Frost Dates

 As I wrote in a recent blog post, it’s clear that the summer is winding down.  This is our first year gardening in this area so I’m wondering about our first frost date. I read recently that it takes roughly two months time between the fertilization of a tomato flower and its ripening into a red tomato, so any blossoms we have now are not likely to make it.  In anticipation, I’ve pinched back the tops of our tomato plants so that they’ll focus on ripening the tomatoes that have already formed rather than developing new leaves and fruit. 

I’ve been amazed at the differences between the growing environment here and the ones I’ve experienced in the past, in New Mexico and Maryland.  In those two states, I’d given up on cucumbers, greens, and radishes.  In both places, the intense summer heat makes these things ripen very quickly, and they’re often bitter and unpalatable.  On the other hand, as long as you supply sufficient water, tomatoes and peppers flourish.  

Here, however, I’m realizing that temperatures are moderate enough that cool-weather crops like peas produce for most of the summer, greens don’t bolt, and cucumbers ripen relatively slowly—allowing them to keep their sweetness.  On the other hand, the shortness of the growing season and moderate temperatures are a challenge for things like peppers, tomatoes, and tomatillos.  I do have ripe jalapeños and some of the tomatillos have filled out their husks, but the tomatillos and tomatoes still have some ripening left to do.  

I’d also like to plant some cool-weather crops such as kale and spinach, and a second round of things like basil and cilantro.  All this leaves me wondering when exactly to expect the first frost.  

Unfortunately, there’s no cut and dried answer to that question.  It’s a matter of probabilities.  The best I can do, l looking at the data, is to say with a high degree of certainty, that our first frost will happen between September 16th and October 12th.  
This range in possible frost dates makes sense once you think about it, of course, because the date of the first frost varies considerably from year to year.  Calculating the date of the first frost date in a given year is a matter of probability based on dates from previous years.  

The source I consult is the National Climatic Data Center (http://www.ncdc.noaa.gov/).  You can find data for your general area within each of the fifty states.  The data they provide for first frost date is given in terms of probability:  10, 50, and 90 percent.  In other words, they predict that the first frost date will happen earlier than a given date one out of every ten years, five out of ten years, and nine out of ten years.  

In my area, there’s a ten percent chance that the first frost will happen before September 16th.  There’s a fifty percent chance it will be before September 29th, and a ninety percent chance before October 4th.  

The data they used to calculate their probabilities is based on frost dates from 1971-2000.  I know that the USDA has issued updated plant hardiness zone maps (maps that predict the lowest temperatures likely to be reached in a given area) to reflect changes in climate that have taken place in the last decade (http://planthardiness.ars.usda.gov/PHZMWeb/).  

Many areas within the U.S. now have higher lows (in other words, it doesn’t get as cold as it used to) and have consequently changed from one climate zone to another.  I’m wondering if the frost dates have similarly changed—are frosts happening later than they used to?  What has the data been like since 2000?  Unfortunately, I was not able to find any aggregated data about frost dates in various regions for the last fourteen years or probabilities of first frost dates based on that data.