Climate change has important implications to us as rosarians, not only in terms of what cultivars we plant, but also in how we winter-protect our gardens and how we care for them in the hotter summers. As such, this is quite important to every rosarian in the country.
While this article is specific to the Twin Cities, it can easily be interpolated to other sections of the country by studying the accompanying National Arbor Day Hardiness maps, which I believe are more accurate than the recently published USDA data. As I point out in the article, the USDA data are already out of date by seven of the warmest years in history.
Winter is Warmer Now
By Jack Falker
You might argue why it’s warmer in Minnesota these days, but you can’t deny the fact that it is warmer, and that has important implications for Northern Gardeners.
In January 2012, the USDA released a new Plant Hardiness Zone Map for the first time in 20 years. It covers the 30 year period from 1976 to 2005, so it’s already seven years out of date and, importantly, those seven winters were among the warmest in the last 50 years. More about that later, but that makes the new USDA data pretty unreliable. The new map (shown below) is nonetheless interesting, so let’s take a look at what it says about Minnesota.
The USDA says that all of southern Minnesota has moved from Zone 4a, with annual extreme minimum temperatures (EMT) of -30 to -25, to Zone 4b with annual EMT of -20 to -25. If you look very carefully at the map, you can see a small area in the Twin Cities, mainly Richfield and Bloomington (including the airport), which is in Zone 5a (EMT -15 to -20). This seems counterintuitive to the “heat island” theory, since the downtown areas are not included. It looks rather like the area along the Minnesota River Valley has been measured. This is much easier to see if you look at the interactive map the USDA provides, and zero-in on the Twin Cities. Here is the address: http://planthardiness.ars.usda.gov/PHZMWeb/Default.aspx . If you play with it a little, you can actually get down to the streets in Bloomington and Richfield that are shown in Zone 5a. In that regard, it is also counterintuitive that Zone 5a doesn’t extend into South Minneapolis and Edina!
Those of you who read my article “Are the Twin Cities in Zone 5 or Zone 4?” published here several years ago, will recall that I cited an extensive study that was done in 2006 by the National Arbor Day Foundation showing that essentially the whole Twin Cities area, including the inner-ring suburbs, plus the areas of southern Minnesota along the Mississippi and St. Croix rivers, are all in Zone 5a. Based on my own record keeping and gardening experience in Edina, as well as regular observations at the Lake Harriet rose garden, this is correct. The 2006 National Arbor Day Foundation Zone Map is included at the end of this article. It is also available on line at: http://www.arborday.org/media/zones.cfm . This further raises the question: Why is the USDA still so far behind the curve?
Let’s take a look at the historic EMT data for Minneapolis/St. Paul from the University of Minnesota Climatology Working Group site (http://climate.umn.edu/doc/prelim_lcd_msp.htm). For example, if we take the same 30 year period as the just released USDA study (1976-2005 inclusive), the average annual extreme minimum temperature (EMT) was -19.4 degrees, which is right on the low edge of Zone 5a. However, if we update these numbers for the 30 year period just ending (1983-2012) the average annual EMT becomes -18.6, which is more convincingly in Zone 5a. That average is definitely being “pulled” by the warmer winters of the last 10 years. For the 10 years 2003-2012, the average EMT is -16.1 degrees, which is closing in on Zone 5b (EMT -10 to -15). Although the winter of 2012 is still with us, it appears that our EMT was -11 degrees on January 19th, which is edging close to Zone 6a (EMT -5 to -10). Having lived in Zone 6a (Detroit) until 1977, I can attest that this winter reminds me a lot of southern Michigan!
To see just how much things have changed in the last ten years, it is very interesting to look at the 50 year period 1963 to 2012 (inclusive), when the average annual EMT was -21.1, ( Zone 4b). But again, the last ten years pull the average, so if you look at the 30 year period 1963 to 1992, you get a better historical picture, with an average annual EMT of -23.4 degrees. What jumps out at you, as you look back at the 1960s and 1970s, is just how much colder it was back then. For example, there was a period in 1970, between January 17th and January 21st when it didn’t go above zero for 4 ½ days, with a low temperature of -34 and a high of -3. Now that was a zone 4 winter! Another thing that jumps out at you is how many -20 degree days occurred in December and February, during those decades. This is something we haven’t seen in several years. Here is a graph of that 50 year period. Notice the upward slope of the graph, right out of Zone 4 into Zone 5. And, If you follow the statistically valid trend line, it takes us into Zone 6 within ten years.
How This Affects Northern Gardeners
First, we need to remember that we are still in Minnesota and it can get pretty cold for a few days in January, even though the EMT averages indicate that the Twin Cities are now on the northern edge of Zone 5. Note on the chart above that the temperature dropped to -21 in 2009 (one degree lower than Zone 5), but it didn’t stay that way for long, like it would have 30 years ago.
Probably the most important message to take away from this discussion is that, unless you live well outside the Twin Cities area, the “Minnesota Tip” method of winter protection amounts to a lot of unnecessary, back-breaking work. The tenderest of hybrid tea roses, planted with their bud unions below the ground surface, should survive very nicely in the Twin Cities, with some mounding and a cylinder of leaves or a leaf bag, with its bottom opened, pushed down over the cut-back and tied-up plants. These are tried and true methods of winter protection in Zone 5, Chicagoland (which is now, by the way, in Zone 6). For roses that are a bit hardier, such as the Bucks, a three-inch layer of wood mulch should be sufficient. My favorite Buck rose, "Earth Song", is rated Zone 5 hardy, and I have experimented successfully with just wood- mulching and natural snow cover over the last few years. The plants die back to the snow cover or the mulch, but grow aggressively, as soon as they are pruned and fertilized in the spring. If I want to save more of the canes, I push a leaf bag over certain plants.
While the long-term implications of global warming are ominous, the short-term benefits for Northern Gardeners are obvious. In addition to less-rigorous winter protection for our roses, and the resulting lighter workload, we can also look forward to planting a greater variety of perennials and trees in the future. And, let’s face it, walking around and playing outside in the winter months has gotten a lot more pleasant than those years when it didn’t get above zero for days on end. Enjoy it!