Widely grown as a temporary annual flower for the cool seasons, the garden pansy (Viola x wittrockiana) is a short-lived perennial plant. Created through various complex hybridising of at least four different viola species, the pansy will grow for a couple years if soil remains moist and temperatures don't get too warm. Only when summer heat soars above 26.7 degrees Celsius and nighttime lows become sultry to pansies falter. Pansies also do not survive summer drought.
Gardeners grow pansies and other cool-season flowers during the cooler weather of fall, winter or spring. Many modern hybrids survive winter low temperatures that drop briefly to -12.2 degrees C and then rebound if weather gets above freezing. Prolonged subfreezing temperatures, even during the day, eventually kill pansy leaves and roots. Conversely, pansies tend to get leggy and stop blooming when temperatures get above 26.7 degrees C. Excessive heat kills plant, too.
Pansies are grown as annuals in the Midwest, Desert Southwest and South in the United States. Excessive winter cold and drying winds kill plants in the Midwest, while intense summer heat destroys pansies in the South and Southwest. In regions with cool, mild weather year-round, a pansy remains evergreen and persists, such as in the San Francisco Bay area. Across the northern U.S. and at higher mountain elevations, pansies planted in spring may readily persist and flower all summer if it doesn't get too hot or dry.
Effects of Heat on Pansy
In cool weather, a pansy plant becomes a rather low, spreading mound of leaves and stems. Flowers rise just above the mass of leaves. When temperatures warm, the growth rate and habit of pansies change. Stems elongate and fewer leaves hide the stems so the plants begin to grow more upright and leggy. Flower production diminishes in overly warm temperatures. Eventually, hot days and warm nights create weak plant tissues and stems collapse and leaves shrivel up.
Pansy Seed Germination
Pansies that are not deadheaded to remove old flowers often set seeds. Young plants sprout up where the seeds reach cool, moist soil. In regions with hot summers, both plants and germination are limited. Even in cool summer areas, the number of plants in the garden may stagnate or dwindle since pansy seeds need darkness to germinate. In summer, the shorter nights inhibit seed sprouting as well as soils that are warmer than 18.3 degrees C, according to Texas A&M University.