In most temperate places in the Northern Hemisphere, leaves begin to fall sometime between late September and November, with northern and higher spots going first. Trees treat the shortening days as their main calendar, and cooler weather adjusts the exact date.1Blooma Tree Experts. How and Why Leaves Change Color in Fall The tree prepares for it, salvaging what it can from each leaf before letting go.
What Tells a Tree It Is Time
Leaves contain light-sensing receptors that let the tree measure how long the days are.2Blooma Tree Experts. How and Why Leaves Change Color in Fall As nights lengthen after late summer, the tree registers that the season is turning.3Blooma Tree Experts. How and Why Leaves Change Color in Fall
Day length is a safer cue than temperature alone, since a warm spell in October would fool a tree that only watched the thermometer. Temperature then nudges the date earlier or later by days to weeks.
Short days and low temperatures together shift the hormone balance inside the leaf.4PubMed Central. Responses of Autumn Phenology to Climate Change and the Correlations of Plant Hormone Regulation Hormones that keep a leaf young drop, and a stress hormone that pushes ageing rises.5PubMed Central. Responses of Autumn Phenology to Climate Change and the Correlations of Plant Hormone Regulation The leaf then begins its planned, programmed shutdown.6PubMed Central. Responses of Autumn Phenology to Climate Change and the Correlations of Plant Hormone Regulation
What you see first is often a faint yellowing or dulling in late summer. Trees on the edges of lots, or ones under stress, tend to turn first. A cold snap can suddenly speed the change across a whole street. Color usually shows days to weeks before leaves actually drop.
Many people think the first frost makes leaves fall. In fact the shutdown is already under way before most frosts arrive. Frost can speed things up, or kill leaves before they finish the job, and leaves killed suddenly by a hard frost may turn brown and cling to the tree instead of coloring well.
Why Leaves Change Color Before They Drop
As autumn approaches, the tree breaks down chlorophyll and proteins in its leaves.7UBC Science. How trees recycle nutrients to produce their autumn colours It then moves the freed nutrients and energy reserves into its branches and roots for next spring.8UBC Science. How trees recycle nutrients to produce their autumn colours Up to about 90% of the nitrogen recovered from a dying leaf comes from dismantling its food-making machinery.9PubMed Central. Why Leaves Turn Red in Autumn. The Role of Anthocyanins in Senescing Leaves of Red-Osier Dogwood
The colors come from pigments. Yellow and orange pigments are in the leaf all summer, hidden by green chlorophyll. When the chlorophyll is broken down, those yellows show through. Birches, aspens and many ashes turn mostly yellow for exactly this reason: they are revealing something already there.
Freed chlorophyll, though, can react with sunlight and damage the leaf’s cells.10PubMed Central. Why Leaves Turn Red in Autumn. The Role of Anthocyanins in Senescing Leaves of Red-Osier Dogwood So some trees, such as maples and dogwoods, make new red pigments that act like sunscreen during the salvage.11PubMed Central. Why Leaves Turn Red in Autumn. The Role of Anthocyanins in Senescing Leaves of Red-Osier Dogwood This shading helps the tree pull back nutrients more efficiently, especially in cold, bright weather.12PubMed. A comparative analysis of the photoprotection hypothesis for the evolution of autumn colours The same tree can be redder on its sunny side than its shaded side. Brown is what’s left once the pigments break down.
The red pigments are made fresh in autumn from sugars still in the leaf. That’s why reds appear later and more patchily than yellows. Making them costs the tree energy, so it only pays off for species and conditions where protecting the salvage is worth the expense.
This also explains why some autumns are more vivid than others. Sunny days and cool, not freezing, nights tend to favor brighter reds. A cloudy, warm fall usually gives duller colors. Drought can cut the season short before colors develop fully.
How a Tree Actually Lets Go of a Leaf
As leaves age, levels of a hormone that keeps leaves attached fall.13EBSCOhost. Leaf abscission A hormone that promotes shedding then rises.14EBSCOhost. Leaf abscission The balance between these two hormones decides where and when the leaf separates.15New Phytologist. Signals in abscission Cells at the base of the stalk form a separation layer, a line of weak cells where the leaf breaks off.16EBSCOhost. Leaf abscission
Once that separation layer forms, a leaf hangs by a thread. A windy or rainy day then brings down many leaves at once, which is why a single storm can seem to strip a tree overnight.
