In the garden, fall is a time of transformation and preparation. Chilly weather and shorter days mean fall leaves and harvest fruits. It’s time to finish pruning, protect tender plants, and batten the hatches while nature takes its winter course. The arrival of frost means having a plan for overwintering your favorite perennials growing in containers.
Perennial plants respond to these shorter daylight periods and dropping temperatures by slowing down their growth, and with frost, many enter seasonal dormancy. These adaptations allow perennials to survive cooler months in the ground or containers.
This energy-conserving season makes it simple to try overwintering perennials in pots (albeit “simple” doesn’t mean foolproof, especially in frigid climates). We’ll explore the best options for ensuring winter plant protection.
Why Container Garden with Perennials?
Container gardening with perennials offers versatile garden solutions, providing long-lasting interest.
Perennials grace the garden with their reemergence in spring or fall, depending on their dormancy period in hot or cold weather. Unlike annuals, which we expect to toss after their growing season, perennials offer multi-season interest in endless container combinations.
Perennials, too, can head to the compost pile after they’ve fulfilled their garden duties, but why not try overwintering them for a lasting container display next warm season? While it can be challenging to overwinter perennial plants in containers in cold climates, we’ll go over a few “how-tos” for the best chances of success.
Step 1: Know the Basics of Overwintering
The main goal when overwintering perennials in containers is to safeguard the roots and crown from cold.
In overwintering perennials, the primary goal is to protect the roots and crown from extreme temperatures, frost, winds, and winter sun. Woody perennials (think lavender and rosemary) have stems and shoots that may be cold-hardy, but the roots are most susceptible to damage in a container.
Container plants lack the benefit of surrounding soil mass, which provides insulation to in-ground plantings. Because of their stand-alone, above-ground location, pots are exposed to the surrounding air temperature with only slight protection (a thin wall of a pot does little against freezing winter temps). The soil inside a pot will likely freeze and heave – expand and contract with freeze/thaw cycles – potentially leading to root disturbance and damage.
In mild climates, like USDA zone 8 and warmer, overwintering containers and pots is relatively worry-free. However, these regions can still experience bouts of freezing temperatures. A simple thermal blanket or burlap cover can provide enough insulation to protect the container plant from frost and wind. Don’t despair if you live in a colder climate – there are several keys to successful overwintering.
Step 2: Select Which Perennials to Overwinter
Allow newly potted perennials six weeks to establish strong roots before winter.
If you have the time and resources, it’s worth experimenting with different perennials to determine their winter survival success. There are, however, components to gauge which plants have the best chance. Tender perennials must be protected or brought indoors.
Most importantly, know your USDA hardiness zone – then opt for a perennial one to two zones hardier. For example, if you live in zone 7, you can overwinter a perennial listed as hardy to zone 5 or lower. Experts agree that the best success in overwintering perennials in a pot comes from winter hardiness. Those potted roots that lack in-ground resources will need to be hardier than their usual in-ground counterparts.
You’ll also want to ensure your plant has enough time to grow a robust root system before winter arrives. If the perennial is new to your pot, plant it six weeks before the first frost date. This will give the roots time to establish, strengthening them against freezing damage. If the perennial has been growing in a container all summer – or longer – it should be set to prepare for winter.
Step 3: Choose the Right Container
Select larger containers to allow for ample soil and root protection.
Not all containers are hardy against winter extremes. With freeze/thaw cycles, containers are subject to expanding and contracting like the soil they hold. Terracotta and clay pots are porous and may crack or shatter in cold temperatures. Glazed pots are typically fired at a higher temperature and may withstand freezing temperatures better than terracotta. Wood, concrete, composite, and metal containers make long-lasting options to withstand winter variations.
When planting, opt for a container large enough to hold the perennial and give the roots room to grow – the larger the container, the better when it comes to overwintering. More soil mass means more root protection.
To further protect both pot and plant, insulate the container with thermal material, fleece, bubble wrap, or burlap secured with plastic wrap. This can help prevent the container from cracking while providing one more layer against cold air temperatures.
Step 4: Care for Overwintered Perennials
Overwintering dormant perennials involves avoiding fall fertilization to focus on root growth, pruning, and providing occasional watering.
