Repeated freeze-thaw cycles can cause temperature fluctuations that damage roots and cause other problems you may not see until plants start to break dormancy in the spring.
Photo courtesy of ECG

Assuming your fall potting went according to plan and your perennials are finishing nicely, the only thing left to do is guide them through the winter and sell them off to some lucky customer this spring. It’s that easy, right? True, unless you consider that your cold frame retains heat like a compact car, your freeze-thaw cycle is on repeat, and dormant plants aren’t fighting back against diseases while they sleep. Depending on your location, overwintering can be no small challenge, for very different reasons. But with a few tips and some simple pre-planning, overwintering can be outstanding for your business and your perennials.

Problem: Long-term cold, wet conditions, but short-term below-freezing temperatures.

In the south, conditions can often be cold and wet during the winter months, but with temperatures that drop below freezing for just a few days at a time. This becomes a problem when you combine cold-but-not-freezing temps with high moisture.

If you’re growing outside, that can translate to lots of issues, but perhaps the biggest is when dormant plants are subjected to long periods of wet and cold. Because dormant plants can’t actively grow and recover from a disease outbreak the way actively growing plants do, root rot can run rampant. This takes its hidden toll in winter and waits for spring to show up.

Solution: Before dormancy, do a preventive broad-spectrum fungicide drench. Watch plants carefully, and if signs of root rot appear, consider repeating with a second drench to help reduce losses. You might consider using a better-draining mix as well if you are planning on wet conditions.

Problem: Cold frames turn into warm frames.

In the south, overwintering perennials in cold frames will help solve your moisture issues. It can be the right answer for certain crops, but if you’re trying to vernalize your crops, cold frames can reduce the amount of cold hours you can provide. That means you may not be able to provide enough hours to encourage flowering if that’s your goal.

Farther north, cold frames are also a great option for keeping plants protected from the elements but still providing nice, cold temperatures at night.

But in either scenario, cold frames can heat up from the sun’s intensity during the day, then cool back off once the sun goes down. In colder climates this can cause plants to thaw during the day and refreeze at night. Repeated freeze-thaw cycles can cause temperature fluctuations that damage roots and cause other problems you may not see until plants start to break dormancy in the spring.

Solution: Get a simple thermometer for your cold frame. It doesn’t have to be anything fancy, just something that’s easy to read and provides a basic temperature reading. Watch your temperatures carefully – in fact, the sun’s intensity is more important than the temperature outside. A clear, sunny day can quickly elevate daytime temperatures above freezing in a cold frame, even when it’s still frigid outside.

To prevent the freeze-thaw cycle, make sure you have a way to vent your cold frames when the sun comes out. Don’t be afraid to let your pots stay frozen during extended cold periods.

Throughout the winter, watch for loss or plant death around pot-to-pot edges – those pots will thaw first and freeze and thaw more often. That’s a sign you need to vent your cold frame.

Problem: Getting plants covered or uncovered before they’re damaged.

If you’re growing outside and are covering your crops with some sort of winter cover for extended periods, a little work on the front end can help fend off problems down the road.

Solution: If you know you’re going to have fluctuating temperatures and moisture during the winter, a preventive fungicide drench can help keep root issues in check. Removing dead foliage before covering will also keep rotting plant tissue from becoming a potential source of disease.

It’s important not to underestimate the time it takes you to cover your crops for overwintering. You don’t want to spend two days pulling perennial cloths, then turn around and spend the same amount of time uncovering them, but sometimes that’s what it takes. Watch the weather and gauge your geographic area’s trends to leave your plants alone as long as you can. Again, don’t be afraid to let perennials stay frozen and dormant – it’s their own built-in protection mechanism.

Josiah Raymer is head grower and general manager for Emerald Coast Growers, one of the country’s largest ornamental grass and perennial producers.