Growing Seasons in Alaska
It's almost impossible to ripen tomatoes without a greenhouse on the Kenai Peninsula, but the right varieties ripen most summers in Fairbanks, more than 5 degrees farther north. Yet the frost-free season in the coastal cities of the Kenai Peninsula is longer than that of the Interior, and many perennials grow on the Kenai that cannot survive in Fairbanks. Obviously, there is more to a growing season than latitude or the number of days between frosts, but what?
A lot depends on what you want to grow. Some plants, such as tomatoes and corn, require air temperatures well above freezing to grow, and need relatively warm soil as well. Even if the frost-free season is long enough for the life cycle of these plants, they will mature only if there is enough truly warm weather. Corn even requires warm nights to mature properly.
Two possible ways of measuring the suitability of a climate for these warm climate plants are the number of days per year with minimum temperatures above 50 degrees F (10 degrees C) and the number of degree-days above 50 degrees F. (Degree-days above 50 degrees F are calculated by adding up all the positive differences between the daily temperature and 50 degrees F in the course of a summer.) Soil warmth is also influenced by the number of degree days above freezing. Successful Alaska gardeners often manipulate soil warmth either by insulating underneath the beds (so the summer's warmth is concentrated in less soil) or laying clear plastic over the soil to trap the warmth.
For annual plants, the main remaining climate requirements are enough sunshine and sufficient water. If the plants are watered artificially, most will do best with as much clear, sunny weather as possible. The length of the day may have a major influence on whether the plant makes bulbous roots (a problem with beets, for instance), has male or female flowers, or produces seed. This is one of the reasons variety selection is so important in Alaska.
Perennial plants have additional climatic needs. To start with, they need a cooling-down period before the first frost in the fall, so they can go dormant properly. Many are also programmed to start dormancy when the days shorten by a specific amount, and this is often later than is needed in many areas of Alaska. Thus the date of the average first hard frost (say 28 degrees F or -2 degrees C) is important.
Nursery catalogs generally delimit hardiness zones by the lowest temperature expected in an average winter. Of course it makes a difference whether the plant dies down to the roots in winter so that it is affected only by the soil temperature, is a small shrub covered by snow, or has its branches exposed to the winter air.
There are other factors affecting perennial survival. Most perennials prefer to stay cold during the winter, and they can be severely damaged or killed by frequent thawing, heaving soil, or ice layers. An even more surprising fact is that some imported perennials require too much winter chilling for Alaska. Many trees and shrubs require a minimum number of hours when the temperature is below about 40 F (5 C) but above freezing before they will come out of dormancy. We had an elm tree in the yard in Kansas that had too low a chilling requirement for that climate. It always leafed out during the first warm spell in January and then had its first crop of leaves frozen. In the interior of Alaska, both cooling in the fall and warming in the spring proceed so rapidly that the chilling requirement of plants properly adapted to the Kansas climate may not be met until July. These imported plants leaf out so late that they cannot complete the next season's growth.