Chrysanthemums, affectionately known as mums, are widely cultivated flowering plants that bring a riot of color to gardens and porches. These composite flowers, belonging to the genus Chrysanthemum, are celebrated for their diverse petal forms, sizes, and brilliant hues, ranging from deep reds and golds to soft pinks and whites. Despite their ubiquity, many people mistakenly associate their bloom time with the spring season. Understanding the science behind this popular plant reveals why its spectacular display is reserved for a later, cooler time of the year.
Why Mums Are Not Spring Bloomers
Mums do not bloom in the spring because this period is designated for vegetative growth, focusing energy on developing a strong root system and dense foliage. Spring is the time when perennial mums emerge from the soil and begin to establish the framework that will support their later flowers. If a plant does show color in the spring, it is often an exception, typically seen in stressed plants responding to unusual weather patterns.
The mums most commonly purchased in the fall are grouped into garden mums and florist mums, both programmed for late-season flowering. Florist varieties are routinely manipulated by commercial growers using controlled light and darkness cycles to induce blooms for specific holidays. A garden mum growing outdoors follows a strict, innate schedule, spending the long days of spring and early summer building up leaf mass to collect solar energy for the reproductive phase that follows.
The Short-Day Requirement for Fall Flowers
Mums are the signature flower of autumn due to photoperiodism, a biological process where flowering is triggered by changes in day length. Mums are classified as “obligate short-day plants,” requiring extended, uninterrupted darkness to initiate flower bud formation. This is more accurately described as a long-night requirement, where the dark period must exceed a minimum, typically around 10 hours, for several consecutive weeks.
As summer moves into late summer, the nights naturally lengthen, signaling the seasonal change. This extended dark period causes a protein known as florigen to be produced in the leaves and transported to the shoot tips. Florigen signals the plant to switch from vegetative growth to reproductive growth. Depending on the cultivar, this process usually begins around late August or early September, with blooms appearing six to ten weeks after the induction signal is received.
Essential Spring and Summer Care for Robust Fall Blooms
To ensure a spectacular fall display, gardeners must promote vigorous, bushy growth during spring and summer. A practice known as “pinching” is employed to encourage branching, which is necessary because mums are terminal bloomers, producing flowers only at the tip of each stem. Pinching involves removing the top inch of new growth from each stem, interrupting the plant’s natural tendency to grow tall and leggy.
Gardeners should begin this pruning when the plant reaches six to eight inches tall in late spring and repeat the process every four to six weeks. Each pinch forces the stem to divide into multiple side shoots, increasing the number of potential flower tips and resulting in a denser plant. It is advised to cease all pinching by mid-July to give the plant sufficient time to set flower buds before the nights lengthen.
Proper care during this vegetative phase requires full sun exposure (a minimum of six hours daily) and consistent watering. Feeding established mums with a balanced liquid fertilizer from late spring through early summer supports strong root development. However, all fertilization should stop once buds begin to form to prevent the plant from reverting to a growth cycle.