Chrysanthemums, commonly known as mums, are a signature flower of the autumn season, typically purchased as mature, budding plants. While most gardeners rely on nursery starts, achieving a vibrant fall display by growing them from seed is entirely possible but demands significant foresight and a precise timeline. Success hinges on providing a long, uninterrupted period of vegetative growth before the plant shifts its energy to flower production. This article focuses on the timeline and care required to cultivate a robust, seed-grown chrysanthemum that will burst into bloom as temperatures cool in the fall.
Calculating the Optimal Sowing Time
Chrysanthemums are perennial plants that require an extensive period of development, needing six to nine months of active growth from germination to establish the structural maturity necessary for an autumn bloom. This long lead time means seeds must be started indoors in late winter or very early spring. A good rule of thumb is to begin the process approximately six to eight weeks before the last expected frost date in your region.
To calculate the ideal sowing window, determine when the mums are expected to flower, generally in early to mid-fall. Working backward from a desired mid-September bloom requires starting seeds as early as January or February, depending on the climate. The goal is to maximize the vegetative phase through spring and early summer, allowing the plant to develop a strong root system and multiple branches before transplanting outdoors once the danger of frost has passed.
Essential Steps for Indoor Germination
Starting mum seeds indoors requires a sterile seed-starting mix in trays or small containers to prevent the introduction of pathogens to the delicate seedlings. Chrysanthemum seeds are minute and require light to germinate, making proper sowing technique important for success. Seeds should be surface-sown directly onto the moistened mix and pressed lightly, or covered with only a very thin dusting of vermiculite or fine soil.
Maintaining a consistently warm environment encourages rapid and uniform sprouting, with an ideal temperature range between 65 and 75 degrees Fahrenheit. This warmth, combined with continuous moisture, typically results in germination within one to three weeks. Once the seedlings emerge, they must immediately receive supplemental light from a dedicated grow light system to prevent them from becoming thin and weak, a condition known as legginess. Providing 14 to 16 hours of light daily is necessary to mimic the long days of late spring and promote compact, healthy growth during their indoor tenure.
Shaping the Plant for Autumn Blooms
After the seedlings have developed a few sets of true leaves, they should be transplanted into slightly larger pots to continue their growth indoors until they are ready for the garden. Before moving the young mums to their permanent outdoor location, they must undergo a hardening off process, which gradually acclimates them to direct sunlight, wind, and cooler night temperatures. This transition is accomplished by placing the plants outdoors for increasingly longer periods over one to two weeks, preventing shock.
The most important structural intervention for developing a full, dome-shaped mum is the practice of “pinching,” which removes the soft, newly grown tips of the stems. This action sacrifices the terminal bud and encourages the plant to produce side shoots, dramatically increasing the number of potential bloom sites. Pinching should begin when the plant reaches about six inches in height and be repeated every few weeks throughout the late spring and early summer. This process must be stopped abruptly around mid-July to signal the plant to cease vegetative growth. Stopping the pinching by this time allows the plant sufficient time to set the buds that will mature into the flowers of fall.