Most gardeners think of a hardy plant as one that will survive cold winters. Hardy annual seems like an oxymoron, but the term, along with half-hardy annual and tender annual, are actually distinctions used in England to classify the relative cold tolerance of newly planted annual seeds. In the U.S., we’ve taken to applying the terms to plants, as well. That is often the case, but not always. And then again, sometimes the terms overlap. They’ve become a gray area, but this is what the terms were originally meant to define:
- Seeds: Hardy annual seeds can handle being frozen in the soil and are often planted in fall or early spring. Most self-seeding annuals would be considered hardy seeds. For example: Alyssum, Dianthus and Viola.
- Plants: Hardy annual plants can handle a slight freeze and are good choices for early fall and late spring planting. Sustained freezing temperatures or a really good dip in temperature will do them in. Hardy annual plants will fair better if planted in the ground, rather than containers and plants that have had time to adjust to increasingly cold weather will be hardier than those that suddenly encounter it.
- Seeds: Half-hardy seeds can be direct sown after all danger of frost. They don’t like being frozen in the ground, but don’t need to wait until the soil warms. Sometimes it’s just easier to start these seeds indoors and move them out as plants. Examples of half-hardy annuals include: Comsos, Gazania and Petunias.
- Plants: Half-hardy annual plants can survive a couple of brushes with chilly night temperatures (35 - 45 degrees F.) and light frost, but anything colder will turn them to mush. Just like hardy annuals, the longer they’ve had to get used to the changing temperature, the hardier they’re likely to be.
Tender AnnualsMost so called tender annuals are really tropical perennials. Tender annuals would include: Begonias, Impatiens and Zinnia (a true annual).
- Seeds: Don’t plant outdoors until the soil warms and nigh time temperatures are reliably well above freezing.
- Plants: Tender annuals can’t handle anything colder than about 55 degrees F.
As with all things gardening, there are no absolutes. Some plants will surprise you, for better or worse. When in doubt, it’s always a good thing to protect your susceptible plants with a row cover, at night.