When do trees grow their leaves back?

After months of bare branches, the green comes back in a rush. But a tree doesn’t just wait for the first warm day. Here’s what has to happen first, the stages a bud goes through as it opens, which trees lead and which lag, and how spring moves north and up the mountains, the reverse of fall color.

What a tree waits for

A tree that leafed out during a January thaw would lose its new leaves to the next freeze, so trees use more than one signal before they commit.

Enough cold. For many temperate trees, winter cold is an important part of releasing bud dormancy. Some species have a minimum chilling requirement, while in others the amount of chilling affects how much spring warmth is needed before bud break. Chilling is usually counted in hours between about 32°F and 45°F, and the amount varies by tree: Purdue gives sugar maple 1,000 to 1,200 hours, tulip poplar 800 to 1,000, and sweetgum 600 to 1,000 (Purdue Landscape Report). There’s more on that winter rest in what trees do in winter.

Enough warmth. As winter ends, accumulating warmth becomes increasingly important. Scientists often track that warmth with growing degree days, a running total of how far each day’s temperature rises above a base, and bud break occurs after a species has received the combination of chilling and warmth it needs. The two work together: a tree that got plenty of chilling needs less warmth to get going, and one that came up short needs more, which can mean late, uneven bud break after a mild winter (Purdue Landscape Report).

Long enough days, for some. Some species also wait for the days to lengthen, which may protect them from leafing out too early; others ignore day length and go when it’s warm (Wisconsin DNR). In a study of 173 woody species, only about a third used day length as a cue (Zohner and others, Nature Climate Change). Beech is the best-known stickler. In one experiment, beech buds kept on 8-hour days broke 41 days later than buds on naturally lengthening days, and each bud seemed to sense day length on its own (Zohner and Renner, New Phytologist).

The stages of leaf-out

Phenologists, the scientists who track the timing of seasonal events, break leaf-out into stages so observers everywhere record it the same way. These follow the USA National Phenology Network’s definitions where it has them.

1. Dormant bud

All winter the bud is tight and hard, its scales overlapping like shingles. Inside, the leaves for this spring are already formed. In oaks, the shape of the leaf can already be seen inside the bud months before it opens (Arnold Arboretum).

2. Swelling bud

As the bud wakes, it plumps up and the scales loosen, often showing lighter-colored edges. In Vermont, red and sugar maple buds are usually first to swell, followed by yellow birch and beech, with ash later (Northern Woodlands).

3. Bud break

The scales part and a green leaf tip shows at the end of the bud. The USA National Phenology Network calls this “breaking leaf buds.” The first leaf hasn’t unfolded yet (USA National Phenology Network).

4. Leaves unfold

The young leaves push out and open. By the network’s definition, a leaf counts as unfolded once its whole length has emerged from the bud and you can see where its stem meets the twig. The bud scales soon drop, leaving the ring of scars you can find on the twig in winter.

5. Leaves expand

The new leaves grow quickly but stay thin and soft, often pale or yellow-green. Many hardwoods put out reddish young leaves, colored by the same kind of red pigment, anthocyanin, that shows up in fall. They turn green as the leaves build chlorophyll, and cold or cloudy weather keeps them red longer. The red may help young leaves cope with cold and screen out ultraviolet light (Northern Woodlands). The network calls this stage “increasing leaf size,” when most leaves are still short of full size.

6. Full leaf

The leaves reach full size, toughen, and darken, and the canopy closes overhead. How fast trees get there varies: in Vermont, red maple starts earliest but takes the longest to reach full leaf, while ash, one of the last to start, finishes fastest (Northern Woodlands). Some trees, such as spruces, carry all of the season’s leaves in the bud. Others, such as balsam poplar, keep adding new leaves into late July or early August (University of Alaska Fairbanks).

From here the leaf does its summer work, described in what leaves do for a tree.

Which trees go first

A study of nearly 1,600 woody species growing in botanical gardens found that leaf-out can stretch over as much as three months, and that species leaf out in nearly the same order wherever they’re planted. Shrubs went about 10 days before trees. Honeysuckles, willows, lilacs, and apples were among the early groups; oaks, beeches, honey locusts, and grapes were among the late ones (ScienceDaily, on research led by Boston University).

