How Do Trees Know When to Leaf Out in the Spring?

How Do Trees Know When to Leaf Out in the Spring?

Illustration by Adelaide Tyrol

Thankfully, most northern trees aren’t dead in winter. They’re dormant – very much alive but relatively inactive. They still burn energy maintaining life, but little goes on in the way of growth. It’s just too cold. And water, being frozen, is unavailable. Dormancy is how trees avoid such unfavorable environmental conditions without moving to Florida.

The strategy makes sense. In most northeastern trees, the buds that burst open in spring were formed the previous summer. Inside these buds are the miniature beginnings of this coming year’s new shoots. They too were made last summer, but they spend the winter dormant and protected under bud scales until favorable growing conditions return in spring. And in what might well be the height of arboreal prudence, these dormant buds don’t even break during midwinter thaws. It’s a good thing, too, considering the alternative.

Imagine Joe Q. Sugar Maple dormantly waiting out November, December, and a nasty run of cold in early January. Then comes the thaw. You know the kind. It starts with some warm rain followed by spring-like breezes and temperatures above freezing for days. But if that sugar maple, impatient for spring, breaks bud and sends out those fleshy young shoots into the unseasonable warmth, its leaves could get zapped and blackened by the returning freeze. It’s a perfect way to lose the coming year’s growth, victimized by a cruel winter hoax. Happily, this rarely happens.

Trees avoid such midwinter mishaps not because they are programmed to simply wait a fixed time period until conditions are right. If that were the case, trees would break bud on the same spring day every year and, as we’ve all seen, the date of bud opening can vary by weeks from year to year. And clearly it’s not just warmth that stimulates budbreak, as January thaws demonstrate. Somehow, trees seem to “know” when true spring arrives and respond by breaking bud. Sure, some forest trees get nipped by late frosts, but generally they get it right.

It turns out the all-important environmental cue for spring budbreak is cold. That’s right, trees must first go through prolonged exposure to chilling temperatures (-5 C to 10 C) before subsequent exposure to warmth will force bud break. These environmental cues trigger physiological responses in trees, altering the balance of hormones and enzymes involved in promoting and inhibiting growth. Of course, the exact chilling temperatures and duration of exposure needed to break dormancy vary by species, individual, and geographic location. Some species need weeks of chilling, some need months. Though their actual dates of bud-break vary, you can count on trembling aspen every year, for example, to be among the first in New England to break bud and white ash to be among the last. Sugar maple tends to be in the middle, needing as many as 2,000 hours of exposure to low temperatures before it will flush. And whereas a red maple growing in the Upper Valley may require a few months of cold, the same species growing natively in Florida may require no cold at all.

Presumably, native trees growing on their natural sites have evolved a synchrony with the annual temperature cycles of their site. Everything’s just fine so long as trees are not planted out of their natural range and nobody messes with the temperature cycles. We’ve got the first part reasonably well in hand. Most nurseries, for example, pay close attention to frost hardiness and avoid stocking and selling southern source trees for planting in the north. The second part – messing with temperatures – is a bit more dicey.

Over generations, trees have proved adaptable to gradually changing environmental conditions. It’s when environments change rapidly that trees, like most of us, get into trouble. One alarming possibility about global climate change is its potential effect on the timing of dormancy in trees. If the pace of global warming exceeds a native tree’s ability to adapt, it may well lead to some very strange things happening in the woods. Some tree physiologists have suggested that certain trees might not receive enough chilling to break dormancy (much like sugar maples from Vermont that don’t break dormancy when grown in Georgia). Research continues, and although absolute predictions are few, there are suggestions that different tree species will respond differently to climatic warming and this could seriously alter their competitive abilities, their survival, and, eventually, their regional distribution. Happy spring.

Michael Snyder is a forester and freelance writer in Vermont.

  1. Tim
    May 09, 2013

    It is May 9th and all the trees here look like January. What is going on? Chemsprays (geoengineering) killing trees? Never ever has it been this late. They look dead.

  2. Theresa Parker
    Jun 21, 2013

    We have a Brandywine Maple. It is still dormant other than suckers. What if anything can we do to save it?

  3. Janina
    Mar 19, 2015

    I have 2 1-year-old Japanese red maple trees I potted last year. I brought them indoors when it started to get cold outside, and they got rid of their leaves in my apartment. They have been indoors in my warm apartment all winter long. They are now growing leaves again. I don’t think the cold timeline has anything to do with them regrowing their leaves. It may be a certain type of sunlight that influences regrowth?