The tree also seals the wound. Even before the leaf falls, cells on the branch side lay down cork-like, waterproof material.17Nature’s Depths. When the Leaf Falls The spot left behind, the leaf scar, is protected from water loss and infection.18Nature’s Depths. When the Leaf Falls The barrier also cuts off water and nutrients to the leaf, finishing it off.19Project Learning Tree. How Trees Survive Cold Winters
You can see these scars in winter. Look at a bare twig for small shield- or crescent-shaped marks. Each one is a sealed leaf scar, and next year’s bud sits just above it.
Why Trees Drop Leaves at All
Deciduous trees drop their leaves as protection against winter damage.20Project Learning Tree. How Trees Survive Cold Winters Broad leaves keep losing water when frozen soil means the roots can’t replace it. Bare branches also catch far less heavy snow and ice, so they are less likely to break.
Evergreen trees keep their leaves, but those leaves are tougher and longer-lived.21AoB PLANTS. Leaf economics of evergreen and deciduous tree species along an elevational gradient in a subtropical mountain Needles are narrow and waxy, so they lose little water. They still drop old needles, just gradually over years. Deciduous leaves are thinner and richer in nutrients, so they work faster in summer. It’s a trade: fast, cheap leaves replaced every year versus slow, durable ones.
Cold isn’t the only hard season. In tropical dry forests, trees go bare when the rains stop, around late October or November. Water shortage, not cold, is the trigger there.
Why Leaf Fall Comes Early or Late Some Years
Location sets the normal date. Peak color comes earlier farther north and higher up a mountain,22Remote Sensing. Linking Spaceborne and Ground Observations of Autumn Foliage Senescence in Southern Québec, Canada because temperature is a main driver of how fast fall progresses.23Remote Sensing. Linking Spaceborne and Ground Observations of Autumn Foliage Senescence in Southern Québec, Canada Moving east or west alone doesn’t change the timing much.24Remote Sensing. Linking Spaceborne and Ground Observations of Autumn Foliage Senescence in Southern Québec, Canada
Your town can still differ from a place a short drive away. Cities are often a bit warmer than the surrounding countryside because pavement and buildings hold heat, which can shift timing in street trees. Low spots collect cold air on clear nights, so trees in a valley bottom may change at a different time than trees on a nearby slope. And different species keep different schedules, so a street planted with one species can look ahead of or behind a mixed woodland nearby.
Stress years can bring leaves down early. A very warm, dry late spring and early summer can make leaves color and drop weeks early.25Agricultural and Forest Meteorology. Premature leaf discoloration of European deciduous trees is caused by drought and heat in late spring and cold spells in early fall A cold spell in early fall can do the same.26Agricultural and Forest Meteorology. Premature leaf discoloration of European deciduous trees is caused by drought and heat in late spring and cold spells in early fall Drought-sensitive trees like beech are especially prone.27Agricultural and Forest Meteorology. Premature leaf discoloration of European deciduous trees is caused by drought and heat in late spring and cold spells in early fall Early shedding after heatwaves and droughts is more widespread than once thought.28Remote Sensing in Ecology and Conservation. Widespread drought‐induced leaf shedding and legacy effects on productivity in European deciduous forests
If your own tree browns and drops leaves in August after a hot, dry summer, drought stress is a common cause, though it isn’t the only possible one. Watering deeply during dry spells can help young or stressed trees. Early drop that repeats year after year is worth asking an arborist about. Scattered early drop on a single branch can mean damage to that branch.
Warming has pushed the average later, with a twist. Over the past century, fall color in some North American trees has shifted 9 to 16 days later.29PubMed Central. A century of climate warming results in growing season extension: Delayed autumn leaf phenology in north central North America Yet warmth before late June tends to make leaves age earlier, about 2 days per degree Celsius,30PubMed. Effect of climate warming on the timing of autumn leaf senescence reverses after the summer solstice while warmth after late June stretches the season out, about 2.6 days per degree.31PubMed. Effect of climate warming on the timing of autumn leaf senescence reverses after the summer solstice
Streetlights can delay leaf fall too. Leaves on branches next to streetlights can change 2 to 3 weeks later than the rest of the tree.32Moravian Geographical Reports. Effects of light pollution on tree phenology in the urban environment Artificial light makes the days seem longer to the leaf. One side of a street tree may stay green after the rest have fallen. This lingering is harmless for the tree in most cases, though those late leaves may get caught by frost before they finish.
