How Hardy Are Your Annuals?

Plants that we call annuals are the eye candy of a summer garden in Pennsylvania. They include flowers that we all know and love, such as petunias, sunflowers, and zinnias. Some annuals can withstand spring cold snaps and fall frosts, but not the heat of summer. Others will shine only when the summer heats up. Let's look at these in more depth so that you will be able to make more informed seed and plant purchases.

What is an Annual?

An annual is defined as a plant that grows, flowers, sets seed, and dies within one growing season. But we also use many plants as annuals that would be considered perennials in a warmer climate—they simply do not live through the winter in our area.

What is a Hardy Plant?

Sometimes "hardy" is a word used to describe a plant that grows well and survives in adverse conditions. In the horticultural world, hardiness is technically a plant’s ability to survive extreme winter and summer climatic conditions.

Hardy vs. Tender Annuals—What's the Difference?

Technically, annuals are divided into three categories: Hardy, half-hardy, and tender. For the purpose of understanding when they can be planted, I am looking at them in two categories: Hardy vs. tender (and half-hardy). We define annuals as hardy if they can withstand light frost in winter and spring. On the other hand, a tender annual will be seriously damaged or killed by frost. Knowing the difference is important because even if the weather seems to have warmed up in spring, a frost will likely kill tender and half-hardy annuals that have been set out too early. Hardy annuals will withstand frost but may not be attractive when temperatures heat up. Here is a sampling of popular bedding plants and how they are categorized:

Hardy Annuals

Although some of these plants are technically perennials, they are traditionally grown as annuals here in Pennsylvania.

Figure 2.  (Left): Snapdragon by Susan Marquesen, Penn State Master Gardener; (Right) Pansies by Mandy L. Smith, Penn State

• Bachelor button (Centaurea cyanus)
• Bells of Ireland (Moluccella laevis)
• Dusty miller (Jacobaea maritima)
• Love-in-a-mist (Nigella damascena)
• Pincushion flower (Scabiosa spp.)
• Pinks (Dianthus spp.)
• Pansy (Viola × wittrockiana)
• Snapdragon (Antirrhinum majus)
• Sweet alyssum (Lobularia maritima)
• Sweet peas (Lathyrus odoratus)

Tender (and Half-Hardy) Annuals

Here again, some of the plants on the tender annual list are perennials, but they will only grow year-round in a much warmer climate, so we treat them as annuals. Some, like sunflowers and marigolds, are true annuals.

Figure 3. (Left): Petunia and Calibrachoa by Susan Marquesen,  Penn State Master Gardener; (Right) Zinnias by Mandy L. Smith, Penn State

• Ageratum or floss flower (Ageratum houstonianum)
• Begonia (Begonia spp.)
• Calibrachoa or million bells (Calibrachoa spp.)
• Coleus (Coleus scutellarioides)
• Geranium (Pelargonium × hybridum)
• Impatiens (Impatiens walleriana)
• Impatiens, New Guinea (Impatiens hawkeri)
• Lantana (Lantana camara)
• Marigold (Tagetes erecta, Tagetes patula)
• Petunia (Petunia x hybrida)
• Sunflower (Helianthus annuus)
• Zinnia (Zinnia elegans)

When Should You Plant Hardy versus Tender Annuals?

Hardy annual seeds are best planted outdoors in fall, about six weeks before the first frost. If they are purchased as bedding plants, they can be planted in containers or beds outdoors in early spring.

If tender (and half-hardy) annuals are started as seeds indoors in the spring, start this process about four to six weeks before the last frost date. When planting tender annual seeds or plants outdoors, wait until the danger of frost has passed. These are general guidelines. Seed packages will state more exact planting instructions.

The last frost date should be used for planning when to plant tender annuals. In Pennsylvania, the last frost date varies from the end of April through late May, depending on the location of your garden. This date is based on historical patterns. Pay attention to your near-term local weather forecast before planting outdoors. Forecasted frosts, high winds, or heavy rains should delay planting.

Whether hardy or tender, annuals purchased as bedding plants should be hardened off before going from gentle indoor to harsh outdoor conditions. For more information on this process, see the Penn State Extension article on Hardening Transplants.

Do Changes in the USDA Hardiness Zone Map Affect Annuals?

The USDA Hardiness Zone Map was revised in 2023, indicating that about half of our country is half a zone warmer than in 2012, including most of Pennsylvania. Since many annuals are actually perennials from a warmer climate zone, a few of these annuals may live longer in the fall and perhaps even through the winter. In particular, look for dusty miller, pansies, and snapdragons to survive the cold. Of course, this also depends on the hardiness zone of your garden, the severity of a particular winter, and how protected the plants are from factors such as wind and extreme temperature fluctuations.

If in doubt about the first and last frost dates in your area or the growing requirements of a specific annual, consult the Penn State Master Gardeners' Garden Hotline in your county. They will have the facts necessary to answer your questions as well as the benefit of their own gardening experience.