USDA Zone 4
Average minimum temperature: -20 F to -30 F
Spring: April 15 - June 15
Average last frost date: May 1 - 30
Average first frost date: September 1 - 30
Fall: September 1 - October 15
USDA Zone 4 encompasses a wide area along the northern U.S. and around the Great Lakes. Plants have to endure long periods of deep cold and sometimes surprisingly hot dry summers. In many places, ancient glaciers created deep, rich, dark soil. On average, the growing season is five to six months long. Depending on the area, rainfall can measure as low as 10 inches or as high as 40.
A usually constant snow cover in the colder areas helps protect plants, and there's a wide variety of perennials and other herbaceous plants that do well here--peony, coneflower (Echinacea), liatris, some bell flowers (Campanula), lamb's ear, foamflower (Tiarella), sedum and sunflower (Helianthus), to name a few. Although fewer woody ornamentals grow here (as compared to warmer zones), sugar maple (Acer saccharum), spruce (Picea), fir (Abies), panicle hydrangea (Hydrangea paniculata), lilac, Chinese juniper (Juniperus chinensis), viburnums and Vanhoutte spirea (Spiraea x vanhouttei) are among the hardy. Wheat, oats, cherries, apples, potatoes and beets are some of the common food crops.
The USDA hardiness zone map is based on average minimum winter temperatures, each zone representing a 10-degree difference from the ones above and below it. Each zone is further divided into "a" and "b" areas. The difference between
each of these sub-zones is a matter of five degrees. The average minimum temperature in winter in Zone 4a is -25 to -30 F. In Zone 4b, it's -20 to -25 F.
A myriad of factors—together called the microclimate—affect a plant's ability to withstand winter in a certain location, including exposure (or lack of it) to sun, proximity to a building, protection from winter winds, amount of soil moisture, etc. In that way, many plants can often be "pushed" into overwintering in an area that's technically too cold for them. Our plant descriptions take "a" and "b" subzones into account. If you want to play it absolutely safe, buy plants that are hardy to your zone, not just marginally hardy to the "b" subzone.
Sample Cities in USDA Zone 4a
Minneapolis-St.Paul, Minnesota
Lewistown, Montana