Gardening in Alaska: zones, frost dates and when to plant

Three gardening climates

UAF Cooperative Extension’s 16 Easy Steps to Gardening in Alaska (HGA-00134) describes the state as several distinct gardening climates rather than one.

• Southeast Alaska is rainy and cool. Drainage is the first concern, and Extension recommends planting on ridges and mounds, or using ditches and tile drains, to shed water and warm the soil.
• Coastal Southcentral and Southwest, including the Anchorage area, the Kenai Peninsula and Bristol Bay, has summers moderated by wind off the water. Soils stay cold and rainfall is not always enough, so full sun and clear plastic mulch matter.
• The Interior, which the publication says includes the Tanana and Matanuska valleys, is dry in spring and early summer. Soils warm up better than on the coast, but moisture has to be conserved.

At high elevations and in much of the state north of the Yukon River, Extension says growing through plastic film may be the only way to bring a crop to maturity. Check the frost dates for your city above, and remember that site matters as much as region: a sloping, south-facing spot often gains days of growing season over a level frost pocket.

Cold soil, long days and wind

The limiting factor in most Alaska gardens is soil temperature, not air temperature. Extension’s Interior variety list explains that cold soil slows root growth and that warm-season crops such as tomatoes and sweet corn need soil-warming methods. In much of Alaska, clear plastic mulch is needed all season for corn, tomatoes and squash. It lets sunlight heat the soil and keeps wind from carrying that heat away. Free seedlings as soon as they emerge, because most will sunburn under the plastic.

Long days are both a gift and a problem. Plants photosynthesize nearly around the clock in midsummer and grow fast, but day length can push beets, spinach and radishes to bolt. Extension’s variety lists flag bolt-resistant choices for that reason.

Wind is a real cost in open sites. A windbreak 4 to 6 feet tall raises both soil and air temperature. Extension describes a three-sided frame of heavy clear polyethylene that blocks the prevailing wind without shading the garden, with the low side left open so cold night air can drain away.

What grows well and what needs protection

Cool-season crops are Alaska’s strength. UAF’s Recommended Variety List for Southcentral Alaska (HGA-00031) says potatoes, carrots and the cabbage family thrive, while beans, cucumbers and tomatoes struggle in open, unprotected gardens and do far better under high or low tunnels. Short-season varieties generally outperform long-season ones. Many of the listed varieties were trialed at UAF’s Matanuska Experiment Farm in Palmer.

A few picks from the Extension lists, by region:

• Interior: Sub Arctic 25, Stupice and Early Tanana tomatoes, with best results through IRT plastic mulch or other season extension; Earlivee and Early Sunglow sweet corn, rated dependable in cooler conditions; and Royalty Purple Pod snap beans, noted for doing well in cool soils. Eggplant is marginal in the Interior even under plastic.
• Southcentral: Grand Rapids lettuce, described as best adapted to low light; Ithaca head lettuce, less prone to tipburn; Alaska Frostless potatoes, which keep growing after frost kills other varieties; and Alasclear potatoes for scab resistance.

Radishes, turnips, carrots, leaf lettuce and peas can be seeded before frost danger has passed. Cabbage, cauliflower, broccoli and Brussels sprouts do better as transplants. Extension suggests planting cole crops in the lowest part of the garden, where early frost is most likely, and beans and other warm-weather crops on the highest, best-drained ground. Order seed in January or February, before seed houses get busy.

Pests and soil

Alaska gardens have few pests, but two do real damage. Root maggots attack every member of the cabbage family, and onions as well. Slugs are a coastal problem and, according to the Interior list, are moving northward. Rotating the garden between two plots helps control root maggots, and the Interior list recommends growing turnips under low tunnels. The Southcentral list suggests kohlrabi as a substitute where turnip root maggots cannot be controlled.

Soil pH varies widely across Alaska, and some soils are very acid. Low pH ties up phosphorus. Many soils are also low in organic matter, which breaks down slowly in cold ground. Extension’s target for most vegetables is pH 6.0 to 6.5. Potatoes do well at 5.0 to 5.4, where the lower pH also helps hold scab in check.

Never add lime without a soil test. Extension recommends sampling the top 6 inches from eight to ten spots in fall before freeze-up and retesting every other year. Your local Extension office has directions for submitting a sample.