Zone AI: Alaska's Colest Climate—Fairbanks and the Interior
Encompassing most of interior Alaska in a region between the Alaska and Brooks ranges, Zone A1 is Alaska’s gardening surprise. During summer, plants benefit from long, warm days, while in winter, gardeners can usually depend on snow to insulate plants. And in permafrost areas, the ground usually thaws to below root level during the warm months.
The keys to beating temperature extremes—with everything from strawberries to snapdragons, beans to sweet corn— include taking advantage of microclimates (especially south and west exposures), boosting soil temperatures with mulches or IRT plastic sheeting, and choosing the right plant varieties. Many birches, for example, reach nearly unparalleled proportions in Fairbanks, as do hardy perennials and a long list of annuals. It also helps to plant in raised beds, whose soil warms up earlier in spring. Average winter minimums are –10 to –20°F (–23 to –29°C),with occasional dips to –60°F (–51°C). Summer highs are in the 70s,with rare spikes to 90°F (32°C). The average growing season in Fairbanks is 113 days.
ZONE A2: The intermediate climate of Anchorage and Cook Inlet
The Alaska Range protects this area from continental extremes to the north, while the Kenai and Chugach Mountains strip most of the wind and moisture from storms blowing in from the Gulf of Alaska. The salt water of Cook Inlet moderates temperatures as well. However, seasons are well defined and permafrost hides here on north-facing slopes and in sheltered hollows.
Plants like showy mountain ash, late lilac, Siberian larch,Amur chokecherry, Swiss stone pine, and Colorado blue spruce do well in Zone A2. But success depends much on each area’s microclimate. Winter lows average 6°F (–14°C) at Anchorage Airport, 0°F (–18°C) in Palmer and Wasilla, with drops to –20 or–30°F (–29 or –34°C) once in a while. Summer days are usually cloudy and in the mid- 60s,with occasional jumps into the high 70s. The growing season ranges from 105 days on the Kenai Peninsula to 138 days in Anchorage. When sowing seeds, take note that plants grow more slowly here than seed packets indicate.You can speed annuals along by starting seed indoors a few weeks before transplant, or outdoors in a cold frame.
Anchorage microclimates: Though Anchorage is zoned A2, the city is composed of many microclimates. Each of these affects the types of plants you can grow in a particular location. Banana belts (Bootleggers Cove and southeast into downtoan for a couple of miles, for example) can be 25º F warmer in winter than the coldest neighborhoods around Bicentennial Park. But in summer, Bicentennial Park can be 10º F warmer on clear days. On cloudy days and at night, summer temperatures are similar throughout Anchorage. If you garden in one of the city's cold pockets, grow plans zoned for A1. If you're in a warm spot, you may succeed with plants that few people would expect to see north of Homer (A3). In moderate locations, use plants zoned for A2.
ZONE A3: The mild maritime climate from Kodiak to Juneau and Prince Rupert
This zone includes southeastern Alaska north of Sitka clear to Skagway, plus Kodiak Island, Homer, Seward, and Prince William Sound. It also touches Prince Rupert, where cold interior air drains down the Skeena River.
Summers are cool and cloudy, while winters are typically windy and rainy. Annual precipitation runs from 80 inches at Kodiak to 200 inches near Sitka. The ground freezes every winter, and repeated freeze-thaw cycles in spring play havoc with cold-hardy plants like hybrid tea roses. Hardy rhododendrons, azaleas, Japanese maples, and dwarf conifers do well here. The easiest bedding plants to grow include pansies, violas, and snapdragons. Planting in raised beds of light, sandy soil assures good drainage in wet areas and quick warm-up of soil in spring. Winter minimums average 20 to 30°F (–7 to –1°C),with occasional drops to –5°F (–21°C). Summer highs are in the low 60s,with occasional jumps to 80°F (27°C). The growing season runs from 113 days in Cordova to 162 days in Haines. But cool summer temperatures offset the advantages of summer day length. Plants take longer to grow than seed packets describe.
ZONE 4: Mild-winter areas of Alaska and British Columbia
One of the West’s most narrow, linear climates, Zone 4 runs from high in the coastal mountains of Northern California to southeastern Alaska, losing elevation as it moves north. It gets considerable influence from the Pacific Ocean, but also from the continental air mass, higher elevation, or both .As it extends north, the zone first touches salt water in northern Puget Sound and is almost entirely surrounded by salt water in southeastern Alaska.
In the contiguous states, Zone 4 has more cold than neighboring Zone 5, more snow, and a shorter growing season. Compared to neighboring zones in Alaska and Canada, however, it has less winter cold and a longer growing season. No zone grows better perennials and bulbs; people who like woodland plants and rock plants love Zone 4. But beware: though you can grow winter vegetables in the southern part of Zone 4, it doesn’t get enough winter sunlight in Alaska to sustain them.
Average winter lows in Zone 4 range from 34°F (1°C) down to 28°F (–2°C),with extreme lows averaging 8 to 0°F (–13 to –18°C). The growing season is 150 to 200 days long, but because Zone 4 summers are temperate (highs average from the low 60s to the 70s), plants take more time to develop. If you’re growing vegetables, for example, add at least 50 percent to the days-to-harvest figure listed on the seed package, or start your garden from transplants.