Selecting Annuals: Tender, Hardy, or Half-hardy

Question: A seed catalog labels Petunias as a half-hardy annual. How is that different from a tender annual and a hardy annual? Can I grow all of them here in Central Virginia?

First let’s define annuals and explain the terms: hardy, half-hardy and tender. These categories are determined based on tolerance to cold temperatures.

Annuals are plants and flowers that live only for one growing season; germinate, grow, flower, and produce seeds. Annuals must be planted or directly seeded every year, although certain varieties can reseed themselves. Examples of annuals: begonia, coleus, cosmos, geranium, verbena, hollyhock, petunia, and red fountain grass.

Hardy annuals are the most cold tolerant. They will survive a light frost and moderate freezing without being killed. They do not need to be started indoors and most can be planted in Fall or Spring before the last frost. Most hardy annuals die with the onset of hot summer temperatures. Examples of hardy annuals: calendula, cornflower, foxglove, annual larkspur, and pansy.

Half-Hardy annuals can tolerate cool temperatures and cool soil, but are damaged by frost. They require a longer period of growth, so they should be started indoors 4-8 weeks before the last frost date to give them extra time to mature before being planted outside. They can tolerate periods of cold damp weather, but are frost tender and shouldn’t be transplanted until all danger of frost has passed. Examples of half-hardy annuals: baby’s breath, bells of Ireland, blue sage, forget-me-knots, and strawflower.

Tender annuals are native to tropical regions, are very sensitive to cold soil temperatures, and are easily damaged by frost. Examples of tender annuals: begonia, coleus, impatiens, marigold, nasturtium, nicotiana, and verbena.

Annuals that are started indoors or purchased from greenhouses are considered tender and should be “hardened” or acclimated to outdoor growing conditions before transplanting them into the garden. To do this, place plants in a shady protected site, then gradually expose them to longer periods of direct sun.

Annuals are sometimes categorized as cool or warm season. Cool-season annuals, such as geraniums, petunia and snapdragon, like it between 70 and 80 degrees. Warm-season annuals, such as blue daze, four o’clock, vinca, and pentas, like it between 80 and 90 degrees.

Other factors to consider are the location for light exposure. Some annuals prefer full sun or partial sun, but usually they will not flower in shade. Remember to water, fertilize, and deadhead to enhance their appearance.

The average last frost date, according to the Old Farmer’s Almanac, at the Monticello Station, VA, in 2018 was April 12. According to Virginia Cooperative Extension home garden vegetable planting guide, the range for the last Spring frost for hardiness zone 7a (Albemarle County/Charlottesville area) is from 4/15 to 4/25.

To answer your question, petunias are sometimes categorized as half-hearty (Illinois Cooperative Extension) and sometimes as tender (NC State Cooperative Extension). Yes, you can grow them in Central Virginia, but to be on the safe side, treat petunias as tender annuals, especially since most are started indoors.

References:

“Annuals: Culture and Maintenance,” Ball, Elizabeth & Relf, Diane, Virginia Cooperative Extension Publication, 426-200, 2009.

“Gardening with Annuals: What is an Annual?,” University of Illinois Extension.

“The Old Farmer’s Almanac,” Yankee Publishing Company.

“Virginia’s Home Garden Vegetable Planting Guide: Recommended Planting Dates and Amounts to Plant,” Alex Hessler, Virginia Cooperative Extension Publication, 426-231, 2019.

“What is an Annual?,” Sherry Rindels, Iowa State University Extension & Outreach, Horticulture and Home Pest News, 1996.

“When to Expect Your Last Spring Freeze,” NOAA, National Centers for Environmental Information.