by Tim Baker, northwest region horticulture specialist
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Fall flowers are always favorites, giving us one last chance for the year to see beautiful plants before winter arrives. Perhaps the favorite fall flower is the chrysanthemum. While many other plants have finished blooming for the year, and are getting ready for winter, the chrysanthemum is just getting started with its colorful display of flowers. The plant is remarkably diverse, with many colors and forms available.
Why do mums wait until autumn to begin flowering? As with many plant seasonal responses, the answer is found in the day length. Chrysanthemums don’t start forming flower buds until the days start getting shorter. As the days become shorter still, the buds continue to develop, eventually opening up under the shorter days of fall. The timing of this sequence of events is modified by other environmental conditions, such as heat and drought.
Mums come in many shapes and sizes, ranging from one inch in diameter to over six inches. Varieties which are available for garden planting usually have smaller flowers, but produce them in abundance. Some varieties have been developed for size, such as the football mums or spider mums. These are produced by removing all lateral flower buds, leaving only one bud on the plant, at the growing tip. All the energy produced by the plant goes into that one flower bud, which results in a large size. Not all varieties are capable of this size increase, so removing buds may not produce the large size you expect on your common garden chrysanthemum.
The cooler temperatures of autumn are ideal for chrysanthemum growth and color development. Even light frost will often not harm the plants. When frosts become heavier, some flower damage may occur, but unopened buds may escape damage if the temperatures are not too cold.
If you buy potted mums to transplant in the garden, avoid heavy soils. If in doubt, add plenty of organic matter to the soil at the site where you plant them, and mix thoroughly. If you want to overwinter them, be sure they are well mulched. But don’t suffocate them with mulch. Make sure the mulch is loose enough to allow some light and air penetration. If it has been a dry fall, be sure to keep them well-watered. Mum varieties will vary in their ability to withstand freezing temperatures. If in doubt, you might want to over winter them in a cold frame.