Cover or Uncover?

Virginia Brubaker
How to manage temperature in outdoor perennial production.  
Commercial horticulture has seen a tremendous amount of change in the past decade and the perennial market hasn’t been left behind. The demand for perennials has increased, as consumers educate themselves and seek out new and improved varieties.
Perennial transplanting time depends on labor, greenhouse space and the availability of plant material. Overwintering perennials can become an important profit center. Fall planting makes better use of available labor for many growers. Depending on your area, finish transplanting no later than early to mid-October to allow the plants time to establish good root systems. The farther north your operation is located, the earlier transplanting needs to be accomplished. Growers in southern climates have a larger window in which to get their perennials planted and bulked up for winter.
Outdoor perennial production is subject to all of the natural elements and, just like the weather itself, doesn’t follow a specific set of rules. The main goal is to avoid extreme temperature fluctuations. Perennials that are warmed too much become more sensitive to cold nighttime temperatures and more susceptible to cold damage.
Keep temperature swings in check by preventing daytime temperatures from heating up your perennial crops. Let the weather report be your guide; closely monitor the forecast to help you determine when to remove or reapply coverings to your crop.
When outdoor temperatures begin to rise above 40F (4C) and new plant growth starts to develop, this is where you need to decide what to do … keep covered or uncover. It’s important to ventilate the hoop houses or uncover the beds to promote good air circulation. Doing so ensures that new growth doesn’t become stretchy or soft. Avoiding heat build-up on sunny winter days is very important!
Greenhouses with roll-up sides are ideal to maintain cool temperatures if outdoor temperatures rise above freezing. White poly and/or positive ventilation can also be used to keep daytime air temperatures cool. “Winter protection fabric” is a convenient way to cover and uncover your perennials; it can come in 12-ft. or 15-ft. widths.
Once warm weather arrives in the spring and remains above freezing for a period of time, removal of the thermal blanket coverings and white hoop house poly can be permanent. When you’re done with them for the season, let them dry, fold them up and store them for next season.  
Experience teaches: “When in doubt, keep them covered.”
After uncovering and looking closely at the areas you haven’t examined for a while, it’s important to address any decayed or damaged plant material that resulted from overwintering. Thorough cleanup is essential to preventing problematic foliar diseases, including Botrytis, powdery mildew, rust and downy mildew.  
Even with the best cultural controls, fungicide sprays are often still needed. Fungicide sprays—such as Affirm, Mural, Orkestra Intrinsic, Pageant Intrinsic or Palladium—are helpful in controlling these foliar diseases. Specifically for outdoor use, Concert II is another good broad-spectrum option. Organic options for these diseases are offered through BotryStop, Camelot O, Cease, Milstop, Nordox 75, Regalia PTO, Triathlon BA and Zero-Tol 2.0.
A preventative fungicide drench can also be applied to add extra protection from root, stem and crown diseases. Banrot, Segway, Subdue MAXX, Terrazole or Truban are recommended options to keep Pythium at bay. To prevent Rhizoctonia, consider Cleary’s 3336, Emblem or Medallion. Organic options also include Cease and Rootshield Plus WP.
Think of it this way: Uncovering your perennials is just another way of giving spring permission to begin. GT
Virginia Brubaker is GGSPro Technical Support Supervisor for Griffin. She can be reached at