Pansies thrive in cooler temperatures, providing a profusion of low-maintenance colour to beds and planters when other plants aren't yet blooming. While pansies are tolerant of cold weather and freezing, they must be planted at the correct time to prevent frost damage. Heat is also a concern as hot weather inhibits flowering and may damage or kill the plants.
Pansies grow well during cool spring and early summer weather. Hot weather inhibits flowering and prolonged hard freezes can kill the plants. Plant pansies in early spring when the average nighttime temperature is 4.44 degrees Celsius. Cover the plants with a layer of mulch if temperatures drop below freezing to protect the foliage and roots from the cold weather. The flowers begin blooming once the average daily temperature rises above 15.6 degrees Celsius.
In areas where the ground doesn't remain frozen all winter, fall planting is preferable. The pansies produce a vigorous root system over the winter months that leads to healthier plants and abundant blooms in spring. The pansies also add colour to fall and winter gardens when planted in the fall. Plant the pansies outside in late August so the roots have at least six weeks to establish before the first freeze. The pansies may not flower right away if the weather is still hot but will begin producing blooms once the temperatures cool. Mulch over the plants after the first freeze to protect them from winter temperature fluctuations.
It takes pansies about nine weeks to reach a transplantable size when grown from seeds. Start the seeds indoors in late winter if you plan to plant in spring. Plants started in January are ready to transplant in mid-April. Start the seeds indoors in late June for fall planting so that the pansies are mature enough for transplanting in late August. You can start spring pansies indoors or outside in a cold frame to protect the seedlings from frost. Fall pansy seeds should be started indoors to protect the plants from summer heat.
Both seedlings and mature bedding plants are available from nurseries and garden centres. Seedlings take longer to produce flowers and are more prone to frost damage. This isn't a concern with most fall plantings, but spring seedlings may require later planting to avoid severe frost danger. Mature plants aren't endangered by a few nights of hard frost in spring, especially if you mulch the plants well. You can plant mature plants later in fall as they only require four weeks to establish in the bed and become frost tolerant.