Most gardeners think of a hardy plant as a perennial that will survive cold winters. The term "hardy annual" seems like an oxymoron, but hardy annual, along with half-hardy annual and tender annual, are actually distinctions long used in England to classify the relative cold tolerance of newly planted annual seeds.
In the United States, we’ve taken to applying the terms to plants as well as seeds. This can be confusing because seed germination and plant cold hardiness do not always overlap––a seed that can tolerate freezing ground temperatures may not grow into a plant that tolerates cold temperatures and vice versa.
The terminology has certainly become a gray area, but this is what the terms generally mean.
Hardy annual seeds can handle being frozen in the soil and are often planted in fall or early spring. Most self-seeding annuals are considered hardy since they overwinter in the soil and germinate the following spring; these hardy annual flower seeds actually benefit from snow cover and frost. Hardy annual seeds include:
Hardy annual plants are the most cold-tolerant annual plants. They can handle a slight freeze and are good choices for early fall and late spring planting. However, sustained freezing temperatures or a really dramatic dip in temperature will do them in. Hardy annual plants will fare better if planted in the ground, rather than in containers, since the ground will insulate roots better than the small amount of soil in a container. And plants that have had time to adjust to increasingly cold weather will be hardier than those that suddenly encounter it. Common hardy annual plants include:
- Sweet alyssum
Half-hardy seeds can be direct sown after all danger of frost. They don’t like being frozen in the ground, but you don’t need to wait until the soil warms. Half-hardy annual seeds have a longer growing period, so in colder climates, they're typically started indoors four to eight weeks before the last frost date to ensure that they'll flower before the season is over. Examples of half-hardy annual seeds include:
Half-hardy annual plants can survive a couple of brushes with chilly night temperatures (35 to 45 degrees Fahrenheit), but they are damaged by frost and 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. Many varieties of half-hardy annual plants will decline in the heat of summer, then perk up and rebloom in the fall. Half-hardy annual flowers include:
- Baby's breath
- Bells of Ireland
To make things even more confusing, there are also half-hardy perennials. These are treated as annuals in colder climates because they won't survive the winter unless brought indoors. Half-hardy perennials include dahlias, gerberas, and geraniums.