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Hardy, Half-Hardy, and Tender Flowering Plants

Calendula officinalis or Pot Marigold is a hardy annual for cool to cold weather growing

Hardy is a term used to describe a plant’s ability to survive in a specific climate zone—even through the cold of winter. Commonly gardeners use the term “hardy” to describe a plant that can survive a freeze, frost, or a cold snap.

Half-hardy is a term used describes plants that can survive only limited or light frost—meaning just an hour or two of frost or near-freezing temperatures. Half-hardy plants must be protected from anything more than a touch of frost.

Tender is a word used to describe plants that are injured by frost or cold weather. Tender plants must be protected from temperatures near freezing.

It is important to note that these terms can be applied relative to the climate where they grow; a plant may be hardy in one zone but not hardy in another. A gardener may say a plant is hardy in her garden—it will survive the winter, but a second gardener in a colder region will that same plant is not hardy.

It is important to note that there are no similar words or phrases to signify a plant’s ability to live through heat, wet, or drought—so all of these conditions are sometimes, but not always, included under the word “hardy’. Thus, a plant hardy to Quebec, which will usually die of heat in Florida or of drought in Arizona may be called “not hardy” in those warmer climates.

In practice, however, gardeners speaking of hardy plants commonly mean those which survive the winter in northern regions. Plants not hardy are called half-hardy or tender.

Hardy Annuals

Hardy annuals are those of which seed may be sown early in the open garden, the seedlings being able to withstand spring frosts, such as poppies, as opposed to half-hardy and tender annuals such as zinnias, which must be started indoors, or if outdoors, sown after all danger of frost is past.

A hardy annual flower can withstand a light spring frost and can, therefore, be started from seed sown in the open as soon as the soil can be worked.

Rose campion, Lychnis coronaria,
Rose campion, Lychnis coronaria, is a hardy annual

Hardy Annuals

Botanical NameCommon Name
Brassica oleraceaFlowering cabbage and kale
Calendula spp.Pot marigold
Centaurea americanaBasket flower
Clarkia spp.Satin flower
Cleome spp.Spider flower
Coreopsis tinctoriaCalliopsis
Cynoglossum amabileChinese forget-me-not
Dianthus barbatusSweet William
Digitalis purpurea ‘Foxy’Foxglove
Eschscholzia californicaCalifornia poppy
Gallardia pulchellaBlanketflower
Helianthus spp.Sunflower
Iberis amaraRocket candytuft
Iberis umbellataGlobe candytuft
Lianthus spp.Mountain phlox
Linaria maroccanaToadflax
Linum grandiflorumFlowering flax
Lobularia maritimaSweet alyssum
Lupinus texensisTexas bluebonnet
Lychnis coeli-rosaRose of heaven
Lychnis coronariaRose campion
Matthiola incanaStock
Myosotis sylvaticaForget-me-not
Nemophila menziesiiBaby blue-eyes
Papver rhoeasField poppy
Rudbeckia hirtaGloriosa daisy
Viola tricolorJohnny-jump-up
Viola x wittrockianaPansy
Felicia amelloides or Blue marguerite
Blue marguerite, Felicia amelloides, is a half-hardy annual

Half-Hardy Annuals

Botanical NameCommon Name
Alcea spp.Hollyhock
Anchusa carpensisSummer forget-me-not
Antirrhinum spp.Snapdragon
Arctotis spp.African daisy
Brachycome iberidifoliaSwan River daisy
Callistephus chinensisChina aster
Celosia cristataCockscomb
Chrysanthemum coronariumGarland chrysanthemum
Convolvulus spp.Bush morning glory
Cuphea igneaCigar flower
Dianthus chinensisChina pink
Diascia barberaeTwinspur
Eustoma spp.Lisianthus
Felicia amelloidesBlue marguerite
Gazania rigensTreasure flower
Gerbera jamesoniiTransvaal daisy
Gomphrena globosaGlobe amaranth
Gypsophila elegansAnnual baby’s breath
Helichrysum bracteatumStrawflower
Limonium sinuatumStatice
Lobelia erinusEdging lobelia
Mirabilis jalapaFour-o’clock
Moluccella laevisBells of Ireland
NemesiaNemesia spp.
Nicotiana alataFlowering tobacco
Nierembergia spp.Cupflower
Oenothera speciosaEvening primrose
Papaver nudicauleIceland poppy
Penstemon gloxinioidesPenstemon
Petunia spp.Petunia
Phlox drummondiiPhlox
Portulaca spp.Moss rose
Salvia farinaceaMealy-cup sage
Salvia splendensScarlet sage
Scabiosa atropurpureaPincushion flower
Senecio cinerariaDusty miller
Verbena spp.Verbena
Three cultivars of Coleus grow outdoors in summer; Coleus is very tender

