Flower planting zones refer to hardiness zones set out by the United States Department of Agriculture. These zones give gardeners an idea of how hardy a plant needs to be to survive in a particular zone given the climate. Gardeners knowing these climate zones can appropriately choose flowers that can survive their area's low temperatures. Though helpful, the zones aren't perfect guides, since factors such as elevation or humidity also affect climate and, thus, a plant's survival chances.
Zones 1 and 2
Zones with the lowest numbers have the lowest average temperatures. Plants must withstand average minimum temperatures that fall below minus 50 F in Zone 1. Fairbanks, Alaska, and Canada's Northwest Territories fall in Zone 1 on the USDA's Plant Hardiness map. The Lapland rhododendron can survive Zone 1.
The USDA divides Zone 2 into 2a and 2b. Zone 2a's annual minimum temperatures fall between minus 50 and minus 45 F, while Zone 2b's average lows fall somewhere between minus 45 and minus 40 F. Zone 2 still falls in Alaska and dips south into the mainland of the United States, covering part of Minnesota. The bunchberry dogwood grows here, producing white flowers on a shrub a half foot tall at maturity.
Zones 3 to 4
Lows in Zone 3a fall in the minus 40 to minus 35 F range; Zone 3b's temperature lows are from minus 35 to minus 30. Alaska, Minnesota, Wisonsin and Montana all have locales in Zone 3.
Montana also has places in Zone 4, as does Nebraska. Minnesota's Minneapolis/St. Paul also falls in the zone, Zone 4a specifically, which has temperatures that fall to minus 30 to minus 25 F. Zone 4b has minus 25 to minus 20 F minimums.
To Zone 3 belongs the Tatarian honeysuckle and the panicle hydrangea. Zone 4 is the home of Japanese barberry, which produces yellow flowers on a tree that reaches about 6 feet high.
Zones 5 and 6
The temperate states of Illinois, Pennsylvania, Iowa, Missouri and Tennessee fall into Zones 5 and 6. Zone 5a's temperature minimums fall between minus 20 and minus 15 F, Zone 5b between minus 15 and minus 10 F, Zone 6a from minus 10 and minus 5 F and Zone 6b from minus 5 to zero F.
Rosa multiflora can survive winters in Zone 5. So can the flowering dogwood Cornus florida. Zone 6 is survivable by the cherry laurel. Indeed, in these temperate zones, gardeners have many flowering plants to choose from.
Zones 7 and 8
Zones 7 and 8 begin to cover the South in earnest, ranging across Virginia, Oklahoma and Florida. Minimum average temperatures in Zone 7a are zero to 5 F; in 7b, the lows plants must tolerate average from 5 to 10 F, while in Zones 8a and 8b, the lows for gardeners to worry about are from 10 to 15 F and from 15 to 20 F respectively. Azaleas grow commonly in these zones.
Zones 9 and 10
The sunniest mainland locations in the United States, Southern California and Florida, fall in Zones 9 and 10. Lows in Zone 9 are still below freezing, from 20 to 25 F in 9a and 25 to 30 F in 9b. In 10a, the lows hover around the freezing point at 30 to 35 F. Zone 10b possesses temperature lows that average annually from about 35 to 40 F. Flowers in these zones include fuchsia and bougainvillea.
In Hawaii, as well as places in Mexico, the average annual low doesn't drop below 40 F. This warm climate is covered by Zone 11. The Lantana ovatifolia flowers here, attracting butterflies with its gold flowers. So does the African iris, which blooms year-round in white.