As an Arboretum horticulturist, my daily routine always starts with the weather forecast. The forecast dictates priorities and deadlines for horticultural tasks. As the forecast shifts, new goals arise and others are eliminated. Pests and diseases come and go with seasonal turns in the weather, and access to our plant collections is determined on a daily basis, especially in the wet and muddy weeks of spring.
Past weather conditions, however, also play an important role in directing our horticultural care. By the start of meteorological winter in December 2016, concern over the health of the collections was mounting. Following an exceptionally dry 2015 season, record-breaking heat and drought made 2016 an especially tough year for plants. From June through October, hundreds of feet of hose were dragged through the collection, and every water tank on hand was filled and refilled in an effort to alleviate drought stress. Water cannons pumped six million gallons of water into the landscape, and irrigation systems were on a tight overnight schedule. The efforts of the horticulture crew to carefully monitor and provide supplemental irrigation in the searing heat of 2016—the hottest summer on record for the city of Boston (but the second hottest according to Arboretum data)—cannot be overstated. As we entered 2017, what would it take to end the drought?
Temperatures warmed during the winter months, and according to thirty-year averages calculated by the National Oceanic and Atmospheric Administration, precipitation during January and February of 2017 was well above normal for the Boston area. Due to the severity of the ongoing drought, expectations of major plant dieback and death were on the forefront of our minds as we awaited spring leaf out. Abundant rains fell during March and April, amounting to more than five inches above normal. Buds swelled, leaves emerged, and a green landscape was a welcome sight. Rains continued to fall. Soils were plenty moist, and the recovery of the collections was exceptional. Not only did the accessioned trees, shrubs, and vines flourish during the growing season but turf and weeds also seemed to grow exponentially, keeping horticulture crews busy. Arboretum staff, always attentive to the impact of weather on plant health, could not have asked for a better year following the preceding drought.
Our annual weather summary tracks the four meteorological seasons and reveals, at least in part, moments when weather demanded adjustments in horticultural care. For ease of interpretation and statistical analysis, meteorological seasons are broken into three-month periods based on annual temperature. Winter is defined as the three consecutive months with the lowest average temperatures, corresponding with December, January, and February in the Northern Hemisphere. Spring, summer, and autumn follow accordingly, each comprising the next three-month series.
Winter: December 1, 2016 to February 28, 2017
December was seasonable with slightly mild temperatures. Highs dropped below freezing on only four occasions. An outbreak of arctic air on the fifteenth and sixteenth forced temperatures to a low of 3°F, and on the seventeenth, we received five inches of snow. Most melted away as conditions rebounded to 57°F on the eighteenth. The majority of precipitation fell as rain during seven major events.
The month of January brought unusual warm conditions. Despite a three-day cold spell that brought single-digit temperatures from the seventh through the ninth, January’s average temperature was 7.1°F above normal. We hit a record-breaking 62°F on the twelfth, far above the average high of 36°F for that day. Precipitation was abundant throughout the month with seventeen days of recorded rain or snow. Of this, 3.65 inches fell as rain. Another 9.8 inches arrived as snow, which mostly fell during the three-day cold spell. Major rain events were long, steady, and light, allowing moisture to infiltrate into the soil and recharge groundwater levels. A high-powered coastal storm arrived on the evening of the twenty-third, bringing prolonged rain over the next twenty-four hours. Gusts reached 39 mph, scattering limbs throughout the landscape and completely destroying an oak and a willow. Overall, the warm and snowless month allowed access into more remote areas, providing opportunities for horticulture teams to prune deadwood in the beech collection and thin trees in Central Woods.
Warm conditions continued into February, as temperatures averaged 6.1°F above normal. The horticulture crew took advantage of snowless days early in the month by pruning and rejuvenating most accessions in the Bradley Rosaceous Collection. We ultimately received above-average precipitation, including 19.2 inches of snow that mostly fell between the seventh and the thirteenth. A nor’easter blizzard, the first since January 2015, delivered 11.5 inches of this total on the ninth. Conditions remained cold and cloudy in the immediate aftermath. With this deep snow cover, horticultural priorities shifted to scouting for signs of invasive insects in the collection. A warm spell sent temperatures soaring between the twenty-third and the twenty-fifth. We reached 74°F on the twenty-fourth, the highest February temperature ever recorded in Boston since recordkeeping began in 1872. Daily records were also hit on the neighboring days (69°F on the twenty-third and 72°F on the twenty-fifth). All remaining snow melted over this period, and the horticulture crew returned to the rose collection, mulching all sixteen beds. Early spring blooms appeared on red and silver maples (Acer rubrum and A. saccharinum) and on hybrid witch-hazel cultivars (Hamamelis × intermedia ‘Arnold Promise’, ‘Diane’, and ‘Jelena’).