Dormant perennials need little care, but there are a few best practices regarding overwintering. When temperatures turn cool and daylight shortens, the plant prepares for dormancy and stops actively growing.
Prune to Prepare
Cut back woody perennials by about a third to prepare for overwintering. Pruning in late fall directs energy to the roots rather than the shoots. Remove any die-back for perennials that have dropped leaves to prevent fungal infection during overwintering.
Make sure not to fertilize into fall beyond the active growing season so these cues aren’t confused, and the plant directs energy to the roots rather than growing shoots, stems, and blooms.
Water is a beneficial element when conditions are frosty. Irrigate outdoor containers deeply before the first frost. When water freezes, it produces heat, adding insulation to the plant. Dry soil freezes more quickly, which can damage roots.
Periodically check the water stored in perennial overwintering containers. While the plants aren’t actively growing in winter, they still need occasional watering. Prevent soil from drying out, but also be mindful of overwatering, which can lead to diseases like root rot.
About once a month is generally the watering rate, but check on a pot-by-pot basis. Ensure the pot has good drainage and err on the dry side instead of keeping the soil too wet.
Step 5: Choose Your Best Method
Experiment and choose the method that suits your garden space and resources best.
Fortunately for containerized perennials, a few techniques exist to preserve them through winter. Consider your garden space and resources and choose the best method for you. Like many garden practices, this involves a bit of experimentation. With the proper winter protection, perennials will thrive again.
Bring potted perennials into an unheated space like a garage, basement, shed, or cold frame when possible. An enclosed space offers the best protection against winter elements and the best chance of survival. Plants benefit from a little natural light in these spaces and require occasional watering without natural moisture from rain or snow. The ideal indoor temperature range is between 30-40℉.
A simple cold frame can be arranged using hay bales to form four walls and placing an old window, thick plastic, or plexiglass on top. Nestle plants together within the frame; don’t forget to check water needs during the season. On warm days, remove the top for ventilation.
Leaving the container outside? If you have the garden space, consider digging a hole for the pot to sit in. Place the entire container in the hole with the rim above the soil line. Then, return the surrounding soil to bury the pot. This technique mimics in-ground planting and provides that warming soil mass container plants lack. Add mulch around the top of the pot for added protection.
Another outside option is to cluster or huddle pots together. Place the most tender perennials and/or smaller pots in the group’s center and surround them with hardier specimens and larger containers.
Nestle plants against a building, structure, or evergreen hedge for protection from winter winds and temperatures. Make a windscreen by staking burlap if necessary. Containers can also be placed in a box or a chickenwire frame. Fill the box or frame with insulating mulch materials – a key to winter survival.
Insulate with leaf mulch, woodchips, straw, or shredded bark. Mulch heavily all around the group, or use bagged leaves for easy removal in the spring. Evergreen boughs and thermal blankets are other insulating options.
Some gardeners plant, or “store,” the perennial in the ground to overwinter. Come spring, it can be repotted for its container display. Plant the perennial before first frost and mulch with shredded bark for this option.
Trenching is another technique for larger shrubs like roses, where the whole pot and plant are turned on their side, placed in a trench, and covered. This insulates both the branches and container roots. Herbaceous perennials may not require such measures, but if you have a large plant to protect, prune it back and try this in extreme conditions.
Step 6: Prepare Plants for Spring
To prepare overwintered perennials for spring growth, gradually remove winter protection and harden them off.
Overwintered perennials may break dormancy earlier than their outdoor counterparts. Take care to harden them off for spring by gradually removing winter protection.
If indoors, open a window. Outside, remove frost blankets, insulation, and mulch/straw in phases. Move plants to their spring location after the threat of the last frost passes. Enjoy the flourish to come!
Overwintering perennial plants in pots and containers can be worthwhile but challenging in colder winter climates. In mild winter zones, containerized perennials may need little else than to move to a sheltered location or be covered with thermal blankets during cold snaps.
In cold climates, move tropicals and select annuals to grow indoors before the first frost. Bring tender perennials to an unheated enclosed space, and outdoors, protect hardy perennials with insulation through frost blankets, soil, mulch, and other pots. Follow these steps for success in overwintering, get cozy, and await the lasting garden display to come!