Planning cold-weather protection for potted plants

Last year I groused about not succeeding with container gardening on a high-rise balcony. (Sure is hard to garden up high in shade and wind.”) The annuals — coleus and impatiens mostly — didn’t grow or outright died in the railing planters. In the balcony floor planters, the perennials looked more promising, provided they survived the winter.

This year was a different story.

Coleus doing well this year

Coleus doing well this year

My annuals are doing so well that I don’t want to leave them outside to die over the winter. Coleus, a bust last year, is exploding. I think the difference was starting with larger, more established plants. Sweet potato vine and vinca major are also flourishing. These annuals will come inside for the winter so they won’t have to be replaced next spring. (Luckily, Lizzy the cat leaves plants alone.)

Unidentified grasses and ferns that were given to me are thriving, too. Since I don’t know whether they’re annual or perennial, they will also come inside.

The perennials aren’t growing as enthusiastically, but they’re alive, and I’d like to keep them that way outside in winter.

In previous years, I threw a clear plastic tarp over the perennial pots for the winter, having read that advice somewhere. A recent article by a Morton Arboretum employee advised the opposite: Wrap the pot but not the whole plant in plastic. Plastic traps moisture and heat; the moisture can lead to disease, and the heat might cook the plants on sunny days. Since the perennials didn’t do well under plastic last winter, the latter advice sounds right.

What else can be done to keep perennials alive in pots on a cold balcony? Here are steps that I found recommended online or learned from sad experience.

* Wrap the sides of pots in bubble wrap or other insulating material.

* Push pots together and against the building for shelter. If there’s room between the pots, tuck in bags of foam peanuts.

* Make sure plants are watered before freezing temperatures arrive, and monitor the moisture level and water if necessary during winter. (I didn’t do this diligently last winter — probably another reason the perennials did poorly.) Plants don’t need frequent watering in winter but shouldn’t be allowed to dry out. But be judicious about watering; too-wet soil can kill a plant as readily as too-dry. Pots should have drainage holes and be raised off the ground on pot feet or something similar.

• Choose container plants that can survive winters even colder than ours. Chicago is in USDA hardiness zone 5, but because plants in pots need to be tougher than those in the ground, container plants able to survive in zone 3 — down to 40 degrees below zero — are recommended.

Choosing cold-hardy plants isn’t a guarantee that they’ll survive. Many zone 3 and even zone 2 plants have died on my balcony, including wild ginger, lamium and lamiastrum, bergenia, ajuga, creeping jenny, lady’s mantle, dwarf goatsbeard, catmint, cranesbill, common violets, and sweet woodruff. Even some hostas and lilies of the valley, which I used to think were unkillable, didn’t make it.

What survived last winter were daylilies, spiderwort, one hosta, one lily of the valley, and one columbine. This short list isn’t much to work with, but now that I have a better handle on winter protection, I’m going to try again with some of the plants that failed — more to the point, that I failed to protect. With the above tips as a guide, I hope to have a better success rate. If you have a balcony that gets more sun than mine (only 2½ hours a day), you’ll have more choices of zone 3 and zone 2 plants.

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