Why Some Trees Keep Dead Leaves All Winter
Many oaks, beeches and hornbeams keep dry brown leaves through winter. In some oaks, the sun-exposed upper leaves stay working later, then die suddenly, possibly with cold, before a separation layer finishes forming.33Plant, Cell & Environment. Marcescence and senescence in a submediterranean oak (Quercus subpyrenaica E.H. del Villar): photosynthetic characteristics and nutrient composition These leaves still give up their nutrients just like leaves that drop.34Plant, Cell & Environment. Marcescence and senescence in a submediterranean oak (Quercus subpyrenaica E.H. del Villar): photosynthetic characteristics and nutrient composition
Proposed benefits include discouraging deer from browsing twigs.35Ecosphere. Not all temperate deciduous trees are leafless in winter: The curious case of marcescence Held leaves may also shield buds from frost or drying out.36Ecosphere. Not all temperate deciduous trees are leafless in winter: The curious case of marcescence And dropping leaves in spring means they land when they rot faster and lose fewer nutrients to rain.37Ecosphere. Not all temperate deciduous trees are leafless in winter: The curious case of marcescence
Young trees and lower branches hold leaves most often. The leaves usually drop in spring as new buds swell and push them off, so there’s no need to pull them off by hand. Held leaves on their own are not a sign of disease.
Telling this from a tree in trouble comes down to the twigs. Dry brown leaves attached to otherwise healthy twigs with plump buds are the normal pattern in oaks and beeches. A struggling tree shows other clues, such as dead twigs with no buds, bark that is peeling or cracking, or branches that never leaf out in spring. If a species that normally drops its leaves keeps them on one branch, that branch may have been damaged and deserves a closer look.
What to Do With Fallen Leaves
Decaying leaves supply nutrients to soil life and build soil fertility.38PubMed Central. Nutrient dynamics of decomposing litter and their effects on subsequent mass loss and nutrient release in a subtropical forest Leaves from broad-leaved trees release more nitrogen as they break down than larch needles do.39Journal of Ecology. Litter decomposition and nutrient release from monospecific and mixed litters: Comparisons of litter quality, fauna and decomposition site effects
Leaves also shelter insects. Removing leaves from yards cut the variety of moths and butterflies emerging the next spring by about 40%,40PubMed. Removing autumn leaves in residential yards reduces the spring emergence of overwintering insects because many species spend winter in fallen leaves as caterpillars or pupae.41PubMed. Removing autumn leaves in residential yards reduces the spring emergence of overwintering insects Leaving leaves under shrubs and in beds helps pollinators, and the birds that eat caterpillars.
A middle ground works well. Rake thick mats off lawns, since they can smother grass. Shred leaves with a mower and leave them as mulch, or move whole leaves into garden beds and around tree bases. Keep paths, drains and gutters clear for safety. Large leaves like maple or sycamore form the densest mats when wet. Thin scatterings usually break down without harm, but clear or shred a mat before it sits on the grass for weeks.
How to Know When Leaves Will Peak Near You
Satellite data can now track and forecast fall color stages, from low color to past peak, in near real time.42Agricultural and Forest Meteorology. Prototype for monitoring and forecasting fall foliage coloration in real time from satellite data These readings closely match what people see on the ground.43Remote Sensing of Environment. Evaluating the potential of MODIS satellite data to track temporal dynamics of autumn phenology in a temperate mixed forest Citizen reports from hundreds of sites help map when color peaks.44PubMed. Peak autumn leaf colouring along latitudinal and elevational gradients in Japan evaluated with online phenological data
For a leaf-viewing trip, check state or regional foliage reports weekly from mid-September. Head to higher elevations earlier and valleys later. A dry or hot summer may move peak earlier than usual, and strong wind after peak can end the show quickly.
Foliage maps often sort areas into stages such as little color, patchy, near peak, peak and past peak, so look at the stage rather than a single date. Reports generalize across a broad area, so a particular road or park can be ahead of or behind the map. Compare each report with the last one you read: a change of a stage or two in a week tells you how fast things are moving.