What Do Tree Roots Do in Winter?

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Tree roots are inscrutable. While their importance to the aboveground parts of trees and forests is well appreciated by forest scientists, tree roots have always been notoriously difficult to study, obscured as they are by duff, soil, rocks, and darkness. And that’s just in summer; the problem is only exacerbated by winter’s snow and frozen soil. Consequently, while researchers literally have been pondering roots for centuries, there is very little direct documentation of tree root activity in winter.

Still, forest ecologists are a curious and plucky bunch. They continue to build on that legacy of effort, by measuring roots of container-grown trees, painstakingly excavating living roots in forests, and, more recently, using modern imaging technology to watch roots grow in place. And most have come to agree on a generalized view of tree roots through the seasons.

These ecologists describe root activity as periodic, with maximum growth in early summer – especially in deciduous species – and pulses of additional growth occurring occasionally in early fall. And complicating things further, they indicate that not all roots grow at the same time. Even within a single tree, some roots may be active while others are not. However, by all accounts, tree roots in our region are thought to spend the winter in a condition of dormancy. This means they are not dead but rather they overwinter in a resting phase with essential life processes continuing at a minimal rate. Full-on root growth resumes in spring, shortly after soils become free of frost, usually sometime before bud break.

But unlike the aboveground parts of most trees that pass the winter in a prolonged dormancy – marked by unbroken inactivity until spring – tree roots seem to maintain a readiness to grow independent of the aboveground parts of the tree. That is, roots remain mostly inactive but can and do function and grow during winter months whenever soil temperatures are favorable, even if the air aboveground is brutally cold. While roots tend to freeze and die at soil temperatures below 20°F, minimum temperatures for root growth are thought to be between 32 and 41°F. So, if soil temperatures warm to or stay above this minimum, winter roots can break dormancy and become active.

This winter quiescence – where roots are resting but ready – is extremely important for the health of individual trees and, by extension, for forests in general. Indeed, it is this trait that allows evergreens to absorb soil water and avoid winter desiccation in their needles, and it is this trait that allows all species, including deciduous hardwoods, the opportunity to expand their root systems in search of water and nutrients in advance of spring bud break.

But there is an important tradeoff. To maintain this quiescence, a tree’s roots necessarily tend to be much less cold hardy than its stems and branches. This is fine, so long as the soil is sufficiently insulated by a covering of snow against extremely low air temperatures. A good early season snowfall – if it persists – can keep soil unfrozen throughout the coldest of winters. In such years, sustained winter root activity may replace previously damaged roots, may ready the tree for spring bud break, and may translate into excellent aboveground growth during the following summer.

Conversely, a deep snowpack coming later in winter, after the soil is already frozen, can also insulate the soil – but in a different way. These late snows actually keep soil frozen for extended periods – even during January thaws and despite the heat of the earth’s core. The surface layers of forest soils do commonly freeze, and when they do, it is not good for roots or the stems and branches dependent on them. Not only do the roots remain inactive under such frozen conditions, but the freezing, heaving, and cracking of winter soils physically damages roots – particularly the fine feeder roots in the uppermost organic layers. This can trigger a cascade of effects on overall tree and forest health. By reducing a tree’s ability to take up water and nutrients, particularly during spring bud break, winter root damage limits subsequent stem and branch growth in summer. In turn, this can contribute to tree mortality and may even explain pockets of dead trees.

Winter injury to feeder roots is an inherent – and natural – part of forests in northern climates. And, through its effects on individual tree health, winter root ecology is an important determinant of overall forest composition, dynamics, and productivity – even though it is difficult to see and measure directly.

Michael Snyder is the Chittenden (Vermont) County Forester.

 
Discussion
  1. Charla Weiswurm → in San Antonio, Texas
    Mar 06, 2010

    Can you give me any info on tree root growth in warmer climates.  My mother had a problem with her sewer line and a plumbing contractor came out and charged her to clear the line.  Two weeks to the day, the line blocked again.  The plumber was going to charge her again to clear the roots from the line.  He told her that they could grow back and block the line every two weeks because of constant growth.  Would this be true?

  2. dave
    Mar 11, 2010

    All parts of a tree grow relatively slowly, so no, if you trim a tree’s root system, the roots you cut will not grow back in two weeks. I’m not sure i understand the whole scenario here, though. I’d call a different plumber and get his/her take on your mom’s situation.

