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To garden successfully is to know your local conditions, following the precept “right plant, right place.”
You need to know how cold or hot, wet or dry each spot in your garden is over the course of the year, which the plants will tell you by their reactions to their placement. I know that one corner of my yard retains moisture because pulmonaria love it and hostas grow leaves as big as my head. However, like the garden itself, the conditions around it are shifting today, more quickly than ever in the modern age.
YOU ARE HERE
A key measure many gardeners start with is the U.S. Department of Agriculture (USDA) Hardiness Zone Map, which divides the country by their coldest temperatures in five- to ten-degree increments based on 30-year averages. (Ten degrees between full zones, five between half-zones designated “a” and “b”. Lower numbers indicate colder temperatures, with Maine being 1 and Puerto Rico 13. This map is the basis for the zone range shown on nursery plant tags. Updated every ten years, the new map has been released for 2023, shifting Seattle to Zone 9a, the same zone as the northern end of Florida. According to a USDA press release, about half of the country shifted to the next warmer half zone since the 2012 map. You can see the current map at planthardiness.ars.usda.gov.
The same release says that it may be a leap to assume these changes are due to climate change, because climate change data is based on overall average temperature which is less variable than the extreme minimum temperature, and that new data is coming in based on mapping technology and added weather stations. Nonetheless, it’s important data to have as a gardener.
THE WAY WE WERE
For some context, you can find older zone maps at Plantmaps.com for the country and the state. In the 1990 map (), the only parts of Washington that were 9a were the northwestern tips of the Olympic and Kitsap peninsulas from Port Townsend to Port Ludlow, and Seattle was 8a. By 2012, Seattle was 8b, a five-degree change, and 9a was migrating east along the edge of the Olympic peninsula around the corner to Cape Flattery.
You can check out older iterations of the maps here:
1990: plantmaps.com/interactive-washington-usda-plant-zone-hardiness-map.php
2012: plantmaps.com/interactive-washington-2012-usda-plant-zone-hardiness-map.php
What does this mean to Seattle gardeners? You may be able to grow some plants more successfully, and others less. Plants formerly treated as windowbox annuals, like geraniums (Pelargonium species) and calibrachoa, may overwinter where they never did before. On the flip side, plants that need some winter cooling period or dormancy may languish or fail to bloom, like herbaceous peonies and tulips. We may not be able to count on winter to cull pest populations, as the proliferation of the stink bug across the country seems to show. More reason to choose plants that support beneficial insects and leave the leaves as much as you can. Warmer conditions can help fungal diseases spread, too, so looking for varieties that resist such diseases may be wise.
ZONE-SURFING: GARDENING AT THE EDGES
There are a lot of things zones don’t tell us. They don’t cover growing season length, amount of reliable snow cover, or timing and severity of rain, heat or frost events, all of which profoundly effect which plants you can grow and when.
Then there are microclimates. Zone deniers are common in Seattle, especially for those of us enchanted by uncommon plants from New Zealand, South Africa, and Chile. We like to play at the edges of those zones by using microclimates. Microclimates are all over your yard, like my moist, shady corner with the pulmonaria.
Where the wind flows, the sun hits, and the water stays all impact your microclimates. Walls or hard surfaces retain and reflect sunshine, making a hot spot. Low spots of slopes tend to be colder, and slower to wake up in spring. A retaining wall on a hill can hold moisture. If it’s sunnier and drier, you may find your rosemary thrives there and blooms earlier than a shadier, possibly wetter spot. As always, it’s key to match the conditions the plant needs with your hyper-local situation.
Is a spot too cold? Mulching over the winter will protect the roots, and for further insurance, wrap the plant with breathable row-cover fabric or burlap.
Are you seeing browned leaves? Maybe a spot has become too warm for a cherished plant. Note the direction where the sun touches that plant and add some shade by planting a taller tree or shrub to block it or install a sculpture or even a parasol for faster shelter.
By planting the same plant in various spaces, I get indicators about the soil temperature. Daffodils bloom earliest on the South-facing side of a wall behind my driveway surrounded by reflected heat and raised up to catch the sun. If I plant that same cultivar in a semi-shaded spot, I can extend the bloom season in the yard.
Ultimately, the zone numbers are only a starting point – you still have to garden where you are, for each plant. If you can observe your conditions carefully, you can surf the microclimates in your yard and make them work for you and your plants – to create a garden that thrives in whatever zone you are in.