Chrysanthemums (mums) are a popular choice for adding vibrant color to the garden, especially during the fall season. The duration these plants can remain outside is primarily determined by their specific variety and the care they receive to mitigate environmental stresses. Many mums are perennial plants whose outdoor survival depends on proper preparation for changing seasons. Maximizing their time outdoors requires understanding their temperature limits, managing their transition to the elements, and providing consistent maintenance.
Temperature Tolerance and Seasonality
The most significant factor determining how long mums can stay outside is their resilience to cold temperatures, which varies between the two main types. Garden mums (hardy mums) are bred to withstand colder weather and can be grown as perennials in USDA Hardiness Zones 5 through 9. Florist mums, conversely, are typically greenhouse-grown for short-term display, lack necessary root establishment, and are generally treated as annuals that cannot survive a harsh winter outdoors.
An established garden mum can usually tolerate a light frost (around 30°F to 32°F) with only minor cosmetic damage. However, a hard freeze (below 28°F) poses a serious threat because it can penetrate the soil and damage the plant’s shallow root system. The optimal temperature range for active growth and vibrant blooms is between 60°F and 70°F. Mums planted in the ground are generally more cold-tolerant than those kept in containers, as the soil offers natural insulation.
Acclimating Mums to Outdoor Conditions
Mums grown indoors or in a greenhouse require a gradual transition process, known as “hardening off,” before permanent outdoor placement. Moving a plant directly from a stable environment to outdoor sunlight, wind, and temperature shifts can cause severe shock, leading to leaf burn or stunted growth. This acclimation allows the plant’s tissues to toughen and better handle environmental stress.
The hardening-off process should take one to two weeks. Begin by placing the plants in a protected, shady location outdoors for just a few hours a day. Over the course of the week, progressively increase their outdoor exposure and slowly introduce them to more direct sun, focusing on morning and evening light. After about a week, if nighttime temperatures remain above freezing, the plants should be ready to stay outside full-time.
Essential Care Requirements Outdoors
Once mums are established outside, maintaining their health requires specific attention to light, water, and nutrients. Chrysanthemums are photoperiodic, meaning they bloom in response to the shorter days of autumn. They require a minimum of six hours of direct sunlight daily to produce the best blooms. Planting them in full sun, away from artificial nighttime lighting, is important because light pollution can disrupt their natural flowering cycle.
Mums have a shallow root system, making consistent watering necessary, especially during high heat. The soil should be kept evenly moist but never waterlogged, as poor drainage quickly causes root rot. Water deeply when the top inch or two of soil feels dry, ensuring the water drains completely. Mums are heavy feeders and benefit from a balanced fertilizer application during the active growing season, which should be discontinued by mid-summer to prevent frost damage to new growth.
Preparing Mums for Winter Dormancy
Preparation for dormancy is the final step for overwintering hardy garden mums as perennials. The most significant threat to the roots during winter is not the cold itself, but the repeated cycle of freezing and thawing. This cycle causes the soil to heave, which can push the shallow roots out of the ground. To combat this, do not cut back the foliage immediately after the flowers fade. Instead, wait until a hard frost has killed the top growth and the plant has entered true dormancy.
The dead foliage acts as a natural insulator, protecting the crown from extreme temperatures. Once the foliage has browned, cut the stems down to four to six inches above the ground. Immediately apply a thick, loose layer of mulch, such as straw, pine needles, or evergreen boughs, around the base of the plant. This insulating layer should be several inches deep and helps maintain a stable, consistently cold soil temperature, preventing frost heaving and protecting the roots until spring.