We’ve been starting to think about this year’s garden which is the first step towards actually doing something about it.

When planning at this early stage, the milestone that comes up most often is the average date of last frost. Seed choice, time of sowing, period of hardening off, and date to transplant all work backwards (or forwards) from this important seasonal transition.
So, when is it?
In the old days, we would ask an old-timer or consult an almanac. I don’t know anyone around here who falls into the former category but there are a variety of almanac websites (and, presumably, one can still buy a print version), most of which provide a list of cities and dates. Looking at the Farmers’ Almanac, for instance, I found a map of the US and after clicking on New York, a short list of cities popped up. The nearest to us is Albany for which the average last spring frost is May 2.

That was very simple and convenient but Albany is significantly farther north than we are. Also, the notes indicate that there is a 50 percent chance that frost will occur on a later date, which sounds risky. With further research, I thought I might be able to determine whether the risk of frost—with a higher confidence level—might be expected to end earlier.
Other websites attempted to refine their estimate by combining information from several nearby cities, based on our zip code. For example, Moon Garden Calendar presented data for Poughkeepsie (to the north of us) and Yorktown Heights (to the south). For a 10 percent probability of exceedance (the site allows a choice), the dates range between May 14 and April 29; a linear interpolation would give us May 7. At Dave’s Garden, three cities were presented (Mohonk Lake and Middletown, both across the river from us, and Poughkeepsie) and their information was averaged for me. The “almost guaranteed” date after which frost will not occur is May 4.

These dates still seemed late to me so I went looking for other sources. I next tried the Cornell Cooperative Extension (CCE), which provides a map on which rough contours have been plotted. Based on this map—which is extremely low in its resolution—we are on the border between a last spring frost in the range of April 30 to May 10 and April 20 to 30, with proximity to the Hudson River clearly accounting for the latter zone. We are in a transitional area with a microclimate that depends not just on the river but elevation as well. Apparently, I needed a more detailed map.
A little web searching led me to PlantMaps. This site is a great example of a geographical information system (GIS), where a lot of available information is linked to a physical location. In this case, after entering my zip code, I was presented with a zoomed-in interactive map of New York hardiness zones (see July 28, 2011), along with a list of other available maps, among them a Last Frost Date map. I clicked on this link, and was directed to another color-coded contour map.

This one appeared to be in greater detail than the CCE version—after all, it is powered by Google Maps—but the problem with it is that because of all of the detail (roads, terrain, satellite imagery, etc.), the contour colors are difficult to match with the legend. As best as I can tell, we are located somewhere between a last frost date of May 1 to 10 and April 21 to 30. This is essentially the same result as before but with higher resolution.
Which should not be a surprise. The source cited by all of these websites is the National Climatic Data Center, a part of the National Oceanic and Atmospheric Administration. They maintain data for a long list of locations in each state, compile and average them, and maintain contour maps that present the information visually (the raw data is also available in text form). Their map (which is low resolution, just like CCE’s) puts us squarely in the May 1 to 15 zone of last spring frost; the nearest city listed in their database is West Point, with a last spring frost date of May 3. The Farmers’ Almanac had it right after all.

Based on all of this, we are approximately 14 weeks before the last frost. Therefore, we’re already behind! Of course, I knew that before I even started this exercise.