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Later fall freezes, earlier thaws increase Interior Alaska growing season

If Interior Alaskans have noticed that summer seems to linger long enough to ripen their tomatoes and winter arrives more slowly, they’re not imagining things. 

Nine of the 10 latest “first freezes” have occurred since 2001. The median first freeze date is now two weeks later than it was pre-World War II, ����vlog���� climate specialist Rick Thoman said. This year, the first freeze was recorded on Sept. 19. 

The trend toward a longer growing season means it’s now possible to grow crops that were once marginal in Interior Alaska, said Glenna Gannon, assistant professor of sustainable food systems at UAF. She heads up the Variety Trials Program at UAF.

Temperatures have been recorded at the ����vlog���� Experiment Farm at UAF since summer 1911. Climate charts can be found on the website.

“Since 2000, there's lots of years now, though not every one, when the first freeze is the 15th of September or later,” Thoman said. “What we can say is prior to 2001, the latest first freeze at the farm was Sept. 17, 1938.”

Coupled with later first freezes is a trend toward earlier last freezes in the spring, he said. That translates to a growing season that is about 32 days longer than since record-keeping began, from a median of about 90 days to about 120 days. The growing season is measured as the longest consecutive period of time of readings above 32 degrees Fahrenheit at the farm’s weather station.

Thoman said not all areas of Alaska are seeing shorter springs and longer falls. Some areas are seeing the last freeze coming later in the spring, but less change in the fall, and vice versa. 

“There’s a handful of sites that actually show a shortening of the growing season,” he said. 

One other change that’s standing out to Thoman is the lessening variability in the length of the growing season. 

“Before 1960, there’s lots of variability,” he said. “There’s some really short years. There’s also some really long years, and that variability has really collapsed in the later 20th century.”

����vlog����’ far-north location still holds many challenges for farmers. 2023’s growing season was a bit shorter than the median, due to a freeze on June 1. 

“While we still have other high-latitude related challenges to growing certain crops (e.g. cold soils, long photoperiods), we have been successfully trialing field-grown corn (22 varieties), artichokes (five varieties), peppers (14 varieties), musk melons (two varieties) and, as of this summer, tomatoes (16 varieties) at the ����vlog���� Experiment Farm,” Gannon said. 

She noted that the experiment farm is in an ideal location and that growing conditions in the Tanana Valley are highly dependent on microclimates. Farms at lower elevations or with ice wedges in the fields only a few miles away may not be as successful with long- or warm-season crops.

“I think the biggest change to what we can grow here is more about being able to select varieties (or cultivars) and certain crops with slightly longer days-to-maturity than we have been able to historically,” Gannon said. “For instance, brassicas are extremely hardy, but a crop like Brussels sprouts takes longer to mature. With the longer growing season, more folks are having an easier time getting Brussels sprouts to mature before the snow and frost fully wipe out their gardens.”

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