When Can Bougainvillea Be Put Outdoors in Zone 6?

Zone 6 Frost Dates

• USDA zone 6 cuts a swath through the center of the U.S. from New York to Kansas, then winds throughout the Mountain West. It experiences average annual extreme minimum temperatures between minus 10 and zero degrees Fahrenheit. The length of the growing season is 180 days, with the last frost date in spring occurring around April 15 and the first frost of winter falling near October 15. The right time for bougainvillea to safely stay outdoors depends on when the last killing frost occurs in spring, but you'll need to adjust for your particular geographical location.

Temperature Variability

• The USDA hardiness figures are an average over the 30-year period from 1976 to 2005, and don't indicate extreme lows or occasional benign winters that may occur. The Illinois State Climatologist's Office discusses actual conditions in that state as an example, showing that while the Champaign-Urbana area is classified as nearly zone 6, it dipped into zone 4 conditions a few times. The Chicago urban area creates a warmer island of zone 6 deep within a solid area of zone 5. The climatologist recalls that winters from the 1930s to the early 1970s were more benign. Each state has its counterpart story -- so do research for your exact location.

Dates for Exact Locations

• Unless you've been gardening a long time in the same place and have your own data, you'll need help to determine your last frost date. Ron and Jennifer Kujawski suggest in "The Week-by-Week Vegetable Gardening Handbook" that you contact your county Cooperative Extension agent office for exact data. If you don't have a county extension office, try local radio stations or the office of the state climatologist. The National Climatic Data Center publishes maps and tables giving average last frost dates.

Fine-Tuning the Data

• The National Climatic Data Center also gives a figure for when you can put your bougainvillea back outside with only a 10 percent chance that it will freeze. The average temperature dates give a 50-50 chance for encountering a killing frost after that date. Use the 10 percent figure for a less risky estimate. Also fine-tune for the microclimate of your garden or yard. Make the last frost date earlier if you live on a sunny south slope. Move it later if you live in a cold valley. Keep in mind that there's always the chance of a surprise freeze well past the official last freeze date.