If you live in a non-temperate climate, like the band of Zone 5b and north, and would like to grow mums in containers, there are some mum care details you need to know. Potted mums are difficult plants to overwinter, but it's not impossible. The first step is to start with a hardy variety. Then, follow these best practices to growing mums in containers.
Success starts with choosing the best plant you can. Although just about every grocery store, pumpkin patch, and corner convenience store will carry mums during the fall, you’re more likely to find a healthy plant at a garden center or nursery. Mums at big box retailers tend to be underwatered or have repeatedly been dried out and then soaked, which can cause the plant stress. For the best results, ask when a store is getting a new shipment in and go first thing that day to get the cream of the crop. Never buy a wilted plant, and look for one that has more buds than flowers; you’ll get more bloom time out of it and the plant will likely survive repotting better.
Always repot a purchased potted mum plant. They are usually root-bound, meaning that the roots are taking up the majority of the pot. Replant the mums in a container larger than the one it came in so the roots have room to spread out and breathe. If the plant is root-bound, the roots will have formed a large tangle at the ends. Carefully pull the roots apart before repotting.
Mums thrive in well-drained soil. If you are growing mums in pots for a single season, you can plant mums with other plants in a large container. If you're attempting to overwinter the potted mums, plant them by themselves in a container, and try to plant them in the spring. This gives the roots time to grow and establish well before winter. Mums in flower beds don't usually face the issue of not having enough space, but mums in flower boxes or shallow pots may not have enough room to spread out.
Choose a spot that gets at least six hours of sun a day. Plants that don't get enough sunlight will be tall and leggy and produce fewer, smaller flowers. If you're bringing mums inside for the winter, make sure they are by a window where they get the same amount of sun.
Water newly planted mums thoroughly, and never let them wilt. After they are established, give mums about an inch of water per week. When bottom leaves look limp or start to turn brown, water more often. Avoid soaking the foliage, which encourages disease. Make sure the water seeps into the soil to the roots, otherwise the plant can drown.
Plants set out in spring should get fertilizer once or twice a month until cooler weather sets in. Don't fertilize plants set out in fall as annuals, but plants you hope to overwinter should get high-phosphorus fertilizer to stimulate root growth.
Once the first hard frost hits, move your mums container inside or into your unheated garage. Mulch up to 4 inches with straw or shredded hardwood. Fill in around the entire plant, spreading well between branches. Cover the pot with cloth.
Pinch off dead blooms to clean up the plant, but leave branches intact—mums have a better chance of surviving if you wait until spring to prune old stems. As soon as the weather warms, pull away mulch to allow new shoots to pop up and move your pots outside to enjoy some spring sunshine.