The USDA has a tool that outlines annual extremes for minimum temperature across 13 areas of the U.S.
An example of a zone represented in Southcentral Alaska is zone 4.
The USDA map has minimum temperatures of -30F to -20F for zone 4.
We can use this zone tool to help guide us when adding perennials (plants that live more than two years) and biennials (plants that complete their lifecycle in two years) to our garden spaces.
If I am shopping and find a “zone 7” plant , I may be able to grow this plant as an annual but the rate of overwinter survival will be less as the minimum temperature for a “zone 7” plant is 0F to 10F and the minimum temperatures in zone 4 may be way too cold for this plants survival without certain extreme protection measures.
When we are discussing annual plants (those plants that complete their life cycle in one year) we need know our frost free dates. This is the span of time where we can reasonably expect not to have killing frosts (28F and below).
For my landscape and microclimate growing spaces this is from May 1 (ish) – Sept 30 (ish). As nature doesn’t always fit in the “box” we can expect seasons to extend and sometimes shorten.
Nature is dynamic…not static
Certain plants can withstand much colder temperatures and overwintering with meticulous care of the soil, the enhancement of natural microclimates and a keen understanding of individual plant species coupled with extensive season extension. Taking growing risks is a fun garden challenge I look forward to every year.
We can use these tools as a guide to direct us towards plant choices and landscape design. Check your zone for perennials and biennials and know your frost free dates for annuals.
Here is a link for the USDA hardiness zones: https://plants.usda.gov/hardiness.html
Here is a link for frost free dates: https://www.almanac.com/gardening/frostdates