Chrysanthemums, often called mums, are popular garden flowers celebrated for their vibrant, late-season color. Growing these plants from seed is a rewarding way to cultivate specific varieties or large numbers of plants. Mums require a significant period of indoor development to ensure they reach maturity and flower before the autumn frost arrives.
Optimal Timing for Starting Seeds
The successful cultivation of chrysanthemums from seed relies on giving them a substantial head start indoors. Mums need a long growing season, often taking up to four months from germination to flowering. The best time to sow seeds is 6 to 10 weeks before the last expected spring frost date in your region.
Starting seeds early allows the seedlings to develop a strong root system and sufficient foliage before moving outdoors. This timing ensures the plants are mature enough to produce fall blooms. While direct sowing outdoors is possible after the last frost, it is less reliable, often resulting in later, less vigorous blooms due to the shortened growth period.
Indoor Seed Starting Requirements
Once the optimal timing is established, the focus shifts to creating the precise environment for successful germination and initial growth. Use a sterile, fine-textured seed-starting mix in trays or small pots, avoiding heavy garden soil which can impede root development. The seeds should be surface-sown or covered only lightly with about 1/8 inch of the mix, as chrysanthemum seeds require light to trigger germination.
Consistent warmth is necessary, with the ideal soil temperature for germination ranging between 70–75°F (21–24°C). A waterproof seedling heat mat can provide the steady warmth required to achieve high germination rates; seedlings typically emerge within 10 to 21 days. As soon as they sprout, seedlings must be immediately placed under bright, consistent light to prevent legginess. Providing 16 hours of light per day, often using fluorescent or LED grow lights positioned a few inches above the plants, supports the robust growth needed for a successful transition outdoors.
Transitioning Seedlings Outdoors
The final stage involves preparing the young plants for their permanent home in the garden, which begins after all danger of frost has passed. This acclimation, known as hardening off, toughens the tender indoor-grown tissue to withstand the sun, wind, and temperature fluctuations outdoors. The hardening process should span 7 to 14 days, gradually exposing the seedlings to harsher conditions.
On the first day, place the trays in a shady, sheltered location for just one or two hours, protecting them from direct sun and strong winds. Over the following week, slowly increase the duration of their time outdoors by an hour or two daily. Begin introducing them to direct morning sunlight after the first few days, progressively increasing the sun exposure.
When the seedlings have successfully hardened off, they are ready to be planted into a garden bed that receives full sun for at least six hours daily. Chrysanthemums thrive in well-drained soil, so it is beneficial to amend the planting bed with organic matter like compost to improve fertility and drainage. Plant the seedlings at the same depth they were growing at indoors, spacing them according to the variety’s mature size. Immediately after transplanting, provide a thorough watering to help the roots settle. To encourage a bushier growth habit and more abundant flowers, the top growth tip of the plant should be “pinched” back when the seedling reaches about six inches in height.