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Aug 162012
 
Hardy Annuals

These plants tolerate frost and cool growing conditions.  Many bloom in early spring or can be planted in late fall to add color to the landscape.  Hardy annuals are irreplaceable for those who live in the cool, high-elevation areas of Idaho, but can be good additions to the spring and fall landscapes in the warmer valleys typical of southern Idaho.  Popular hardy annuals suitable for Idaho include:

Common Name Scientific Name Regional Adaptation
Alyssum Lobularia maritime N, SW, SC, SE, HA
Calendula Calendula officicinalis N, SW, SC, SE, HA
Cornflower Centaurea cyanus N, SW, SC, SE, HA
Dianthus (China Pink) Dianthus chinensis N, SW, SC, SE, HA
Dusty Miller Senecio cineraria N, SW, SC, SE, HA
Kale (ornamental cabbage) Brassica oleracea N, SW, SC, SE, HA
Larkspur Consolida ambigua N, SW, SC, SE, HA
Pansy Viola wittrockiana N, SW, SC, SE, HA
Snapdragon Antirrhinum majus N, SW, SC, SE, HA
Sweet Pea Lathyrus oderatus N, SW, SC, SE, HA
Viola Viola cornuta N, SW, SC, SE, HA

Gaillardia Semi-hardy Annuals

These plants are not injured by light frost events (down to 28° F).  They can also tolerate cool weather, although many are adapted to hot summer conditions and will bloom all summer.  Semi-hardy annuals can help extend the season and provide color into the fall.  Popular semi-hardy annuals suitable for Idaho include:

Common Name Scientific Name Regional Adaptation
Carnation Dianthus caryophyllus N, SW, SC, SE, HA
Cosmos Cosmos dipinnatus N, SW, SC, SE, HA
Gaillardia (Blanket Flower) Gaillardia pulchella N, SW, SC, SE, HA
Gazania Gazania rigeas N, SW, SC, SE, HA
Moss Rose Portulaca grandiflora SW, SC, SE
Petunia Petunia spp. N, SW, SC, SE, HA
Phlox Phlox drummondii N, SW, SC, SE, HA
Rudbeckia Rudbeckia hirta N, SW, SC, SE, HA
Salvia Salvia spp. N, SW, SC, SE, HA
Sunflower Helianthus annuus N, SW, SC, SE, HA

BegoniaTender Annuals

These plants cannot withstand any frost an may not grow well during extended periods of cool weather.  Some of the tender annuals are actually perennials, but due to a lack of hardiness will only survive one summer in Idaho.  They can be planted only after the soil has warmed in the spring and will die back after the first frost of the fall, unless protected.  These plants are more suited to the warmer valleys of Idaho, although many can be planted in cooler areas if desired for a short season of summer color.  Popular tender annuals suitable for Idaho include:

Common Name Scientific Name Regional Adaptation
Ageratum Ageratum houstonianum N, SW, SC, SE
Amaranth (Love Lies Bleeding, Joseph’s Coat) Amaranthus spp. N, SW, SC, SE
Begonia Begonia x tuberhybrida N, SW, SC, SE
Celosia Celosia cristata N, SW, SC, SE
Chrysanthemum (annual) Chrysanthemum carinatum N, SW, SC, SE
Coleus Coleus x hybridus N, SW, SC, SE
Geranium Pelargonium hortorum N, SW, SC, SE, HA
Impatiens Impatiens wallerana N, SW, SC, SE
Marigold Tagetes patula N, SW, SC, SE, HA
Nicotiana (Flowering Tobacco) Nicotiana alata N, SW, SC, SE
Verbana Verbena x hybrida SW, SC
Zinnia Zinnia elegans N, SW, SC, SE

Key to regional adaptation notes:
N = Northern Idaho valley locations in USDA zones 5 & 6, Moscow to Sandpoint.
SE = Southeastern Idaho valley locations in USDA zones 3 & 4 from Rexburg to Pocatello. SC = South-central Idaho Magic Valley locations in USDA zones 4 & 5, Burley and Twin Falls.
SW = Southwestern Idaho Treasure Valley locations in USDA zones 5 & 6, Boise area (also Lewiston).
HA = High altitude (>5,000 ft) areas of central, southeastern Idaho and similar locations elsewhere.

Purdue University publishes a frost hardiness chart for annual flowers.

 August 16, 2012