At the Arboretum, an unquiet winter

Don’t let the smooth blanket of snow fool you. Don’t be deceived by the footpaths free of summertime crowds or the trees patiently waiting for spring. Winter at the Arnold Arboretum is a busy time.

For humans, it’s a time for catching up outdoors and forging ahead inside. It’s a time when the frozen ground is a blessing, when heavy machinery — a bucket truck to reach high branches, say — can cover territory that is off-limits in the summer. Meanwhile, horticulture crews have time to focus on oft-neglected patches of natural woodland, targeting invasive species.

Indoors, researchers peer into microscopes and visit greenhouses, examining and cataloging collections from other seasons. Though spring prep work is typically completed in the autumn, planning for the season is ongoing, as is the development of programs for high school and college interns interested in horticulture.

Work aside, winter at the Arboretum is a time of beauty, of marveling at the gnarled branches of the hawthorn collection, of surprise at the fresh blossoms on the witch hazels, and of transport while walking snowy paths under evergreens.

“The firs, spruces, and pines, you really get a chance to appreciate them in the winter better. Juxtaposed against the snow, you can be transported to the Alps, the Rockies, wherever you want,” said Michael Dosmann, curator of living collections.

For horticulture supervisor Andrew Gapinski, the witch hazels are most interesting. The shrubs win the annual competition for pollinators by blossoming when other plants are still sleeping. Their spidery blossoms, which open through winter, are visited by gnats and other insects that become active during the season’s fleeting warm-ups.

Prep time

In the fall, leaves are cleaned up and newly planted specimens are thoroughly watered to ensure they’re hydrated — especially evergreens, such as rhododendrons, that photosynthesize on warmer winter days even though they can’t draw water from the frozen ground, making them susceptible to drying out.

But one of the most important winter-prep tasks, Dosmann said, is a year-round one: ensuring that specimens enter the season in good health.

“We practice tough love here. We don’t have the necessary resources to go through and pamper plants. A healthy plant going into winter will winter [well] and be good in the spring.”

The first step in that process is choosing a site, Dosmann said. With the Arboretum’s 140 years of history to draw on, horticulturists know better than to put in plants that won’t make it through the winter, but in some cases knowing how conditions vary across the terrain is valuable, Dosmann said. Certain locations have microclimates that are a benefit to tender plants. Explorers Garden, a flat area on Bussey Hill, gets ample sunshine on its southwest-facing slope, and its elevation keeps it above winter’s coldest bite, as the chill air drains away to lower elevations.

Winter is a time of particular emphasis on pruning. Not only are staff less occupied with tasks that dominate in the growing season, a plant’s branch structure is more visible because of the loss of leaves. Pruning is also less stressful this time of year because the plant is dormant and because the vigorous growth of spring is just weeks away. There’s also a reduced chance of transferring pathogens in the winter.

Though pests are mostly dormant in the winter, they aren’t ignored. Officials meet to assess the pest situation and plan strategy for the coming year. One January day, horticulturists took in monitoring traps for winter moth — a pest that affects many kinds of trees — to better understand the level of infestation before spring begins.

Frigid temperatures might help on this front. One early winter cold snap hit minus 4 to minus 5 degrees Fahrenheit at the Arboretum, cold enough to help with some pests and close to the minus 10 that knocks out the hemlock woolly adelgid, one of the Arboretum’s least welcome visitors, Gapinski said.

The Arboretum is home to three full-time faculty members, Director William (Ned) Friedman, the Arnold Professor of Organismic and Evolutionary Biology, and recently appointed assistant professors of organismic and evolutionary biology Robin Hopkins, an expert on speciation in plants, and Elizabeth Wolkovich, an ecologist investigating the effects of climate change on plant communities. Faye Rosin, director of research facilitation, works with visiting scientists and postdoctoral fellows.

Analysis of flowers, leaves, stems, roots, and other specimens collected during warm months is a process that amply fills the winter, Rosin said. “People with extensive outdoor work do analysis in the winter months. The indoor work happens all the time.”

For those who can’t wait, such as Friedman, whose research involves a rare, extinct-in-the-wild water lily from Rwanda, the temperature and moisture in the Arboretum’s greenhouses can be adjusted to replicate various outdoor environments, as can the conditions in smaller growth chambers.

“On a cold, dark, winter day in Boston, there is nothing better than taking some time to visit theses water lilies under the bright supplemental lights, high humidity, and warm temperatures that they thrive under,” Friedman said.