Chrysanthemums, often called “mums,” are popular fall-blooming perennials known for providing color when other garden plants fade. Their success and ability to return year after year depend heavily on managing temperature extremes. Understanding the limits of both cold and heat tolerance is crucial for ensuring the plant thrives and survives the dormant season. The precise management of the mum’s environment ensures a vibrant flowering season and the plant’s ability to withstand harsh weather conditions.
Understanding Cold Hardiness and Winter Survival
Garden chrysanthemums are perennials that require dormancy to survive winter. Their ability to endure cold depends significantly on the cultivar and root insulation. Most common garden varieties are reliably hardy in USDA Zones 5 through 9, generally surviving when soil temperatures remain above 0°F. Tolerance to a brief, light frost (30°F to 32°F) differs significantly from true winter hardiness during dormancy.
When temperatures fall below 28°F, chrysanthemums are at risk of damage, especially if newly established or growing in a container. Plants in the ground benefit from the earth’s insulation, allowing roots to handle air temperatures below 20°F. To increase survival, planting should occur in late summer or early fall, well before the ground freezes, allowing for adequate root establishment.
Applying a thick layer of mulch, such as chopped leaves or straw, is a standard practice to insulate the root zone once the foliage has died back and the ground begins to freeze. This mulch layer is meant to maintain a consistent, cold soil temperature throughout the winter. The primary danger is repeated freeze-thaw cycles, which cause the soil to expand and contract. This process, known as frost heaving, pushes the root crown out of the soil, exposing it to lethal drying winds.
Managing High Temperature Stress
Although known for their autumn display, chrysanthemums are fundamentally cool-season plants. They thrive best during the growth phase in temperatures between 60°F and 75°F. When daytime temperatures consistently climb above 85°F to 90°F, the plant begins to experience significant stress.
High heat inhibits the plant’s physiological processes, causing growth to slow and delaying development. Temperatures of 85°F or higher during the short-day period that triggers flowering can cause “heat delay,” significantly postponing the autumn bloom. Prolonged temperatures above 90°F (32°C) also negatively affect flower quality, often resulting in faded or bleached color due to decreased pigmentation.
Mitigating heat stress requires specific cultural practices focused on water management and sun exposure. During a heat wave, the plant’s water requirement increases dramatically for evaporative cooling, necessitating deep, consistent watering to prevent wilting. Providing afternoon shade, either through strategic planting or a temporary shade cloth, prevents intense heat and reduces leaf scorch risk. Reducing or stopping fertilization during intense heat is also advisable, as excess nutrients can stress the roots and worsen high-temperature effects.
Recognizing and Treating Temperature Damage
Temperature extremes manifest in distinct ways, and recognizing specific symptoms is key to effective treatment. Cold damage, typically occurring after a hard frost or during winter, often results in blackened foliage and stems. The tissue becomes limp and water-soaked before turning dark, blackish-brown, indicating cellular rupture from freezing.
If damage is limited to top growth, pruning off the dead, mushy stems and leaves reduces the risk of disease. A complete lack of spring growth indicates the cold killed the root crown, a common outcome if the plant was poorly insulated or suffered frost heaving. For unprotected container plants, the entire root ball may turn brown and mushy, signaling the plant’s death.
Heat damage often begins with wilting, even when the soil is moist, because the plant cannot transport water fast enough to keep up with transpiration. Prolonged wilting leads to leaf scorch, where the edges or tips of the leaves turn brown and crisp. This effectively sacrifices outer tissue to protect the core of the plant. Flowers may also appear bleached, faded, or stunted, indicating heat interfered with pigment development or bud formation.
Immediate recovery steps include adjusting the watering schedule to ensure the soil remains evenly moist, which helps the plant recover turgor pressure. Moving potted plants to a location with guaranteed afternoon shade will lessen the thermal load on the foliage. Avoid applying fertilizer to heat-stressed plants, as the salts can draw water away from the roots and worsen the wilting condition. When the temperature returns to a moderate range, the chrysanthemum will often recover, but severely scorched or damaged foliage should be pruned away.