Remove all summer annuals that have been killed by frost from the planter. If you have some perennial plants in the planter also, of course they can stay. The herb Rosemary is perennial and you can continue taking some for kitchen use over Thankgiving and Christmas. Some of the smaller creeping varieties of euonymous, like Euonymus fortunei, work great in planters and will stay colorful all winter as well and continue to look great year after year. This type takes on a pinkish hue to its foliage in winter. Your new winter plants will do best if they are an area that gets plenty of sun.
Now that you have some room for your new winter plants, go to the nursery and look carefully at the tags on the plants. You want to find plants that will tolerate temperatures as cold or colder than you find in your area in the winter. You may want to buy only plants that tolerate colder temperatures than occur in your area because planters do freeze harder and faster than plants that are in the ground.
Pansies are just about the only flowering plant that you might get to enjoy through the winter. They will likely rebound in the spring, even if the temperatures dip colder than they prefer. The “Icicle” variety is especially cold hardy and comes in a variety of colors.
Even though not many flowers will tolerate cold temperatures, there are plenty of beautiful foliage plants that will tolerate the cold. Ornamental Kale comes in lovely shades of purple, cream and green and has a very interesting texture.
Plant your new fall plants in your planter and give them a good drink. Your plants will be fairly maintenance free after that.