Mum plants are a symbol of fall, with their domes of rich, jewel-tone colors. Mums pop up in gardens centers late in the season and are ubiquitous in fall gardens. To tempt gardeners to purchase these late season bloomers, they are often sold in full bloom. While it is nice to have instant color and even nicer to know exactly what color we are getting, buying flowers in full bloom can mean they are past peak and already declining. That's fine, if all you want is a seasonal decoration, but not good if you are hoping for a "hardy" perennial plant.
Why Hardy Mums Don't Live Through Winter
For mums to be truly hardy, they need time to become established in the ground. Ideally, they are best planted in the spring and allowed to grow in place all season. Unfortunately, the mums for sale in garden centers in the fall have been coddled in nurseries and coaxed to set buds for September blooms. That means they are putting an awful lot of energy into blooming, not growing roots. Planting these out in the garden in late summer or early fall doesn't guarantee sufficient time for the plants to become established. This is not a problem in warmer climates, where a bit of deadheading will satisfy most mums after bloom, but in areas with sub-zero winters, perennial plants need substantial roots to anchor them into the ground. The repeated freezing and thawing of the soil will heave the plant out of the ground and kill the roots.
How to Protect Fall Planted Mums During Winter
For fall planted mums to have a better chance of survival in cold areas, you need to give the roots and crown of the plant's extra protection. First, leave the foliage on the plants until spring. Don't prune them back after frost has turned them brown. Then, either mulch the plants heavily or dig, pot and move the plants to a more protected spot in the garden for the winter. If you choose to move the plants, do so before the first hard freeze.
How to Make Sure Your Mums Bloom in Fall
Spring planted mums will have plenty of time for root growth. Many gardeners are surprised that their garden mums start to bloom in mid- to late summer. If you want fall flowers on your mums, you'll need to pinch the plants back periodically throughout the summer. Start when the plants are about 4-5" tall and repeat every 2-3 weeks until about mid-July. This will cause the plant to get stocky and bushier, and by late summer, it should be covered with flower buds.
Spring planted mums should over-winter reliably in USDA Zones 5 and above, maybe even Zone 4. As with fall planted mums, don't cut them back until spring and provide some extra winter mulch, to prevent heaving. Established plants shouldn't be fed after July, so new growth isn't injured by frost.
Of course, you can always grow mums as annuals. They do provide wonderful fall color and work great at filling in empty spots where summer bloomers have faded. Look for plants with lots of unopened buds, to have blooms well into the fall season.