Tender Annuals

Botanical NameCommon Name
Ageratum spp.Flossflower
Amaranthus caudatusLove-lies-bleeding
Amaranthus tricolorJoseph’s coat
Asclepias curassavicaBloodflower
Begonia semperflorensWax begonia
Browallia spp.Browallia
Coleus spp.Coleus
Heliotropium spp.Heliotrope
Hypoestes phyllostachyaPolka-dot plant
Impatiens spp.Impatiens
Mimulus spp.Monkey flower
Pelargonium spp.Geranium
Perilla spp.Perilla
Salpiglossis sinuataPainted tongue
Tagetes erectaAfrican marigold
Tagetes patulaFrench marigold
Torenia spp.Wishbone flower
Tropaeolum spp.Nasturtium
Vinca major ‘Variegata’Periwinkle
Zinnia spp.Zinnia

Hardy Perennials

Hardy perennials such as columbine die to the ground in winter, but the roots remain alive to put up new growth year after year. A plant is called root-hardy when the upper part of the plant winter-kills but not the roots.

Hardy Trees and Shrubs

Trees, shrubs, and other woody plants are hardy when the entire growth remains alive through the winter. Woody plants are affected in different ways that annuals and perennials. A tree or shrub is said to be bud-hardy when the flower buds formed in summer or autumn to bloom the following spring pass through winter uninjured.

Hardening-Off Young Plants

Hardening-off describes the process of preparing or “toughening” young plants (started from seed indoors) for outside weather conditions, especially when outdoor temperatures are lower. Young plants can be acclimatized to lower temperatures and outdoor exposure in several ways:

  • Young plants can be moved from a greenhouse or indoors to a cold frame and exposed to lower temperatures gradually.
  • Plants in a cold frame can be set outside during the day and covered with a floating row cover then returned to the cold frame at night.
  • Plants in the garden can be protected by cloches or movable fabric or plastic tunnels at night and exposed to the elements during the day.
  • During hardening off, protect plants not only from cold but also from full sun and wind—all of which can cause young plants distress. Use shade cloth a tunnel covered with garden fabric or a lean-to made of lath to buffer wind and sun.
  • Hardening off is commonly done over a period of three to five days before transplanting. Harden off young plants a few hours the first day, add more hours each day until the plant is exposed to the outdoors for six to eight hours.
A plastic greenhouse or tunnel can be used to start seeds in late winter and early spring

Cloches and Tunnels to Protect Young Plants

Protective coverings can raise the temperature around young plants and half-hardy and tender plants by 5 to 20 degrees Fahrenheit or more. Temporary and easily movable coverings are useful where the weather warms up, then is hit by a quick frost, and then warms up again. Place protective coverings over young plants that are hardening off and over the tender and half-hardy plants when nighttime temperatures may dip.

Here are two simple and easy to move and store plant coverings:

  • Cloche. Cloche is any covering that allows sunlight to reach a plant but keeps cool temperatures and winds away. A simple cloche is made by removing the bottom from a plastic jug and setting it over a plant. A cloche traps heat and warms the soil and air under it.
  • Plastic or fabric tunnel. Tunnels made from sheet plastic or garden fabric stretched across arched construction or chicken wire, wire wickets, or over hoops of PVC tubing can cover rows of young seedlings or new transplants. Portable tunnels can be 1 to 3 feet wide and 10 to even 15 feet long. Plastic tunnels can be as much as 10 to 20 degrees warmer than the outside temperature and give half-hardy and tender plants a one to two-week head start on the season.

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