Overwintering: It’s a Love/Hate Thing

Paul Westervelt
I’m in a love/hate relationship with overwintered perennials. I love the early sales, but hate the added risk. I prefer quick turns—the less time a plant is on the ground, the less time I have to screw it up. But when done well, overwintering results in gorgeous plants early to market and an additional turn of the space. It’s been so successful for us that, over the last 10 years, we’ve increased overwintered perennial production by a little more than 600%. Really.

Our biggest challenge to overwintering perennials is water. Too little water and they dry to death. But too much and they rot. In summer, a little too dry on a hot day can render a plant unsellable for weeks due to marginal foliar burn, so if we question whether or not to water in summer, we water. In winter, we reverse that rule. It’s remarkable how dry many perennials can be without damage in the winter and I confess I’ve killed way more with too much water than too little. So when we question whether or not to water in winter, we don’t water.

My ideal for most perennials mid-winter is a moist bottom with a dry top. That keeps the roots happy and keeps my Botrytis threat low on any remaining top growth. We grow in 100% pine bark on the floor of covered hoop houses. The floor is weed fabric over black poly over earth. The houses are all graded to allow drainage, but when covered, the water runs off the uphill side only to run back under the baseboard and under the crop. If the houses were perfectly graded like those drool-worthy sloped flood floors, then everything would water evenly.

But I have the redneck version and water channels a little differently through every house. It’s not as bad as it sounds, as water doesn’t pool anywhere, but we do have to hand water the high and dry spots. Occasionally, we reach a point where the whole house is so uneven that we water everything thoroughly to bring it back to the same saturated starting point before letting them dry down again.

Our second biggest challenge is fertilizer. Plants appear to be in suspended animation for much of winter—not growing at all for weeks or months on end. But it gets warm enough in the hoop houses for our top dressed slow-release fertilizer to release a little along. Since we also run the plants as dry as possible that fertilizer can accumulate and do significant damage when we finally water. Regular pour-throughs keep us in line there and also help us keep an eye on pH. For reasons I still don’t understand, the media in our dianthus crops often becomes more acidic over the season. Regular monitoring allows us to periodically correct it with liquid feed rather than the much more cumbersome liquid lime.

Our third biggest challenge is a new one for me—temperature swings. We had higher losses than usual last winter. Losses in crops that I thought were at least tough if not bulletproof. (My thanks go out to perennial guru Paul Pilon, who solved the mystery in an early version of the Perennial Pulse e-newsletter.) In our part of Virginia, a normal winter night is at or below freezing with days above freezing. We get a few single-digit lows each winter, but not many.

With that in mind, when we experienced repeated temperature swings into the teens or low 20s as plants started to wake up, I didn’t blink an eye. Our plants spend all winter in unheated houses and they’re perennials for goodness sake—they can take the cold, right? While they can take the cold when it gets cold and stays cold, I overestimated their ability to tolerate sudden cold snaps while they’re waking up. This year, we’ll keep a closer eye on those sudden drops in temperature during the critical waking up period and turn on the heat to take the edge off the cold. Not enough to make them soft, but enough to keep them from freezing.

For all its challenges, winter production may be my favorite because when those gorgeous little planties start to break dormancy, I get to experience the magic of spring before everyone else … and I love that. GT

Paul Westervelt is Annual & Perennial Production Manager for Saunders Brothers, Inc. in Piney River, Virginia.