Cypress Knees – What Are They Good For? (Absolutely Nothing?)

29 Dec

Note added in 2018:   The original version of this post dates back a few years.   It has been updated repeatedly due to new thoughts and new information.  If you are looking for a definitive answer on the adaptive significance of cypress knees,  not here.

If you’d like a short answer, personally I strongly suspect the knees allow the living tissues of the root to come up for air, providing a large surface area for the vascular cambium and more importantly the sugar-pumping phloem,   like surface pumping stations along a pipeline.   But much fun stuff to discuss, read on…

Observations, speculations, interpretations and questions about cypress knees date back about as far as botanical writing concerning the Southeastern U.S.    And beyond:

Any interpretation of the knees must extend beyond Taxodium distichum in and near the Southern U.S.   Other Taxodiaceae/Cupressaceae, including Asian representatives, can form knees.   Whatever functions the knees serve must pertain broadly in time, space, and related species.

It is surprising, after centuries of interest, how much interpretation is based on field observation, and how little hard data exist on knee anatomy, cellular structure, and physiology.  Not easy to study, let alone to monitor the growth of woody glaciers.   As field equipment becomes smaller, less expensive, and accessible, we may see a new development of data to help settle questions.    The phloem physiology of Taxodium roots and knees has a target on it.

And while in the realm of ifs, and, and buts,  it may be worthwhile to mention that multiple functions are possible, even if we prefer one adaptation as “the” main driving force behind knee evolution.    One function does not necessarily preclude others.    And also conceivable is no particular function.

A wonderful summary of what came before and an in-depth new analysis  is by H. S. Lanborn in the American Naturalist 1890.    There seems to have been a flurry of interest in late 19th Century botany.   Interestingly, after airing the evidence thoroughly, including the common observation that knees tend to form where aeration is needed, Lanborn disfavored the usual interpretation of cypress knees as air-exchangers  in preference for a pair of alternative hypotheses….that the knees function primarily to brace the trees against root failure during storms and to catch floating debris, which would add weight on top of the root system.

taxodium lamborn

Lanborn drew the diagram above, showing how he felt the knees with their broad bases reinforced the roots (root B stronger than root A).   Lanborn recognized that the knees rise  far higher than needed to serve as root reinforcements, which led him to the secondary hypothesis of the pegs as flotsam-catchers.    The brace-flotsam interpretation thus has a fundamental problem by invoking two functions.

Long before and after Lanborn, the conventional interpretation of the knees is as ventilation devices.  Cypress roots run horizontally through anoxic mud and water and thus need air.   Other wetland plants have aerenchyma tissue allowing gas exchange between deep roots and the soil surface (although problematically such tissue is unknown in Bald Cypress). Despite tilting away from ventilation, Lanborn and others, including me, agree that knee formation seems to correlate (here’s a chance for quantified research) with growth where roots would otherwise be smothered under water or anoxic mud.  Assuming this correlation stands up to systematic study,  it  is in itself evidence for a role in gas exchange somehow.  After all, the knees are called “pneumatophores.”

As Lanborn quoted an earlier observer:

“It seems likely, therefore, that some process connected with the exposure of the sap to the air takes place in these protuberances.”

Taxodium cut off

Solid wood.   (I did not cut the knee.)

The ventilation  concept has complications.    At first glance, it is easy to suspect the knees are the above-water snorkels to a generalized root aeration system.   The knees are wood with no apparent ductwork.  The water-conducting cells are tracheids, which have a membrane (pair of adjacent cellulose cell walls) blocking the passageways (pits) from cell to cell.  The pit membranes are not air-tight but there are millions of them in the path of the gas exchange. But are the knees completely  air-tight? Not necessarily.

First of all, aging knees can become hollow with age, obviously then allowing air to pass readily to the base of an older decaying knee.  Further, a recent study shows  increased oxygen in Taxodium roots near the bases of exposed knees as opposed to under submerged knees.   CLICK for an abstract.

