Here’s how to safeguard your container plantings from winter’s chill.
Winter brings fluctuating temperatures that can challenge even the hardiest garden. If you grow vegetables, fruit or flowers in containers or planter boxes, there are a few steps you can take to prolong their lifespan when the weather changes.
Protect your containers.
Most of us have two goals when winterizing container gardens: we want our containers and everything inside them to survive. To begin safeguarding your pots and planters, divide them into two categories:
Frost sensitive containers
Materials susceptible to frost include terra cotta, porcelain and other pottery-style pots. These will need emptying when the temperature plunges, because they will often crack when frozen.
Transfer the soil and any spent plants from these containers to a compost pile. Place dormant perennials in the ground or in another, frost proof pot. They’ll need a sheltered place to weather the season. Rinse pots inside and out, stack upside down and store out of the weather, preferably in an unheated garage or shed.
Frost proof containers
Other materials can tolerate the cycle of freezing and thawing that comes with the changing seasons. These containers–those made from metal, wood, fiberglass, stone or concrete–can stay filled all year long if desired.
For pots that remain filled over the winter, consider the following information.
Bring tender, growing plants indoors.
If you have plants that you’d like to keep growing over the winter, now is the time to move them indoors. This might include potted citrus plants, tender herbs such as lemon verbena, or any perennial plant growing outside its ideal geoclimatic zone.
Place these plants in a sunny location (such as in front of a bright window) in a cool room where the temperature stays slightly and consistently above zero. Reduce watering to once per week, misting leaves occasionally to prevent spider mites.
Cover edibles to extend your harvest.
Planters and pots filled with hardy greens such as Swiss chard, kale or radicchio will survive longer if covered. And while they’ll eventually succumb to cold temperatures, covering will extend your harvest by several weeks. If you don’t have a greenhouse or cold frame, use an insulating layer of diffuse row cover. You can purchase this from your local garden center.
Take care when overwintering outdoor plants.
There are a number of steps you can take to improve conditions for plants that you hope to overwinter outdoors. This includes those growing in window boxes, raised planters and pots.
- Find the best winter location.
Cold winter days may feel sunny in your region, but the sun is lower in the sky and casts long shadows. That sunny place where you kept your planters in summer may now be shaded by nearby trees, fences or retaining walls.
Before the deep chill sets in, check your planter locations. If you have plants that stay active in the winter, you’ll need to ensure they get the sunlight they need to survive. Use a simple sunlight calculator or one of many sun-measuring apps to be certain, and move potted plants where needed.
If your container plantings go dormant, light is less of a concern than shelter. Move pots to a protected side of your house to shelter from winter winds and extreme weather. If you live in zone 8 or warmer, this may be the only step you need to take to overwinter your potted perennials.
- Provide extra protection.
For everyone below zone 8, extra protection may be needed. Plants growing in containers are at a slight disadvantage in winter. That’s because cold air flows more readily around plant roots, which are separated from the elements by a few inches of soil and the thickness of your container. And while many perennials appear dormant all winter long, their roots are still alive and need protection.
Choosing thicker pots will give your plants some insulation from the cold. Grouping pots together will also provide some protection. For extra sensitive plants, consider potting up into bigger containers with more insulation value, wrapping with burlap and mulch, or placing into a pre-dug hole in the ground when temperatures drop.
- Choose the right plants for your area.
Since potted plants experience more temperature fluctuations and extremes that can challenge their survival, it’s conventional wisdom to plant two zones higher than your climatic growing zone. This gives you and your plants a margin of error.
If you’re unsure about your growing zone, opt for winter hardy plants in your region. These come in both annual and perennial varieties, with a good selection of edible and ornamental. Winter hardy annuals will last longer when the cold weather hits but will eventually die back. Winter hardy perennials will survive the cold temperatures after a dormant period, reviving in the spring and summer.
Cold hardy plants for container growing
Here is a small selection of the many suitable plants.
- Ensure the soil is meant for containers.
As a general rule, it’s important to fill containers with soil that’s light, absorbent and not prone to compacting. During the winter, drainage becomes even more important, because heavy rains and fluctuating temperatures make plants more susceptible to root rot.
- Keep watering until the soil freezes.
When fall comes around and signs of dormancy set in, it can be tempting to stop watering your containers. While they may not look like they’re doing much, energy is moving from the tops of your plants to the roots, which are still actively growing. Keep watering until the soil freezes–and beyond if temperatures rise above freezing and the soil dries out.
This is particularly important for evergreen trees and shrubs, which slow down their growth during the winter but may still produce new roots. Check the soil for these plants regularly and water whenever it feels dry.
- Withhold fertilizer.
While pots and planters may need extra feeding throughout the growing season, winter is not the time to fertilize. Dosing plants with organic or synthetic fertilizers in fall could encourage the plant to put on new, tender growth too close to frost. Save the fertilizer for early spring, when new growth begins.
- Watch for frost heave.
Constantly fluctuating temperatures in the freeze-thaw cycle can challenge any plant. Frost heave occurs when water in the surrounding soil expands and contracts so much, it forces plant roots above ground (or pot) level. The roots are now even more exposed and vulnerable.
To prevent this from happening, ensure your pots are well-drained. Plant no later than six weeks before frost, so plant roots have a chance to get established. Mulch around plant stems for added protection.
Green onions and scallions
Choose a soilless growing medium or a potting mix meant for container growing. If you live in an area that receives heavy winter rains, avoid mixes with high proportions of water-absorbing materials. Make sure your containers have drainage holes to avoid seasonal waterlogging and prop up on ‘feet’ or blocks where available.
Frequently asked questions
Which plants will overwinter in containers?
Choose plants rated two zones hardier than your growing area. That’s because potted plants and containers are exposed to more severe temperatures than those in the ground. Common examples of plants that survive the winter in pots include English lavender, salvia, echinacea, wintergreen, mint and pansies.
Can I store potted plants in the garage for the winter?
If your garage maintains a consistent temperature that’s slightly above zero, it’s a great place to store overwintering pots that may not survive a deep freeze. To be sure they first enter a dormancy period, wait until temperatures drop to 40 to 50 degrees fahrenheit and your plants have lost their leaves.
Will plant pots crack in winter?
Pots made from porous and inflexible materials (like terracotta and porcelain) are more likely to crack when left outside during the winter. That’s because water in the soil (and the pot itself) expands when frozen, pushing against a pot’s brittle exterior, causing it to break. Even when the soil in the pot appears dry, it can still expand as temperatures plunge.
How can I prevent tender perennials from freezing outside?
Ideally you’ll plant only winter hardy choices, but if you want to give your plants an extra layer of protection, set your pot in a hole in the ground, group inside a circle of other pots, or wrap roots in a layer of burlap and mulch.
Do I need to empty wooden planters or raised beds in winter?
Wooden planters and raised beds are quite tolerant of temperature fluctuations. They do not need to be emptied in winter. Having said that, emptying a wooden planter that’s not in use and storing it in a dry area will prolong the lifespan of the wood. Other tips include setting wooden planters on hard surfaces so the bottom of the legs don’t come into contact with the ground and cutting garden pathways in late fall to keep wet weeds and grass away from the bottom edges of wooden beds.
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