There is no ideal spring or autumn month for planting nursery pansies (Viola wittrockiana) that fits all climates. No matter where you live, watching soil temperatures is the best way to know when to plant pansies, not the calender. Pansies bloom in spring and fall when day temperatures are roughly 60 degrees Fahrenheit and night temperatures are around 40 degrees Fahrenheit.
Pansies are perennials in U.S. Department of Agriculture plant hardiness zones 7b through 11 but are typically grown as cool weather annuals. They're planted in early spring and grown as annuals in Midwestern and northern states with cold winters. In states with milder winters, they're planted in autumn for winter and early spring blooming. They do not like the summer heat of southern states. In many areas of California they are grown as perennials.
Pansy cultivars come with small blooms, 1 1/2 to 3 1/2 inches wide, medium blooms 2 1/2 to 3 1/2 inches wide and large blooms, 3 1/2 to 4 1/2 inches wide. Pansies with small blossoms typically tolerate heat better than those with large blossoms.
Planting Time and Soil Temperatures
Soil temperatures, the best guide to planting time, vary with the calendar in different USDA growing zones. The best time to plant pansies is when soil temperatures are between 43 and 65 degrees F.
If you plant them too early in the fall or too late in the spring with temperatures above 65 degrees F, they may turn yellow with long, spindly stems and with small rosettes at the ends of new stems. If you plant them after temperatures drop below 45 degrees F, their roots will not grow well, and they may be stunted with pale green leaves and few flowers.
Your local nursery will typically begin selling spring pansies when it's time to plant them. If soil temperatures are right, you can plant blooming nursery pansies immediately. Plant non-blooming nursery pansies for late spring blossoming.
You'll get stockier, healthier pansies if you plant nursery seedlings in autumn to blossom in late winter and early spring. That's when most of them are planted and when nurseries sell seedlings.
Nursery seedlings should have at least four to five leaves and look stocky, but six to eight leaves is preferable. Plant them about the same depth they were in nursery containers and from 6 to 10 inches apart. Take care that the soil doesn't cover the crown of the stem, and press the soil firmly enough so that it makes good contact with the roots but not so tightly that it cakes as it dries.
After you plant them, cover the soil with 4 to 6 inches of pine bark, wheat straw, pine straw or other organic mulch to suppress weeds and help conserve moisture. Water them thoroughly 3 to 8 inches deep. Keep the bed moist for the first three weeks while their roots develop and they begin growing.
Water and Fertilizer
Give pansies 1 inch of water each week during the spring to autumn growing season. Avoid watering in late afternoon to prevent fungal infections on wet foliage. Early to late morning watering will give excess water a chance to evaporate in the afternoon.
Apply 1 teaspoon of 10-10-10 fertilizer per square foot of pansy bed one week after planting and water thoroughly.
Pinch off faded flowers to keep them blooming.