It’s getting close to the time when vacationing houseplants need to be moved back indoors. That includes tender perennial plants, like rosemary and dwarf fruit trees. It’s best to get them back indoors while the windows are still open, so they can acclimate gradually, long before the drying heat comes on.
But what do you do about those perennials you planted in containers. It seemed like a good idea when you say it in a magazine, but will they survive out there in a pot?
5 Things to Consider, Before You Leave Your Containers Out in the Cold.
Material: First, is it a frost-resistant container? Notice I didn’t say frost-proof. Although some pots may be, repeated freezing and thawing have done in some of the strongest pots. In general, you’re safe with: metal, high quality resin and fiberglass (although they weaken with age), and some high end terracotta. Concrete can handle some freezing, but it’s porous, so make sure it is off the ground and able to drain. Ceramic and low end plastics won’t make it through a tough winter.
Zoning: Just because a plant is hardy to your USDA hardiness zone does not mean it can handle your winter above ground. There is far less insulation in a container than tucked in a garden bed. To survive in a container, a perennial should be hardy 2 zones lower than your area. A gardener in Zone 6 would need to look for plants rated as hardy to Zone 4.
Exposure: Freezing and thawing can ruin a container, but the resulting expanding and contracting can also push you plant’s roots above the soil line, where they will surly die. Move your containers to a shady location, where the temperature will remain consistent.
Water: Watering in winter is a tough call. If the soil is frozen, there’s not much point. If it’s not, you need to find the balance between bone dry and rotting wet. The one thing I can say with certainty is your containers should be elevated off the ground, so they can drain when they need to. It also makes them easier to move in the spring, if they aren’t frozen onto the soil beneath them. Also remember to remove and saucers underneath them, where water and ice and collect.
Insulation: It is possible to add insulation to outdoor containers. Some newer pots even come with it. And sheer size and volume will make a big difference. Plants squeezed into a one gallon container are going to have a tougher time than those in a half barrel, with plenty of soil around the roots.
You can insulated individual pots by mulching them with leaves or straw. Some gardeners circle the container with wire and fill in with the mulch. Others will wrap their containers with bubble plastic or burlap. I haven’t had great success with that. Don’t cover the whole plant, unless ridiculously cold weather is predicted. Covering the plant can warm it up enough to cause it to sprout.
If you’re the proactive type, you can line the inside of your container with 1 inch of foam, prior to planting. Then there’s always good old snow, which is an excellent insulator, but unreliable.
Safety in Numbers: If you have a lot of containers, the easiest thing to do is to group them all together. They will help insulate each other and it is a lot easier to corral them in wire fencing and mulch the whole lot. This is what I do, after a hard freeze.
Another option I’ve used a lot is to sink the plant into my vegetable garden for the winter, sometimes pot and all. Since the vegetable garden is partially empty in the fall, I plant my container perennials in there and don’t have to worry about them until it’s time to plant the vegetable garden in the spring. Any plants I’ve potted up for my local spring plant swap get submerged in their pots. They’re easy to lift and the plants are traumatized by the upheaval.
Planning for Unpredictability
We have to remember that weather is mercurial. The only safe option is to move the containers into the garage, and even that has its risks. But gardening is the process, not the end result. Just keep thinking how delighted you will be to see what has survived another season