Yeah, I didn’t even think about that. Weather stations are usually in places that aren’t in the middle of cities, right? And urban areas right in the middle of cities tend to be five, sometimes even ten, degrees warmer because of all those black roads.
If I’m remembering correctly, usually you’ll only see 10 in very urban areas – lots of big roads and lots of tall apartment buildings with no trees for shade, for instance. It’s probably more like a 2-5 degree difference in my neighborhood, which is in the middle of a medium-sized city, has lots of trees, and has normal-sized roads and a mixture of both single family homes and apartment buildings.
“Weren’t” being the operative word. Because in some cases the decision was to leave them where they were because moving them to a different microclimate would screw up the data.
Zones are averages and thus include those weird once in a 20 year events to the hardiness. If something is classed for zone 7 it will likely survive zone 5-6 averages, but not zone 5-6 extreme lows. Another thing is lenght of the growing season. The more time plants have to proberly get ready for winter to more they will tolerate. I remember reading about zone 6 (or was it 7) kaki in edible landscaping that had survived weird siberian front in South Korea or Japan that went down to -40C/-40F. Often they are killed with far less either because season isn’t long or warm enough for them to be ready for winter or during the winter temperature swings are so great that they break dormancy and are killed by late freeze. That late freeze might be far less than winter max. Same applies to any trees, vines and bushes.
The 1990 edition had colder data. Some horticulturists prefer using the older 1990 map as the new map takes the 15 year warm cycle data nightly lows and 15 year cold cycle data nightly lows and divides them by 30 to determine the zones. If they use the warmer modern map they still can encounter winter lows much lower than the zone the new map has assigned for a given area. This translates to plant losses during those colder winters. So to play it safe they use the older colder map for plant selection.
With a cold weather forecast here projecting lows much lower than my new altered USDA hardiness zone, I did some thinking. First, what is this map telling me? And second, what is it good for?
For the first question it seems the answer is the average yearly minimum temperature over the last thirty years.
The average doesn’t tell you much on its own. Other than maybe there’s a reasonable chance the temperature doesn’t go below average in the next winter. Average becomes exponentially less useful for predicting multiples winters in a row.
I would reason to only use the USDA zone for a chance at overwintering something for one year, such as a biennial crop. If you try to use it accurately for perennials you take on excessive risk, because at some point you will blow out the average.
This will be our second year in a row with the winters dipping 1-2 zones below what the USDA is telling me my zone is now. To me that renders the map useless.
A more useful metric for let’s say trees, is the minimum temperature in the last 100 years, although that’s no prediction of the next 100 years it’s better than USDA hardiness.
While it may be true that using the map for perennials means excessive risk, perhaps it also means that in cases of “pushing the zone” for perennials, where you know the average is a small risk, but the unexpected extreme swings are risky, you need to pay more attention to planting site to take advantage of your microclimates. For example… I keep a few “riskier” perennial herbs and flowers in a south-west bed that happens to have fencing and building foundation for temperature protection. I have some lovely gladiolus interplanted there for aesthetics, and I’ve never lifted them for winter despite being in zone 6b (now 7a). The only major loss I’ve had in that bed since 2008 is a rosemary last year that wasn’t mulched before the Christmas polar vortex. My glads survived, and my supposedly-delicate lavenders, but the rosemary was too exposed and couldn’t manage.
Unexpected extremes are only unexpected because they “average out” as the saying goes. The warm years bring the average up but the warm years have nothing to do with the lows in the cold years. By all means, if you can grow something successfully then do it.
Yup, useless for me too. So many non native fruit trees and shrubs need extensive amendments to get established. Most vegetables can be tricked into growing with compost…but just desert sand…that isnt enriched by flooding and silt deposits, is what we start with…not everyone has rich soils by the colorado river.
Do you have access to ChipDrop? If so, covering everything over in wood chips may be a great way to make your land fertile. It takes a lot of hard physical labor, but it doesn’t cost any money. That’s what I’ve chosen to do.
Yes. We get free woodchips from a local arborist…it just takes alot of water to compost them for about 8 months…if not the fresh chips rob the soil of nitrogen. I’ve put four of the 3 cubic yard truck loads into my growing area…it lasted about 2 years, have to do it again this year before summer…then its too hot and dry, and the composting slows way down…we are lucky to get 2 inches of rain for the entire year, so water is a major input for growing. I only have a 40x80 yard and a 20x40 greenhouse … no tractor unless I rent for the weekend…so all is done by wheel barrow. I wish I lived on the river. Its just incredible how fast compost gets digested by the sand. Commercial farmers can’t utilize compost because they flood irrigate and wood chips just float to one corner of the field. Thats why they use liquid amendments to just add to the water, so no soil building. Its just frustrating, I’m venting a bit. My apologies.
I stick food scraps on top of my soil and then stick brand new, fresh wood chips on top of them. No nitrogen tie-up! As long as you add them with a nitrogen source (urine works, too), fresh wood chips are fine.
I don’t give them any additional water. The whole point of the wood chips is to hold in water around the plants’ roots, so that I don’t need to use as much of it. So I just stick six inches to a foot deep on all my beds, and water only when I have water available. So far, it’s working fine! All my fruit trees are delighted at this treatment, and the peas, fava beans, radishes, brassicas, and garlic I stuffed into the top inch of the wood chips all seem vigorous and happy right now. I’ll see how it works for my summer crops later this year!
I hear you about having to do it all by hand. I don’t even have a wheelbarrow, so I carry all my wood chips from my front yard (where I have the arborist companies dump them) into my back yard using five-gallon buckets. It takes many hours of hard physical labor. Good exercise, right? Getting a wheelbarrow may be a good idea at some point, though. (Laugh.)
P.S. Two inches of water a year! Wow! Yeah, you’re definitely in a more arid climate than me. We get no water in our summers, but we get an average of 18 inches of rain a year, almost all of it in winter. So wood chips make a fantastic sponge to keep that winter water available for my plants’ roots for longer.
Getting a wheelbarrow may be a good idea at some point, though. (Laugh.)