Spring: March 1 to May 31, 2017
March was colder than normal with an average temperature below both January and February. Early in the month, lows dipped into single digits on three occasions. Precipitation was abundant and consistent throughout the month, although we experienced some of the driest air of the year between the third and sixth, when relative humidity levels remained in the teens and low twenties. On the fourteenth, a late-season blizzard brought high winds, gusting at 47 mph, our highest of the year. Heavy snow amounted to 6.5 inches and turned to heavy rain that fell at a rate of 0.50 inches per hour. Temperatures plummeted to the teens overnight, turning roads and sidewalks into a veritable skating rink by the morning of the fifteenth. The storm brought down limbs in the conifers and toppled an apricot (Prunus armeniaca). Temperatures remained cool until vernal equinox on the twentieth brought sunny conditions and a high of 51°F. We ended the month with plenty of rain. Buds that had begun to swell due to warm February temperatures suspended their development, waiting to open, while cooler temperatures extended the bloom time of many witch-hazels. The lack of snow cover allowed the horticulture crew to pursue diverse projects: mulching the beech collection, installing new paths, removing invasive plants in natural areas, and cleaning winter storm damage (including a giant willow that was pulled from the meadow).
Abundant rainfall continued into April, further reducing the water deficit from 2016. Temperatures were above average for the month. A storm that lasted from March 31 to April 1 delivered 2.82 inches of precipitation, most falling as rain. We began the third with our last spring frost, which melted as temperatures warmed to 60°F later that day, marking the beginning of the growing season. Excessive rain and melting snow saturated soils, especially in low-lying areas, leaving those areas inaccessible. Ponds filled, brooks flowed, and the forsythia began to bloom. Nursery digging for spring plantings began on the tenth. Warm conditions persisted as we hit 86°F on the eleventh and sixteenth, causing katsuras (Cercidiphyllum japonicum), magnolias (Magnolia), and cherries (Prunus) to burst into flower. Despite seasonal dreary conditions, the landscape looked alive as turf greened up and trees leafed out. Horticulture crews were busy with mowing operations throughout the grounds. We ended the month with temperatures hovering just above 80°F, pushing lilacs (Syringa) into bloom.
May was slightly warmer than average, but we began the month with below-seasonal temperatures and lows in the thirties. A number of fast-moving downpours accounted for over an inch of rain during the first week. According to the United States Drought Monitor, soil moisture returned to normal conditions on May 9, officially ending the drought that had begun on June 7, 2016. Overcast conditions prevailed for the week leading up to Lilac Sunday on the fourteenth. These cooler temperatures extended the blooms of many plants, especially lilacs, but with the threat of soaking rain on the fourteenth, Lilac Sunday activities were held on Saturday the thirteenth. True to forecast, a nor’easter arrived that evening, and by the afternoon of the fourteenth, we had received over 1.5 inches of rain. Temperatures soared over the following days, hitting highs in the nineties from the seventeenth to the nineteenth and peaking at 96°F. This heat wave was one of the earliest for the area, as the most recent with an earlier date occurred May 2–4, 2001. A fast-moving cold front brought a heavy downpour on the evening of the nineteenth: within fifteen minutes, 0.30 inches of rain had fallen and temperatures had plummeted by 10°F. We ended the month with typical spring weather fluctuations and steady precipitation. Soils remained moist, plants were lush and floriferous, and turf continued its rapid growth. Horticulture crews were busy mowing and finishing spring cleanup.
Summer: June 1 to August 31, 2017
A lingering cold front in early June brought temperatures in the fifties. Four days of rain accounted for 2.0 inches of precipitation, almost half of the monthly total. Moist soil conditions made it difficult to access low-lying areas for mowing operations. We experienced our second heat wave of the year between the eleventh and thirteenth, once again hitting 96°F. High soil moisture and hot temperatures led to an explosion of turf and weed growth throughout the grounds. The plant collections flourished, easing concerns over last year’s drought as plants continued to recover. A fast-moving system brought an additional 1.78 inches of rain on the evening of the sixteenth. Conditions stabilized as temperatures remained in the high seventies to mid-eighties for the remainder of the month. Drier conditions were prevalent, and despite seven short rain events, we accumulated only one additional inch of rain. Nonetheless, compared to June 2016, we received almost four times more rain in 2017.
July temperatures were seasonable with slightly below-average precipitation. We began the month with comfortable conditions, highs in the eighties and lows in the high fifties. Two fast-moving thunderstorms on the seventh and eighth brought downpours and a total of 0.94 inches of rain. Thunderstorms returned from the eleventh through the thirteenth, followed by a cold front that brought temperatures in the sixties for a couple days. Temperatures returned to expected highs, and we experienced our third heat wave of the year from the nineteenth to the twenty-first, with temperatures topping out at 93°F. Conditions continued to fluctuate as a slow-moving storm materialized, bringing prolonged soaking rains and temperatures in the sixties on the twenty-fourth and twenty-fifth. Because this addition of 1.47 inches of rain kept soils moist, we had yet to irrigate the collections (except for new plantings). We ended the month with partly cloudy conditions, high humidity, and temperatures in the seventies and eighties.