  4. Sue Bernstein
    Jun 04, 2015

    I have two dwarf Japanese Maples, one for only a year.  While the older tree leafed out a month ago (May), the other has yet to do so even though it has buds and is green beneath the bark.  Is there anything that I can do to stimulate its leaving out?

    Sue B

  5. Diane Mason
    Jun 19, 2015

    I planted a locus tree this past November. It is now mid June and still has not leafed out. When I cut off a bud, you can see green inside. Is there still hope for my tree? Can I do anything to help the situation? Thank you so much.

  6. Michael Landis
    Jul 01, 2015

    I have 5 maple trees that have not leafed out this year and it is now July 1!  When I break apart some of the branch it is still green.  Is there anything I can do?  Do you think they will be ok?

  7. Dave
    Jul 03, 2015

    If the tree was defoliated by insects, or frost, or something external, then it should recover and be fine. If it just didn’t set leaves, it’s a very bad sign. If you still see green you might give it a year just to be sure, but more than likely it’s dead.

  8. David Neuroth
    Feb 20, 2016

    We had a very warm December with no snow and record setting high temperatures, unusually warm for about three weeks.

    Then it turned cold until late January. At that time it started to look like a very early spring so I tapped my 75 maple trees. Sap ran well and I boiled it down immediately but to my disbelief it produced black bitter syrup. I had to throw it out and dumped all my sap. My theory is that the buds were already out and all I got was “bud sap”. Then temperatures went way down setting a new record at minus 36F below zero in Watertown. My questions are: 1) Was my bad syrup due to bud sap and 2) If so, would the super cold kill all the buds and the trees will then begin to produce good sap until new buds form?  Also what will this do to the maple trees?

  9. Dave
    Feb 23, 2016

    My guess is that the warm temperatures weren’t the cause of your bad syrup. I’d try again and see if things are better the next run. It’s hard to say what affect the warm winter will have on the trees. The sap may be less sweet this year because of the warm December. Unless the buds break, they should be able to deal with extreme cold.

  10. Stephen Twohawks
    Apr 10, 2016

    We have a Brandywine Maple about 5 seasons in the ground. This season we have buds but no leaves yet. Tree is not dead - the buds are green and strong. Every other tree in our area has foliage except our Brandywine. Daytime temps have been in the 70’s nighttime mostly in the 50’s. Any ideas on why we have no leaves here in mid-April??

  11. morris
    Apr 10, 2016

    My maple tree is about 4 years old now. Before, at this time of the year, it had leaves on it. So far, no leaves, but it has budded out. However, my larger maple trees are doing well and are all healthy. Can you tell me what is going on with this young tree? Thank you.

  12. jeanette
    Apr 24, 2017

    I planted a maple tree in November. It’s late April and nothing is showing growth.

  13. Alan
    Jul 27, 2017

    I live in an area with lots of mixed hardwoods and conifers. Some have been here for as long a 80 years, others I have planted in the last 8 years.

    I have made some unscientific observations over the last three to five years:
    It seems that our “summer-fall” season is lasting much later into the calendar year than it used to with first frost well into October, winters seem milder, and it is taking much longer to warm up in the “spring-summer” than it used to, with last frost in mid-May.
    Also, especially during the last three years, new growth of many of these trees seems much more profuse than in the past. For example, an approx. 10-year old Austrian pine has this spring added between 10” and 13” of new growth, and an approx 8-year old sugar maple has added between 16” and 24” of new growth. These trees are not given any special treatment like mulch or fertilizer either.

    So, I realize there are many other factors that might affect tree growth from year to year, and I also realize my perception and recollection of what is normal growth may be suspect, but ...
    While what’s happening may be neither good nor bad, it seems to me it is not normal for this area of the world.

    Can you tell me about any similar observations and explanations for why this is happening?

  14. Dave
    Aug 01, 2017

    Alan, Hubbard Brook Experimental Forest in NH—a place with perhaps a similar climate as yours in WI—has been doing long-term studies related to climate change. Check out their site:

  15. Laura Jacobs
    May 23, 2018

    We have a Shantung maple that we’ve had for many years. Precipitation was down over the winter here. It is now spring going into summer and our tree has leaves on the outer part of the tree but the inside is bare. Do you think it might be getting a late start or is it dying? We have never fertilized it, so we are going to do that. Should we give it extra watering?

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