We use the USDA hardiness zones which is the standard rating
guide. We are conservative in our
ratings and often with care and attention to siting, many of our plants can be
grown in a colder zone. Our nursery is
firmly in Zone 7. A half hour walk west
to our friend’s Bill and Sue’s garden finds us in an upper Zone 6 as they are
cursed with the mother of all frost pockets – more like a frost crater. A half hour walk east to downtown Port
Townsend and we’re firmly in Zone 8 with some microclimates a balmy Zone
9. It’s maddening.
A useful map can be found at this link:
www.usna.usda.gov/Hardzone/ushzmap.html If you are web searching hardiness zones,
don’t bother with the zipcode method for finding what your zone is – we tried
and it came up with Zone 5 for us instead of the correct Zone 7.
We cannot stress enough how
many factors beyond anyone’s control determine the hardiness of a plant. Fertility, age of plant, established or not,
hot or cool summer, soil type, timing of cold, wind, near the drier vent, how
expensive and difficulty of replacement all seem to play a role. Also within any given zone and often within
any given garden, a range of zones and microclimates may be found.
The early November arctic blast of 2010 saw unprecedented
damage in our area to supposedly perfectly hardy plants but that is what going from
the 60’s to 15F with strong winds and nothing hardened off will do. Zone 5 Sorbus (Mt Ash) were badly damaged in
the garden for example. The previous
winter was colder but the Sorbus were unscathed as they were hardened off. It had been very wet prior to the arctic
blast of 2009 and plants in the garden were fine but the exposed container
plants were decimated despite having been through colder temperatures in the
past. Losing Zone 3 perennials in
containers that we have grown for 20 years without protection was just not fair
but it does highlight the pitfalls of zonal dependence.
We rate our Crocosmia at Zone 7 but with mulch or dependable
snowcover, these will handle short dips into Zone 6. A few fir boughs placed over a plant or a
temporary wrapping can make all the difference. Remember, plants have feelings too.