managing ventilation (automatically with environmental control systems or manually by opening vents or end doors on Quonset structures) can delay de-acclimation and reduce temperature fluctuations. Aim to keep the temperatures around freezing (30 to 35F/-1 to 2C) inside structures. A best practice is to avoid freeze/thaw—if frozen, stay frozen.
• Protective blankets: Allow perennials being overwintered to experience as much cold as possible and cover with blankets as late as you can. Blankets should only be used during the absolute coldest times in winter.
• Rodent control: Rodents can cause a lot of damage to perennial crops being overwintered. Take the necessary precautions with single-dose baits, but be sure to keep them away from pets or other nearby animals. Some growers have had success with repellants as long as they’re regularly reapplied.
• Winter inspections: Your perennial production team should take an active approach to overwintering and keep an eye on crops, even in winter. Weekly inspections are ideal—to avoid issues related to pathogens or rodents catching you off guard. Check the crown and roots of your plants for signs of disease. Watch for signs of animals feasting on foliage. If you’re using blankets, you’ll need to pull them back from time to time to inspect plants closely. GT