Chrysanthemums, often called mums, are a favorite for bringing vibrant color to the late-season garden, but growing them from seed is a patient undertaking compared to using cuttings or divisions. Success depends on precise timing and a meticulous indoor environment to give the seedlings the extended head start they require. This guide outlines the necessary steps and schedules to ensure your mums establish well and provide a brilliant display.
The Critical Timing Window
The primary factor in successfully growing chrysanthemums from seed is calculating the precise start date for indoor sowing. Mums require a significantly long growing season, often up to 16 weeks, to reach flowering maturity in their first year. This lengthy development period means they must be started indoors well before the last spring frost.
The timing rule involves sowing seeds six to eight weeks before the average date of the last expected frost in your local area. This window allows the young plants enough time to develop a strong root system and two to three sets of true leaves, preparing them for outdoor life. You can find this crucial frost date information through local agricultural extensions or online gardening resources specific to your region.
Starting too early results in leggy, root-bound plants that are stressed before they even reach the garden soil. Conversely, starting too late means the plants may not mature enough to produce blooms until very late in the season or even the following year.
Starting Seeds Indoors
Once the optimal timing window is determined, the process of sowing begins with selecting the appropriate materials. Use shallow trays or small peat pots filled with a sterile, fine-textured seed starting mix. This mix is lighter than garden soil and promotes better drainage. The mix should be pre-moistened to the consistency of a wrung-out sponge before sowing.
Chrysanthemum seeds are tiny and require light to initiate germination, meaning they should not be buried deeply beneath the mix. Scatter the seeds on the surface and cover them with only a very thin layer—about one-eighth of an inch—of the starting mix or fine vermiculite. Gently mist the surface with a spray bottle to settle the seeds.
To achieve germination, which typically occurs within 10 to 21 days, the soil temperature must be consistently maintained between 70°F and 75°F. Utilizing a waterproof seedling heat mat beneath the trays provides the necessary bottom warmth to stimulate growth. Immediately after germination, remove any plastic humidity domes and place the emerging seedlings under bright light.
Seedlings need 16 hours of bright light daily to develop into stocky, healthy plants and prevent them from stretching toward the light source. Suspend grow lights just a few inches above the tops of the seedlings, raising them as the plants grow taller. Once the young mums have developed two pairs of true leaves, they should be transplanted into slightly larger, individual pots to allow their root systems to expand further before their final move outside.
Moving Seedlings to the Garden
The transition from the controlled indoor environment to the unpredictable outdoors is known as “hardening off.” This step gradually acclimates the young mums to direct sunlight, wind, and fluctuating temperatures, preventing severe transplant shock. Begin this process about seven to ten days before the anticipated planting date, which is after the last expected frost.
On the first day, place the seedlings in a sheltered, shady outdoor location for only an hour or two, ensuring they are protected from strong winds and direct sun exposure. Over the following week, incrementally increase the time they spend outdoors by an hour or two each day. You should also progressively expose them to more direct sunlight, which toughens the cellular structure of the leaves.
The plants should eventually be able to remain outside for a full 24-hour cycle, provided overnight temperatures stay above 45°F. Once fully hardened off, they are ready for their permanent spot in the garden, which should receive at least six hours of full sun daily. Prepare the garden bed by incorporating well-rotted compost to ensure the soil is rich and drains effectively.
Dig a hole slightly wider than the seedling’s root ball, placing the plant at the same soil depth it was growing in its pot. Space the plants about 12 to 18 inches apart to allow for mature growth and good air circulation. Water the newly transplanted mums thoroughly to settle the soil around the roots, and continue to keep the area consistently moist while the plants become established.