While it certainly isn’t our first choice, as ornamental grass growers we all at some point have to deal with the inevitability of leftover product. Of course, being resourceful businesspeople focused on profit and the bottom line, throwing it out is our last option—so that means overwintering. Here are ways to prepare your grasses for successful overwintering.
Photoperiod or temperature dependent?
First, understand what type of grass you’re dealing with: photoperiod dependent or temperature dependent. The difference is in their dormancy trigger.
In photoperiod-dependent grasses such as Panicum virgatum and Andropogon sp., dormancy is triggered by reducing day length and will generally occur around the same time each year for a given area.
In temperature-dependent grasses such as Miscanthus sinensis, dormancy is triggered by cold weather and can vary according to the timing of cold weather. Cool-season grasses such as Festuca and Calamagrostis will also generally fall into this category.
Prep and timing
Start by cutting back on fertility as dormancy approaches. Integrating this into your fertility schedule will result in dormancy ready plants in the landscape for your customers, or overwintering at the nursery if you have leftover material. Timing is critical when trying to finish pots before dormancy. You’re trying to get the plants established and finished quickly, but then let them shut down for dormancy.
The overall goal is to not push lush new growth while heading into dormancy. You want the plant to shift to storing energy in the root system in preparation for the coming winter.
Liquid feeding offers great flexibility for controlling fertility. Switch to a lower nitrogen fertilizer, or reduce overall fertility as dormancy approaches.
Slow-release fertilizer doesn’t offer the same flexibility as liquid feed, but if you use the correct release period, you can time fertility to run out near the onset of dormancy. For example, leading into a dormancy that is expected to start in September, apply a two- to three-month slow-release fertilizer in June to run out at the end of August. You can also use a slow-release fertilizer that is scheduled to run out early and supplement with liquid feed as needed.
Cutting back dead foliage after dormancy, but prior to winter, will reduce the amount of organic matter and moisture during the winter and leave you with less of a mess to deal with when uncovering in the spring. It will also remove problems associated with having to cut back old foliage after new growth emerges in the spring.
You can overwinter grasses alongside other perennial crops in cold frames, under cloth or in the open in locations with milder winters.
Leftover pots of grasses can overwinter alongside newer production. If plants aren’t root bound, fertilize them when they break dormancy in the spring to have them ready for early sales. If they are root bound, shift material into a larger size in the fall or the spring. Base your decision to shift them in the fall or wait until spring on whether they have enough time to root out in their new pots prior to dormancy.
If you keep grasses in cold frames during the winter, avoid unnecessary freeze/thaw cycles by opening up the cold frames during the day to avoid high temperatures. (High temperatures will cause the pots to thaw during the day and then refreeze at night. Let them stay frozen.)
And finally, a preventive fungicide drench can help reduce root rot issues in areas with wet winters.