Seasonable temperatures continued in August. High humidity and warm temperatures over the first couple of days culminated in violent thunderstorms that brought heavy downpours, hail, and high winds on the third and the fourth. Almost two inches of rain fell, scattering the landscape with fallen leaves and causing erosion (even in mulched beds) and flooding. A cold front brought conditions in the seventies from the fifth through the eighth, but otherwise, typical New England heat and humidity returned for the remainder of the month. We received only five additional rain events over the remaining four weeks, totaling a mere 0.38 inches. Soils dried, leading to dusty conditions in open grassy areas. Newly established turf began to go dormant, and supplemental irrigation began mid-month. Clear skies were ideal for viewing the partial eclipse, which peaked at 2:48 p.m. on the twenty-first. We reached 91°F on the twenty-second, the single incident of temperatures in the nineties for the month. Although very humid, temperatures were seasonable, remaining in the eighties mid-month and dropping to the seventies during the last week. Serviceberries (Amelanchier), which typically begin to drop their leaves early in the season, remained green and lush throughout the month.
Autumn: September 1 to November 30, 2017
September brought average rain that fell predominantly as quick downpours. The month began with temperatures in the sixties and seventies as remnants of Hurricane Harvey delivered a quarter inch of rain on the third. An additional inch arrived as heavy overnight downpours on the sixth, including 0.30 inches that fell over a fifteen-minute period, the first significant rainfall since August 3. Soils were moist and temperatures favorable when autumn planting began on the eleventh. Hot and humid conditions returned mid-month leading to pop-up thunderstorms on both the fourteenth and the fifteenth. Conditions did not improve as Hurricane Jose approached on the seventeenth, bringing dense morning fog that was followed by wind and sporadic rain. Dreary weather persisted with minimal precipitation until the twenty-third, after which the sun and heat returned. Temperatures rose to the mid-eighties between the twenty-fourth and twenty-seventh, 15 to 20°F above average for this time of year. Summer conditions, however, would not last. Temperatures dropped to a more seasonable 56°F when a storm delivered over half an inch of rain on the thirtieth. With the abundance of precipitation in the latter half of the month, irrigation was limited to recent plantings.
October was the third warmest on record. Highs shot above 70°F on seventeen occasions, exceeding average monthly temperatures by over 7°F. We began the month with warm and dry conditions, reaching a high of 79°F on the fifth. Two small showers arrived on the eighth and ninth. Warm temperatures continued and precipitation remained minimal. By the seventeenth, we entered moderate drought, mirroring conditions last experienced in late March. Conditions were ideal for the horticulture crew, which spent these weeks seeding renovated areas, clearing the natural area behind the hickory collection, mulching new paths, and planting meadow natives. The dry pattern broke when a slow-moving system dropped over two inches of rain between the twenty-fourth and the twenty-sixth. An overnight nor’easter arrived on the twenty-ninth, bringing heavy rain and strong winds with 37 mph gusts. By the morning, 3.41 inches of rain had fallen and the collections experienced moderate damage; ten trees were lost, and many branches dropped throughout the grounds. Low-lying areas were flooded from the 5.5 inches of precipitation that had fallen over the previous six days, and the drought ended as quickly as it had arrived. We ended the month with highs in the sixties, having yet to receive our first frost.
November temperatures were seasonable, despite large fluctuations, and rainfall was below average. We started the month with a continuation of the warm temperatures experienced in September and October, hitting 76°F on the third. These warm temperatures forced a number of spring-blooming shrubs into flower, including Smirnow rhododendron (Rhododendron smirnowii). This would mark the end of an unusually warm autumn; temperatures dropped into the sixties from the fifth through the seventh, before settling in the forties and fifties for the remainder of the month. We finally received a frost when temperatures dipped to 29.5°F on the eighth. This ended our growing season at 218 days. Low temperatures continued to drop as we sunk into the twenties for six straight days, reaching 21°F on the eleventh. These freezing conditions, well lower than expected for this time of year, caused leaves on many trees to freeze and die before abscission cells could fully develop. Temperatures remained unseasonably cool with consistent rainfall. The largest storm delivered 0.88 inches of rain on the twenty-second. Sustained cleanup efforts continued throughout the month, as the crew progressed into leaf cleanup after completing storm-damage removal early in the month.
Early Winter: December 2017
Cold temperatures extended into December. Highs dropped below freezing on the twenty-sixth and then sunk into the teens from the twenty-eighth through the end of the month—the four coldest days of 2017.
The Arboretum experienced an optimal growing season in 2017, yet we cannot close the chapter on the preceding drought before considering the long-term effects of such a prolonged water shortage. Symptoms of persistent plant stress are more often observed years down the road, ultimately causing slow decline and possible death. As plants recover from drought, their ability to defend against disease and insect attacks remains compromised. Bark beetle invasions can be linked to drought stress, as can the onset of Diplodia tip blight and Cytospora and Nectria cankers, but connecting future disease and pest outbreaks to past drought events often proves difficult. Internal plant damage is hidden, and the cumulative effects of long-term drought stress may impact tree health for many years. As we move into 2018 and beyond, vigilance and regular observation will be critical to the overall preservation of the collections.
Sue A. Pfeiffer is an Arboretum Horticulturist at the Arnold Arboretum.