I recently discovered that I can no longer rely on the gardening calendars that once dictated my year’s outdoor schedule.
The two weather events on which it most depends, the average dates of the last frost in the spring and the first frost in the fall, change with climate change. According to the National Weather Service, Massachusetts’ growing season has increased by 10 days since the 1960s, and spring comes a week earlier than it did a half-century ago.
As temperatures rise, so do climate anomalies, events that differ markedly from the average. For example, the expected date of the last spring frost in my garden in southwestern Massachusetts is May 9. Last year, I started frost-sensitive seedlings in my vegetable garden the following week, then had to rush and cover all the beds. With a plastic tarp when the temperature drops into the 20s on May 17. Late frosts are nothing new, of course, but spring weather had already been shortened by the early arrival of summer heat, and you needed to get your spring planting done as soon as possible.
Rainfall has also become erratic. We are seeing more severe storms with greater rainfall in the Northeast, but longer periods between rainfall, leading to more dry spells.
The failure of traditional gardening guides is why I became interested in the paper, “Calendar Plants of Southern Vanuatu,” published in the Journal of Economic Botany last May. In this report, dozens of botanists report on research they have been following on the Pacific island of Vanuatu. They recorded Vanuatu islanders’ use of 111 native plant species as indicators of the best times to pursue a range of interactions with their environment, including foraging, hunting, fishing and gardening.
For more information, I reached out to the article’s lead authors, Michael Balik, Ph.D., vice president for plant sciences and director of the Institute of Ethnobotany at the New York Botanical Garden (the study of how people of a particular culture and their culture interact with each other). The area benefits from native plants), and Gregory Plunkett, Ph.D., director of the NYBG Molecular Systems Program (the study of how different species of plants evolve and relate to each other). The scientists conducting the study focused on Tafea, Vanuatu’s southernmost province, where they collected plant specimens and consulted with approximately 130 Ni-Vanuatu (the indigenous people of Vanuatu) about the names and local uses of each species.
Balik and Plunkett told me that many traditional plant traditions still apply—particularly among older populations. In the case of calendar plants, it has gained increasing importance. This is because plants react in their growth to changes in climate and weather, and are constantly correcting this type of gardening calendar.
In 2022, for example, rainfall was much higher than normal in one area of Tanna Island. Calendar plants used by indigenous people to mark the beginning of the gardening season responded by developing more slowly. Local gardeners followed suit, delaying crop planting. The harvest was successful, although later than usual, and was not damaged by heavy rains.
The characteristics of the calendar plants observed by residents vary from species to species and sometimes by region. In western Tana, the germination of seedlings of a shrub, Orena lobata, determines the time of planting. Further south, on Anetium Island, the flowering of the same shrub signals the end of hurricane season.
There are hints that there was a similar use of the plants in North America as well. For example, I’ve heard an old injunction that corn should be planted when the leaves of the native oak trees are “as big as a squirrel’s ear.”
When I suggested to Plunkett and Palek that American gardeners might benefit from a revival in the use of calendar plants, they agreed. A useful resource for this can be data on flowering times and bud break for a variety of plants across the United States collected by the National Phenology Network (usanpn.org). These dates can be linked to the timing of gardening tasks. I suspect that Native American traditions may include information about calendar plants. In this time of climate change, I think gardeners would do better to look to plants for clues.
To listen to the full conversation with Balik and Plunkett, log on to the Berkshire Botanical Garden’s “Growing Greener” podcast at berkshirebotanical.org.