Just as everything else in the garden fades, along come the chrysanthemums -- cheerful, autumn-blooming perennials in many sizes and colours. Hardy garden chrysanthemums -- "mums," to most gardeners -- produce stolons or underground shoots that help them survive cold winters year after year. While gardeners in warm climates need to cut back mums in the autumn to encourage compact growth and continued blooming, the rest of us need to avoid that practice -- no matter what. Cutting mums back after frost could kill them.
Choosing hardy chrysanthemums
Even "hardy" garden mums can be vulnerable to very cold temperatures, due to their very shallow roots. The first step you can take to help your chrysanthemums survive winter cold is to choose early-flowering types proven to be hardy in your climate. Think twice about planting out any chrysanthemums purchased at grocery stores or florists. These greenhouse-grown mums produce very few if any stolons and are quickly killed by cold.
Increasing their survival odds
Plant your chrysanthemums in spring if at all possible, because these plants will have more time to get established before winter -- which increases their survival odds. If you'll be planting mums in autumn, plant them at least six weeks before the first expected killing frost so they can get settled in. Plant them in the full sun -- and away from street lights or other nighttime lights, which increase the hours of "daylight" and inhibit flower development.
Preparing them for cold
Plant chrysanthemums in an area protected from cold, drying north winds. Sunny southern and western exposures are warmest. Block, stone or brick walls and patios collect solar heat during the day and radiate it back at night, so planting near structures with some thermal mass is like providing them with a nighttime heater -- at least when the sun shines. Stop fertilising mums by midsummer, to avoid tender late-season growth. Keep plants well watered. Ice crystals that form on the leaf surface draw moisture from inner leaf tissue, causing frost damage, but this "freeze drying" is much less severe when plants are fully hydrated.
Mulching for cold protection
Deeply mulching your mums is the best protection from killing frosts you can provide for them. Mulch keeps soil evenly cold after it freezes, preventing the alternating freeze-thaw cycles that can lead to heaving and fatal plant damage. Thoroughly covered soil will also retain moisture. Mulch with 10 to 15 cm (4 to 6 inches) of straw, evergreen boughs or dried pine needles -- but only after the soil surface has frozen.
Avoiding autumn and winter pruning
Garden mums are more likely to survive the winter if their old foliage is left intact all winter. Come spring -- assuming your mums have survived -- pull the mulch away to let the sun warm the soil. When the tender new growth is 10 cm (4 inches) tall, dig up the plant, separate the new offshoots and discard the old centre section. Plant each offshoot 45 to 60 cm (18 to 24 inches) apart. Fertilise with low-nitrogen 5-10-5 or 10-20-10 synthetic or organic fertiliser. Prune mums by pinching back each new stem every time it reaches 75 to 125 mm (3 to 5 inches) long. Stop all pruning by mid-July.