Dear Miss Floribunda,

Although I was interested to find out in last month’s column that the Department of Agriculture has again changed our zone to a warmer one, I’m disgruntled to see it limited to the lowest frost date. While I see that we are in the same frost-date zone as Oklahoma and certain parts of New Mexico and Tennessee, I think it’s important to take into account that we certainly have more humidity than New Mexico and Oklahoma, and we have windier winters than most of Tennessee. It seems odd to be lumped with them in the same zone. 

Does any agency or organization have another way of zoning? Or at least, is there some site that includes such important information as when spring really comes in our region and when to expect the last frost?

What are the hottest temperatures we can expect? Also, what about these odd periods of drought and rain each year? Do they form a pattern in our region, and can they be predicted in any way? 

I’m not looking for information about tidal waves and volcanic eruptions but just what I need to know as a home gardener in my own microclimate.

Don’t tell me I’m being self-centered because I think we home gardeners do a lot to combat climate change just by planting things that take carbon out of the air.

Unapologetic on Oliver Street

Dear Unapologetic,

I share your frustration. I wish there were a nationwide map like California’s Sunset Climate Zones. This system, created by Sunset Magazine and the UC NAR (University of California Division of Agriculture and Natural Resources), takes into account not only winter minimum temperatures, but also summer highs, lengths of growing seasons, and humidity and rainfall patterns. 

Their maps are extremely useful to Californians. Although it’s a lot more work for non-Californians to do this, I tried to get some needed information piecemeal. 

I found that The Farmer’s Almanac has an easy-to-use planting calendar (almanac.com/gardening/planting-calendar) that uses your zip code to let you know when it’s safe to plant different vegetables in spring and when to harvest in fall. However, based on my own experience, it didn’t seem quite accurate to me.

The USA National Phenology Network provides a First Leaf Index and First Flower Index. The Air Sports Net humidity map for Maryland is limited to seven-day estimates, and Weather Street’s drought monitor map does not extrapolate beyond seven days either. I found the National Oceanic and Atmospheric Administration, or NOAA, map that is supposed to include annual high temperatures difficult to understand. 

The Environmental Protection Agency EnviroAtlas Interactive Map is more concerned with the consequences of climate change on public health, rising sea levels, severe weather events and forest fires — along with some information on the changes of ecosystems relevant to the home gardener. 

Although the U.S. Geological Survey site has many maps, they tend to be limited to such matters as volcanic activity, although the site also provides important information concerning the invasive species encouraged by climate change. 

Feeling overwhelmed, I remembered the Chinese proverb: “A conversation with someone wise is worth 10 years of study.” I appealed to Kathy Jentz, editor of Washington Gardener, for help. Right off the bat, she sent me the site for the American Horticultural Society’s heat and humidity map, found at ahsgardening.org. 

I also recalled that living only a few blocks away was Dr. Greenthumb, a climatology expert, and I asked her for help determining the last frost dates in our own little microclimate. The first thing she did was to warn me to be wary of such popular but not rigorously scientific sites as The Farmer’s Almanac. A safer bet for Hyattsville gardeners, she said, is the University of Maryland Extension Service for our area (extension.umd.edu/resource/vegetable-planting-calendar). It is supported by the Cooperative Extension System under the aegis of the USDA National Institute of Food and Agriculture, a partnership that provides research-based information to the public. 

For regional climate change summaries, she also recommended the most recent National Climate Assessments (NCA4 and NCA5). The NCAs consider our area as part of the Northeast region and have summaries of our changing weather and conditions and most recent findings here: nca2018.globalchange.gov/chapter/18 and nca2023.globalchange.gov/chapter/21. She also recommends the National Gardening’s Association’s online frost dates tool,  garden.org/apps/frost-dates.

Dr. Greenthumb gave me her assessment of Hyattsville frost dates, based on her own experience gardening right here in Hyattsville, saying, “I consider April 9 as the 50% chance last frost date (32 F). April 24 is the average 50% chance of 36 F night weather.” She is being exceedingly cautious and circumspect about this, and warns against sudden cold snaps and other weather vagaries, but I feel confident that it’s safe to set out cool-weather plants after April 9 and warm-weather ones after April 24 — soil temperature permitting. It is always good to get an inexpensive soil thermometer to check soil temperature, which is just as important as night-time low temperatures. Soil should be no cooler than 50 F for planting cool weather crops and 60 F for setting out tomato plants.

The Hyattsville Horticultural Society now posts local soil temperatures and gardening timing hints on its main web page, hyattsvillehorticulture.org. In addition, please come to our next meeting, which will take place at the Hyattsville Municipal Building on Saturday, March 16, at 10 a.m.

Miss Floribunda writes about gardening for the Life & Times. You may email her at Floribundav@gmail.com.