By Thomas Christopher
For Capital Region Independent Media
I’m not a great fan of the USDA’s Plant Hardiness Zone map.
For the most recent edition, published in 2012, those who drew up the map more than doubled the length of the weather records used in creating it. That is, the previous map, which was published in 1996, was based on 13 years’ worth of data, while the current map is based on a 30-year stretch of records.
The USDA asserts that this change reflected a desire for greater accuracy. But by including older records, it also suppressed the evidence of recent changes to our national climate: the recent hotter years were averaged together with older, cooler years. And of course, our North American climate has warmed steadily since 2012, making the USDA map even more inaccurate.
When I consult the map, I take its information with a grain of salt.
A similar criticism can be leveled at the National Oceanographic and Atmospheric Administration (NOAA) map of the average local dates of the last spring frost. This, too, is based on 30 years of records (1991-2010) and so is surely not entirely accurate in its prediction of what you will experience in your garden.
I know from personal experience that spring is arriving earlier in the North Country than it did a generation ago. Taking a somewhat longer perspective, ecologists have noted that the wildflowers around Walden Pond are blooming about three weeks earlier than they did when Henry David Thoreau recorded their bloom dates in his journal in 1854.
Why does this matter? If you are a vegetable gardener like me, you know that the recommendations on when to plant various crops are typically tied to the average date of the last spring frost in your region.
The recommendation for tomatoes, for example, is to set them out in the garden two weeks after the average date of the last spring frost. That, in turn, determines when I sow the tomato seeds indoors – I plant tomatoes eight weeks before I want to plant them out.
According to the NOAA map, the average date of the last spring frost on my chilly hilltop in western Massachusetts is May 10. Add two weeks to that, and I should be planting out my tomato seedlings on May 24. If I subtract eight weeks from May 24, I know that I should plant the seeds in the last week of March.
I know, however, that the last frost date map does not accurately reflect anymore what happens in my garden in the average year. It’s too conservative, and probably delays my planting by as much as a week.
Given that my garden has a short and cool growing season, delaying my tomato harvest by a week means fewer fruits will ripen before fall’s cool weather kills the vines. As it is, I have to grow special varieties of tomatoes to get a satisfactory harvest.
Over the years, I’ve tried a number of different varieties recommended for cool climates and have found I get the best results with a Czech heirloom tomato, “Stupice.” This bears relatively small two-inch fruits, but their flavor is good and “Stupice” bears abundantly in my garden.
I’ve also had adequate results with some of the tomatoes developed for cool climates by Dr. James Baggett at Oregon State University such as “Siletz,” which bears bigger fruits, though of lesser quality than “Stupice.”
This year, I’ll also be trying another Oregon state tomato, “Legend,” which is supposed to be resistant to late blight, a fungal disease that is particularly troublesome in my cool, usually moist plot.
Where does the inaccuracy of the frost date map leave me? I am torn between being bold and planting earlier than it recommends, and playing it safe and getting a delayed, lesser harvest.
Typically, I’ll admit, I play it safe. For along with an earlier arrival of spring, climate change has brought erratic, unpredictable weather. An average is just that — an average — and what happens in any particular year is likely to be different. I don’t want to nurture my tomato seedlings indoors for eight weeks and then have them killed by a late frost.
To obtain seeds of tomatoes adapted to cool climates, I recommend Tomato Growers Supply Company of Fort Myers, Florida (https://tomatogrowers.com/). Starting from seed is, of course, more trouble than picking up whatever seedlings are on sale at the local garden center, but I’ll discuss the advantages of growing your own from scratch in my next column.
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Be-a-Better-Gardener is a community service of Berkshire Botanical Garden, located in Stockbridge, MA. Its mission, to provide knowledge of gardening and the environment through a diverse range of classes and programs, informs and inspires thousands of students and visitors each year. Thomas Christopher is a volunteer at Berkshire Botanical Garden and is the author or co-author of more than a dozen books, including Nature into Art and TheGardens of Wave Hill (Timber Press, 2019). He is the 2021 Garden Club of America’s National Medalist for Literature, a distinction reserved to recognize those who have left a profound and lasting impact on issues that are most important to the GCA. Tom’s companion broadcast to this column, Growing Greener, streams on WESUFM.org, Pacifica Radio and NPR and is available at his website, https://www.thomaschristophergardens.com/podcast.