This study you just clicked  contradicts earlier research where aeration was not in evidence. The recent positive results are based on needle-drawn air samples from root wood near the knee bases.   The experiments dug and cut free knees with portions of root attached and transported the knees to a lab, then studied the knees in an aquarium many hours later under varied water  depths.     The experimental technique complicates interpretation of the results:

Most importantly, root xylem under natural conditions usually has negative pressure, “suction,”  as transpiration pulls  water and any dissolved oxygen inward to the trunk and up the tree, away from the feeder roots.  The instant the roots are severed  the negative pressure is eliminated,  and any related gas dynamics are lost, making it tough to know what to make of oxygen levels in such depressurized dying roots.

Further, it is not clear if the needle-drawn air samples were consistent in origin, and each from sapwood, heartwood, wood rays, or pith.     Root damage and changing conditions could elicit a hormonal response with metabolic consequences.    It was not clear if the knees used were solid, partly decayed, or even hollow (probably not, but their structural condition matters).      It was also unclear if oxygen entry was via wood, or alternatively along the porous bark we’ll return to momentarily.   Maybe even from air trapped in the bark during transport to the lab.

cypress knees riverben jb

By John Bradford

Despite all  these worries,  my hunch is to agree that some oxygen may well reach the base of the knee and attached adjacent root.     But then a bigger question:  exactly what living tissue (wood is mostly non-living), and where does the oxygen serve?     Perhaps oxygen makes its way along the length of that woody root snaking through the mud in ways not adequately explained anatomically, and  then somehow ventilates the system.   The concept of effective long-distance air movement in the wood is problematic.

But something else moves the length of the root through known channels under pressure and requires oxygenation: the phloem sap. So maybe the oxygen entering the knee (and possibly reaching adjacent roots) is most important at and near the knee, not to ventilate the root, but rather as a periodic surface-oxygenating rejuvenation station for the phloem, a pumping station.

There is a vague and inchoate hint of this as far back as Lanborn.  Review the quote dating to 1879 above.  It does not refer to aerating the root, but rather to exposing the sap to the air.    The root system pumps sugary phloem sap all the way from the leaves out to the tips of the tiny feeder roots.    This requires oxygen.    Oxygen is in short supply under that water and wet mud.    Knees rising periodically and exposing the phloem (and associated cambium) at the knee surface to a periodic breath of fresh air may keep the sap flowing.

Knees often form on arched roots.

Knees often form on arched roots.



A photo my dent1st would love (exposed roots).   This big old knee, and many other like it, does not say “snorkel” to me.  It says “root hub.”  It sure could be a place where radiating roots get a dose of life-giving gas exchange.

The phloem is a continuous living sleeve covering the entire root.    It has two layers:  a soft spongy outer dead layer (bark) covering the living breathing pressurized inner layer where the sugary sap flows.   Just under that is the likewise living vascular cambium responsible for growth in diameter, and all this exterior to the wood.    Think of the wood as a wire and the phloem/cambium as the plastic insulation covering the copper.

Air entering the knee would contact and presumably benefit the phloem immediately.    The knees has a large surface area potentially exposing a lot of phloem and cambium to a lot of air,  a giant gill, with no need to postulate gas exchange through dead wood under negative pressure in the wrong direction.

This would explain why the knees rise so high…to provide a big exposed surface.   This fits with the acknowledged prevalence of knees in situations  where oxygenation is needed.    And localized gas exchange at and near the knee is consistent with the recent observations of oxygen getting into the knees and adjacent roots, diffusing through wood and/or  through the porous bark, and maybe even carried along as dissolved oxygen in the reinvigorated outbound phloem sap.

Do the knees give the otherwise submerged cambium and phloem a metabolic shot in the arm?