Part of the reason is the wood. Oaks, ashes, and hickories are ring-porous: they move most of their water through wide vessels in the newest ring of wood, and oaks need to build fresh ones each spring before their leaves can open (Wisconsin DNR). In the botanical garden data, ring-porous species leafed out about 9 to 12 days later than species with diffuse- or semi-ring-porous wood, such as maples and birches (Panchen and others, Annals of Botany).

Some trees are so late they worry their owners. Black walnut, hickory, pecan, honeylocust, Kentucky coffeetree, and northern catalpa need more heat to start, and Iowa State Extension suggests waiting until at least June 1 before deciding one of them didn’t make it through the winter (Iowa State University Extension).

At the other end, many invasive shrubs get a head start. In a study of more than 800 shrubs at nearly 400 sites, invasive species kept their leaves up to 77 days longer than native shrubs, by leafing out earlier and dropping their leaves later (USA National Phenology Network).

Flowers before leaves

Some trees bloom on bare branches. Red and silver maples flower while still leafless, in March at Boston’s Arnold Arboretum (Arnold Arboretum), and serviceberry and eastern redbud flower before their leaves too (Penn State Extension). American elm also usually flowers before it leafs out, which lets wind carry its pollen without leaves in the way (Yale Nature Walk). A red haze over a hillside in early spring is usually red maple flowers, not leaves.

Sap comes first

Before any of this shows, the sap is moving. Maple sap runs best when nights fall into the 20s and days climb into the 40s (Northern Woodlands). As the buds begin to swell, the tree starts sending a different mix of nutrients to them, and syrup made from that late sap picks up an off flavor sugarmakers call “buddy.” It signals the end of the sugaring season (The Maple News, Cornell Maple Program).

How spring travels

Spring green-up runs the opposite way from fall color. It starts in the South and the lowlands and moves north and uphill. A century-old rule of thumb from entomologist Andrew Hopkins, known as Hopkins’ bioclimatic law, puts the shift at about four days for each degree of latitude and for each 400 feet of elevation, other things being equal (Alabama Cooperative Extension System). Real springs are messier, but the pattern holds well enough to watch it climb a mountainside over a few weeks.

The spread across the country is wide. The Spring Leaf Index, which models the leaf-out of early-season lilacs and honeysuckles rather than trees generally, reaches Miami around mid-January, and NOAA gives average dates of January 22 in Houston, February 16 in Charlotte, and March 25 in Pittsburgh (NOAA Climate.gov). The index comes from the USA National Phenology Network, which posts each year’s progress as it happens (USA National Phenology Network). Trees come later: in the botanical garden study above, shrubs leafed out about 10 days ahead of trees on average, and at Harvard Forest in central Massachusetts, the main forest trees typically leaf out around the first week of May (Harvard Forest).

Fall runs the other way: color starts in the North and the high country and moves south and downhill. See the fall foliage calendar for that wave.

When frost hits new leaves

The biggest danger of leafing out is a hard freeze afterward. A frost after a warm spell can damage new buds and tender leaves, and the tree then has to leaf out again from backup buds, which takes time (Iowa State University Extension). The cost can linger: a study of 870 species found that a late frost delayed the next spring’s leaf-out by about a week on average, with broadleaf trees hit hardest (USA National Phenology Network).

Spring is coming earlier

The first signs of spring are arriving earlier across much of the country. Using the USA National Phenology Network’s Spring First Leaf Index, Climate Central found the index trending earlier in 212 of 242 U.S. cities, by six days on average since the early 1980s (Climate Central). The EPA reports earlier leaf and bloom dates across much of the country, but later dates in much of the South and part of the Upper Midwest (U.S. EPA).

Does an early spring mean an early fall? A 2020 ETH Zurich study of six European tree species found that trees that photosynthesized more in spring and summer began their fall leaf aging earlier (Life Science Zurich). Richard Primack, a Boston University biologist, cautioned that it might not be a universal pattern and that in New England, moisture may matter more (Popular Science). For how Leaves.com estimates fall timing, see how the forecast works.

Spring leaf-out at a glance