Although pansies (Viola x wittrockiana) perform best in moderate temperatures, they are surprisingly hardy in cold weather. Grown as annuals throughout the United States, pansies are short-lived perennials that sometimes reseed themselves in U.S. Department of Agriculture plant hardiness zones 6 through 10. Although these plants typically cannot survive subzero temperatures, when properly cultivated, they can easily withstand occasional spells of frost and snow.
Pansies can survive brief periods of subfreezing temperatures as low as 2 degrees Fahrenheit; however, they cannot tolerate temperature extremes. They grow best when nighttime temperatures remain between 40 and 65 degrees Fahrenheit and daytime highs do not exceed 75 F. Pansies cannot survive long, harsh winters, nor can they withstand hot, humid conditions. When daytime temperatures remain above 80 degrees, pansies become leggy and cease blooming. When summer heats up, pansies are usually dug up and replaced with summer annuals.
Healthy plants are less susceptible to frost injury, so it's important to start with healthy specimens. When selecting pansies from a nursery, always look for plants with thick, sturdy stems and dark green leaves. Avoid plants with thin, spindly stems, yellow leaves or those that appear overly dry. Smaller plants fare better for fall planting because they have time to grow and become established over the winter.
Giving your pansies a well-supported start at planting time promotes the development of the healthy root systems they need for better cold resistance. Pansies require full sun to part shade and prefer loose, well-drained soil, consisting of approximately 25 percent organic material. Poor, hard soil can be improved by incorporating 4 to 6 inches of composted manure, compost or peat moss into the soil with a shovel. Plant pansies when soil temperatures remain between 45 and 65 degrees F. Soil that is too cold causes root systems to shut down, resulting in stunted growth. If soil is too warm, pansies develop yellow leaves and leggy growth.
In areas with moderate winters, such as the Southeast, pansies are typically planted in the fall, so they can continue to grow and bloom throughout winter and spring. Mulching plants with pine straw, bark or dried leaves helps protect against freezing temperatures and improves moisture retention. If a hard freeze is expected, cover plants with 2 or 3 inches of pine straw until temperatures warm up. Pansies may become pale and wilted when temperatures drop below 25 degrees F, but quickly perk up in warm sunshine. Removing frost-damaged flowers stimulates new growth and helps prevent fungal diseases from developing.
Regular applications of a fertilizer developed for cool-season annuals promotes healthy growth and improves cold tolerance. Summer formulas typically contain slow-release forms of ammonium nitrate, which can cause pansies to become spindly and susceptible to winter injury. Pansies do best when fertilized with formulas that derive nitrogen from potassium nitrate, calcium nitrate or magnesium nitrate. The Utah State University Cooperative Extension recommends monthly applications of a 5-10-5 fertilizer at the rate of 1 pound per 50 square feet to promote healthy growth.