Zone 1a: Gardening in Alaska's Coldest Valleys
Zone 1a is the coldest half zone on the USDA map, found only in a few low river valleys of interior Alaska. Almost no nursery perennials are rated this cold, so permanent plantings are limited to the hardiest zone 2 shrubs and berries in sheltered, snow-covered spots. The real garden here is a summer vegetable garden grown on warmed soil.
- Coldest winter night
- -60 to -55°F
- ZIP codes in the zone
- 5
Frost in zone 1a
Frost decides the growing season here. Only 1 of zone 1a's 5 ZIP codes has a nearby weather station with frost records, too few for a zone-wide frost date; where it is measured, the frost-free season is about 92 days. Valley gardens can freeze in June or July. Enter your ZIP code to see the frost dates of your nearest station.
What a zone 1a winter means
The coldest night in a zone 1a winter is colder than almost anything nursery plants are tested for. Hardiness ratings for trees, shrubs and perennials rarely go below zone 2, so a zone 1a gardener is growing permanent plants beyond their tested range. What survives depends heavily on snow: NDSU Extension notes that snow traps soil heat and limits how deep the frost goes, and that roots can be killed once the soil itself drops below about 10°F. A plant buried under a deep, early snowpack faces a much milder winter than one on a windswept rise.
Where zone 1a is
Only a handful of ZIP codes fall in zone 1a, all in interior Alaska. They lie in low river country such as the Yukon Flats, where cold air pools in valley bottoms during long, still winter nights.
Perennials, fruit and shrubs
Start with plants rated to zone 2 and accept that some will fail:
- Shrubs: NDSU rates common lilac and rugosa rose hardy to zone 2.
- Fruit: Montana State reports the University of Saskatchewan dwarf sour cherries (‘Romeo’, ‘Juliet’, ‘Carmine Jewel’, ‘Crimson Passion’) as cold hardy to zone 2, and UAF lists them among the most reliable cherries in Fairbanks. Short bush forms stay under the snow better than trees.
- Bulbs: UAF’s lily trials found that hardy lilies are hurt more by poor drainage than cold as long as snow cover is adequate, and recommend spring planting in the Interior.
Plant in the most sheltered spot you have, where snow collects, and avoid the lowest ground. Pile extra snow over small shrubs during cold snaps.
Vegetables in a short, cold-soil summer
The growing season is short and the soil stays cold, but long days drive rapid growth. UAF Extension’s advice for the Interior:
- Choose varieties tested in Alaska. Many popular varieties perform poorly under long days and cold soils. UAF publishes a recommended list for the Interior.
- Warm the soil. Clear plastic mulch is needed all season for warm-soil crops in much of Alaska; UAF measured soil under clear plastic 8-10°F warmer than bare soil at Palmer, and row covers add another 3-6°F.
- Plant hardy crops early. Radishes, turnips, carrots, leaf lettuce and peas can be sown before frost danger is past. Cabbage, broccoli and cauliflower do better as transplants.
- Watch for bolting. Long days make beets, spinach and radishes bolt, so pick bolt-resistant varieties.
Where zone 1a is
States with zone 1a ZIP codes, the largest first: Alaska.
Zone 1bAll zones and the zone finder
Questions
What perennials can survive in zone 1a?
Very few plants are rated below zone 2. Your best chances are the hardiest zone 2 shrubs and fruit, such as common lilac, rugosa rose and the University of Saskatchewan bush cherries, planted where snow drifts and stays all winter. Expect losses and treat each one as a trial.
Can you grow vegetables in zone 1a?
Yes. Long summer days drive fast growth once the soil warms. UAF Extension recommends Alaska-tested varieties, clear plastic mulch for warm-soil crops, raised mounds or north-south rows to warm the soil, and frost-tolerant crops such as radishes, turnips, carrots, leaf lettuce and peas.
Why do my plants freeze in midsummer?
Cold air drains into valley bottoms on clear nights. UAF Extension describes a valley garden that dropped below freezing on three days in June and two in July while a nearby upland station did not. In a frost pocket, grow frost-tolerant vegetables and keep row covers handy.