Washington, D.C. is famous for its cherry blossoms, but someday it could also be renowned for its palm trees. That’s according to a new study looking at how palm trees are expanding into northern parts of the world that have long been too cold for the iconic tropical trees.
There are over 2,500 species of living palm trees, and while the vast majority of them survive only in tropical climates, some are more hardy and can grow in places that regularly get snow. As climate change warms the planet, countless plants and animals are migrating into new habitats made more suitable for their survival. Palm trees are no different.
The study, published in Nature’s Scientific Reports, found that a palm tree’s latitudinal limit is determined by the average temperature of a region’s coldest month. If that average is above 2 degrees Celsius, or 36 degrees Fahrenheit, the palm may be able to successfully propagate in the wild. According to Tammo Reichgelt, a Lamont-Doherty Earth Observatory researcher who led the study, this means that at an average of 34 degrees Fahrenheit in January, Washington, D.C. is just a little too cold to host palm trees, “but that you can expect range expansion in the coming decades as average winter temperatures warm up.”
Looks like the National Arboretum might need to make room for some new species.
Average January temperatures in slightly farther south Norfolk, Virginia, and Greenville, North Carolina, have not dropped below 36 degrees Fahrenheit in January since the 80s, according to the study—long enough for a palm to grow to maturity.
“But if that is actually the case also depends on other factors, such as the presence of cultivars and the amount of competition with the natural vegetation,” Reichgelt said. “I would love to see the data that shows how the range of palmettos has changed in eastern North America, or fan palms in California, but unfortunately I do not have that data.”
Reichgelt told Earther that for many palm lineages, the average temperature of a place didn’t matter as much as the yearly temperature range. This could also impact the ability of palm trees to thrive in certain tropical locales as they face increased heat driven by climate change, which “could be detrimental to palms in those places.”
Palm trees grow very quickly and thus their cells cannot go dormant in winter. They are also very aesthetically appealing, and when people plant them in non-native areas it allows the palms to potentially establish new strongholds in the wild. Reichgelt said that in Florida alone there are seven “category 2” invasive palm species that were introduced as ornamental plants but have since established healthy wild populations. For now they are co-existing with native flora, but eventually they could become “category 1” invasives if they begin to harm or displace local vegetation.
David Greenwood, a paleobotanist from Brandon University who also worked on the study, said the authors wanted to see how cold some palms could go. Greenwood told Earther that the Chinese windmill palm (Trachycarpus fortunei) is especially cold hardy, and is commonly cultivated for this reason. Windmill palms have recently been found in the forests of southern Switzerland—the foothills of the Alps—after one such decorative palm escaped and spread “simply because frost is not as prevalent as it used to be,” according to the study.
Greenwood said for a palm to colonize a new region, its seedlings must be able to survive their first winter.
“So just because you can grow a windmill palm or another cold-tolerant palm in your yard in Utah, British Columbia, or Ohio, doesn’t mean that species can colonize the nearby woods,” he said. “But this is of course changing as the climate gets warmer.”
Greenwood said parts of the Northeast, Northwest, and neighboring areas of Canada are close to a winter climate that could allow cold-tolerant palms to become established.
“In all of these areas palms in people’s gardens are flowering and setting fruit, which means the temperatures are now warm enough for that step in their reproduction,” he said. “A little bit more warming will see their seedlings survive winters.”