  3. Roseann
    Apr 18, 2010

    Tree roots grow, and many times pipes are made of clay.  Strong growing roots easily permeate clay.  The roots will not just enter that one spot; they have the potential to be surrounding the pipe.  One solution is to replace the outdated materials (disintegrating clay) of yesterday with schedule #17 pipe.  The roots growing back in two weeks is no different than toilet paper clogging a drain in two weeks time.  Both are barriers, and a stronger barrier is the solution.

  4. peter → in Bronx, NY
    Nov 11, 2010

    any suggestions about root killers such as copper sulfate or rock salt is appreciated for roots in sewer lines—trees are in Havre, Montana

  5. Art Hislop → in Rigecrest, CA 93555
    Feb 15, 2012

    My daughter thinks trees need no water in Winter - none between Oct to Mar. Water is costly & we are rapidly draining our 1,000 year old underground lake source.
    Roots do need water, as I see in literature.
    We have 2 Chase, 1 Catalpa, & 8 Pine trees for shade. A bunch of crepe mrytle bushes for shade.
    My thoughts 1.  Winter- water every 2 to 3 weeks-all are 1 to 5 years old.  2. I think deep watering once a week except when we have long term 100º, 110º +++days. Do you have any papers on winter watering needs? She does not like my opinion! I am a retire 87 year old BSEE engineer from Illinois. This high desert-2,300 above SL is 20-30 nights and 6o to 70 day time in this whole winter. Thanks.
    ah

  6. Chris → in New York
    Mar 04, 2012

    I have a question if anyone has any information. I live outside of new York city. Is there any correlation between the extremely warm winter we are having and the large amount of downed and uprooted trees?

  7. Meghan Oliver → in Corinth, VT
    Mar 06, 2012

    Hi Chris.

    I have not heard anything about the warmer winter causing tree damage of this sort.

    If I had to take a guess, I imagine the large amount of downed and uprooted trees are likely “leftovers” from the early snowstorm much of New England and parts of New York experienced back in October 2011. I know driving to CT from VT a few months ago, I was surprised to see so much tree debris along the roadways, dating back to that October storm. Also, there may still be some downed trees from Tropical Storm Irene.

    If any other readers out there have thoughts on this, please weigh in.

  8. Chuck Maynard → in SUNY College of Environmental Science & Forestry
    Apr 01, 2012

    RE: Watering in the winter.

    I plant lots of trees. I seldom water after the first growing season and never after the second. However, I’m from Upstate NY.  We get a lot of precipitation, essentially year around (We average ~160” of snow every winter.) so contact your local nursery or an arborist that is familiar with your climate. Having said that, I’ll take my best shot anyway:

    It all depends on the tree species and how well-established it is:

    First year- water a lot!, especially for the first month or two after you plant. Research has shown that up to 90% of a tree’s roots can get left behind in the nursery when a tree is dug. It will take several years to re-grow a new root system, so you just about can’t over water in the first year.

    Second & ~3rd year:  The trees should have regrown a substantial root system. So you should be able to get buy with far less frequent, but deeper watering.

    After that, you are (or should be) talking about well-established trees, I think your daughter is correct. You shouldn’t need to water in the winter, or the summer for that matter.

    Again, talk to someone that knows your local climate. You say that Rigecrest, CA is high-desert.  For all I know, you may have to irrigate for the life of the tree!  if so, ya should-a planted cactus!

    LOL

     

     

  9. Garland Sagen → in North Dakota
    Apr 18, 2013

    The plumber cut roots out of my sewer last summer, i have placed root killer, copper sulfate in the line a couple of times that i would be out or the home for a few days.  will root killer, kill the roots in my sewer line?

    thanks.

    Garland

  10. Henry Homeyer → in Cornish Flat, NH
    Oct 10, 2013

    Hi Michael, Would you be willing to be interviewed by me for my weekly gardening column? I’m in several Vermont papers including the Herald, Times-Argus, Reformer and Caledonian-Record. I am keenly interested in knowing more about root growth, particularly in the fall.

    Would you please send me your phone number and a good time for me to call? I’m at 603-543-1307, but am often not there.

    Thanks, Henry

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