A lot of perennials do great in containers, and container gardening has plenty of advantages – you can move the plants wherever you want them, filling in spaces on patios or taking advantage of small patches of sunlight; soil can be more easily amended in containers, so you can grow things that need specific requirements; and containers are more manageable than large expanses of landscaping, especially when it comes to weeding!
One potential disadvantage of containers, though, is that left alone, perennials in containers likely won't survive the winter in most planting zones.
To help your perennials make it through to another year, there are several different routes you can take.
- Plant Them in the Ground
The simplest thing to do, if you have the space for it, and the perennials in question are winter-hardy in your zone, is to plant them in the ground before winter sets in. You can either take the plants out of the containers, or put the containers themselves in the ground. Add mulch or another ground covering around the base of the plants.
Plants in the ground are more protected from temperature fluctuations and extremes, and from frozen water, so this option has a higher success rate than the next one.
- Leave Them in Containers Outside
This option carries more risk, but there are a few things you can do to help your plants survive using this method.
First, use containers that are less likely to break when the temperature drops below freezing, such as freeze-proof plastic or thick terra cotta. Second, fill the containers with additional soil or mulch so there's no room for water to collect there and freeze. Then group your container plants together and place them against a wall of your house or other structure, out of the wind. Cover the pots with straw or mulch to help insulate them.
If you don't have a place to put the pots where they'll have some protection from the elements, you can enclose your plants with chicken wire or other fencing and fill the whole thing with hay, leaves, or mulch to surround and cover the plants. Also, wrapping the pots with heavy burlap, bubble wrap or other heavy material can help to keep the soil and plant roots more stable.
- Build a Cold Frame
Building a cold frame is another way to protect the plants. With a cold frame, though, you do have to be careful not to let the plants overheat when the weather is sunny.