There’s been a lot of coverage recently about the new USDA Hardiness Map. According to the map, I now live in zone 6, while I was previously classed as zone 5.
In some ways, this is really great news for what I can grown here. In other ways, it’s no big honkin deal.
A lot of the sites I’m reading a really overstating the significance of this. The map is based on retrospective data. It might be saying that your land is warmer now than it was 20 years ago when the map was last released. It’s also possible that it’s saying it’s been the same temperature all along, but they did a bad job collecting data 20 years ago, and got the numbers a little wrong for you. What it is absolutely not saying that it is warmer now than it was last year, because last year is when they were collecting the darn data.
So, for instance, if your rosemary died and your peaches didn’t set last year, they won’t do better this year, just because now a chart has a different number on it. (well, they probably will do better THIS year, because we’ve had crazy weather, but not long term)
If, however, you tried to push something that was on the borderline in the past, and have been getting lucky so far, there’s a good chance your luck will hold. a.k.a: maybe you didn’t do as good of a job finding a microclimate as you thought – it’s just warmer than you realized everywhere.
Some people are saying that this data even has repercussions for first and last frost dates. Which is tangentially maybe a little bit true, in that if the climate did change, it is likely both the minimum winter temperature and the dates at which it starts getting cold changed. However, the zone map only measures one of these things, and trying to interpolate the other off of it is just a bad idea. Zone is a terrible way to estimate first and last frost dates.
An awesome way to estimate first and last frost dates is http://www.ncdc.noaa.gov/oa/climate/freezefrost/freezefrost.pdf It has data on first and last observed frost dates for specific towns over a 30 year interval, with great stuff like which date has a 95%, 50% and 5% probability or reaching a certain temperature by. Mmmmm, delicious data. There are a lot of towns used for measuring points, so you are very likely to find somewhere quite close to you to get dates from. It is data from the 80s, but I still think it’s a more reliable source than what zone you are, particularly since with something like a frost date you can watch the weather when you get within a few days and judge for yourself whether it’s currently snowing. These dates were never a contract, and will vary more year to year than they have changed on average over a few decades.
Pluuuus, at least in my part of central ohio, I think winter survival has more to do with wetness from the poorly drained clay and damage from freeze thaw cycles without good snow cover than it does with absolute low temperatures.
Not that I’m not excited to officially be classified as zone 6, (for one thing, it gives me extra hope for my Magnolia Sieboldii). I just have to keep reminding myself that this doesn’t mean that the temperature fairy has made any changes to my actual garden (recently, at least). I resolve to be more adventurous in choosing trees and shrubs rated for warmer climates than I was before, but I also resolve to not expect anything already in my garden to start behaving any differently.