Each year, I receive inquiries about why the leaves haven’t yet dropped. If autumn officially begins on September 1, one would expect winter to be approaching by early November.
However, a photographer documenting the season’s hues in southern England for several years recently shared with the BBC that 2024 seems to be lagging behind. You might have observed this trend as well: it’s increasingly typical for leaves to remain on trees later into the year.
A comprehensive review from 2015 analyzed 64 studies conducted between 1931 and 2010 across Asia, Europe, and North America, revealing that rising global temperatures have postponed leaf fall in the northern hemisphere. To grasp this phenomenon, we must recognize that trees perceive their environment differently than we do.
They rely on various environmental signals to determine when to drop their leaves. Typically, foliage begins to fall gradually starting in October and may continue until early December.
Most tree species native to the UK retain their leaves until temperatures drop significantly, which usually occurs by November. Essentially, once it becomes too cold or too dark, the plant enters a state of dormancy.
Blackberries in June
The process by which deciduous trees prepare for winter is intricate and occurs in three distinct phases. The initial phase begins in early autumn and involves a chemical known as auxin, produced by the buds, which inhibits the tree’s growth.
This substance originates from actively growing tissues and restricts the development of buds further down the branch. As autumn progresses with shorter days and cooler temperatures, the tree also starts to generate abscisic acid.
Both of these chemicals accumulate in living tissues throughout autumn, ultimately halting cell division and expansion within the dormant buds. By mid-autumn, while growth has ceased, leaves may still remain if weather conditions are favorable.
This period allows trees to reclaim nutrients from their leaves—resulting in a change of color—before forming a protective cork layer at each leaf’s base that leads to leaf drop. Once all leaves have fallen, winter dormancy sets in, sustained by reduced light levels and lower temperatures.
Prolonged exposure to cold for approximately two to three months gradually alleviates this chemically induced dormancy. With the arrival of spring warmth and longer daylight hours, trees are spurred back into growth. The shifting climate is altering the typical progression of autumn.
In 2022, certain trees that typically bloom in spring exhibited flowering during autumn instead. This anomaly was attributed to an unusually hot summer, which tied for the hottest on record in the UK and was also among the driest since 1890.
By August, leaves were beginning to turn brown and drop from the branches. The combination of heat and drought hindered growth and caused a surge in abscisic acid levels.
Additionally, fruits such as blackberries, usually harvested from August onward, were spotted as early as June 28 according to Woodland Trust. It appeared that autumn had made its presence felt in the peak of summer.
However, with the onset of meteorological autumn, conditions shifted; while it remained warm (the third warmest autumn recorded since 1884), the drought came to an end. The fifth driest summer since 1890 transitioned into the 13th wettest autumn since 1836, allowing trees to resume their growth.
The Price of Uncertainty
This particular growth cycle was distinct. Typically, trees exhibit multiple growth spurts and periods of dormancy throughout a season, primarily focusing on leaf development. The spring’s initial growth phase is when flowers usually emerge.
Flowering structures are found only in the dormant buds that develop in late summer and early autumn, just before the leaves fall. For these winter-dormant buds to awaken and prepare for blooming, they need two to three months of cold temperatures.
However, in 2022, a mild and wet autumn led many tree species to experience an unexpected burst of new growth; several spring-flowering varieties such as magnolia, cherry, apple, blackthorn, dogwood, hawthorn, horse chestnut, and sweet chestnut bloomed out of season—especially notable in southern England.
Although this occurrence isn’t unprecedented in nature, it marked the first recorded instance for horse chestnuts since 1944. The unusual weather pattern resulted from record high temperatures combined with drought conditions that stressed the trees significantly and led to an accumulation of abscisic acid.
Additionally, growth had ceased abruptly and much earlier than usual; this happened before normal autumn dormancy could take hold. Consequently, the sudden warmth during autumn mimicked late winter conditions for the buds prompting them to grow prematurely.
Unfortunately for these trees, this premature spring was cut short in early December when a cold front originating from Norway—referred to as the Troll of Trondheim—brought extended periods of cold weather and snowfall. This marked one of the most significant cold snaps since December 2010 and severely impacted the young autumnal growth from 2022.
At Anglia Ruskin University’s Writtle campus, some species within our collection have only recently begun showing signs of recovery after suffering damage or loss among approximately 30 typically resilient plants during that harsh end to 2022.
Could this be indicative of future trends? As summer droughts are expected to become more frequent in the UK over time, if these conditions disrupt the growing season for deciduous trees significantly enough their survival may be at risk outside only ideal locations.
Should we be concerned about late leaf drop? I don’t believe so; plants are responding appropriately by preparing themselves for whatever challenges winter may present—they’re simply taking their time with it this year.