Taxodium on shore

Knees on shore


There have been additional knee proposals.  Here are some:

  1. Serve as variable-height  launchpads for tufts of feeder roots.   The idea here is that the water levels rise and fall during the very long life of a bald cypress.  As the water level fluctuates small feeder roots might branch off the trunks and knees to exploit the oxygen-rich water at the water surface.    I have not seen such tufts personally, although supportive observations are reported.  I wonder why so many trees would make so many knees not apparently engaged in this way, seemingly an expensive investment in a rare contingency.  I  wonder similarly if the cluster of knees could make enough of a contribution to justify the substantial investment in those big woody  knees.    If the knee does sprout a tuft of feeder roots in the water  allowing nutritional supplementation (from water?)  this input is separated physically from the exponentially greater mass of feeder roots at the distal end of the root system.   The water and dissolved nutrient input from knee-roots would go into the xylem and would be pulled to the trunk by the inward/upward direction of the transpirational xylem stream, feeding the above-ground tree, not the distal root system.   Readers interested in exploring this intriguing hypothesis should consult, Stahle, D. et al. Quat. Sci. Rev. 34:  7. 2012.
  2. Allow methane to escape. (Here is some info on methane release)
  3. Rise into stumps and extract nutrients (No evidence and no apparent adaptation to this purpose, and knees generally encountered rising into thin air.)
  4. Food storage (no traction)
  5. Will grow into new trees (no)
  6. No function, merely tumors or burls. (See below)
  7. Giant thorns.  (See below)


Cypress knees are often not single units, but rather commonly clustered, hinting that whatever “set off” a knee kicked off multiple knees in one fell swoop.  Some are branched, or are the fusion of adjacent knees creating a branched appearance.  Often the knees start from the high point of looping roots, as if a giant knee formed on top of the St. Louis Arch (photo above).

knee cluster

The could be burls, reaction wood,  hormonal aberrations, or the results of injury.  Interestingly,  Bald Cypress trunks will grow a burl at points of abrasive contact.   The knees are so widespread and so abundant,  their existence as mere burls or growth aberrations is not an appealing hypothesis.



Don’t trip over those “knees.” This painting is by Brian Regal (in The Evolution and Extinctions of the Dinosaurs by D. Eastovsky and D. Weishamel. 1996) . This is based on the Triassic Chinle Formation in Arizona, probably earlier than the appearance of Taxodium, although maybe not before its similar ancestors.)


An intriguing if far-fetched possibility is maybe the knees block large herbivores.   (They sure block canoes.) Even if unlikely, fun to envision.  Members of the Taxodiaceae (or Cupressaceae) date back to dinosaurs, some of them hunky hungry herbivores.   More recently and locally, Mastodons ate Bald Cypress in massive quantities.  In the Aucilla River in the Florida Panhandle remain to this day literally truckloads, tons, of preserved Mastodon dung.  Guess what it is made of mostly:  Bald Cypress twigs.



Yep—that’s it. 12,000 years old yet still fresh. (Photo from UF Museum of Natural History)


Cypress knees in creek 2



Newsom, L.A., and M. C. Mihlbachler. Mastodons (Mammut americanum) Diet Foraging Patterns Based on Analysis of Dung Deposits.  Springer. 2006.



Posted by on December 29, 2014 in Bald Cypress


Tags: , ,

15 responses to “Cypress Knees – What Are They Good For? (Absolutely Nothing?)

  1. Martin

    December 31, 2014 at 9:48 am

    Ooo, George, I like it! If you want to see some dramatic examples of long-term abrasion and repeated human impact on individual knees, you need look no further that the cypress knees in and around the canoe dock/boathouse at the Trapper Nelson site. He (Trapper) built his docks right on top of existing knees, and some that were cut off flush with the dock decking quickly healed, and have been trampled on by thousands of visitors, lo these 50-plus years.

  2. George Rogers

    December 31, 2014 at 5:03 pm

    Good idea! Time to take the boat ride anyhow. Last time I did that, I took my parents there, but they were not interested in the knees. So who killed Trapper anyhow?

  3. Martin

    January 4, 2015 at 5:35 am

    Ask ScaryCheri!

    • George Rogers

      January 4, 2015 at 2:02 pm

      Does she appear only at midnight on cloudless eves?

  4. Steve

    January 4, 2015 at 7:39 am

    Elaborating on the #7 concept, knees may have helped thwart large herbivores from pushing down trees to get at the tasty leaves/needles at the top. I understand that this is a common tactic by Elephants today.

    I wonder what old Tom Gaskins would have thought about this discussion (he was the owner of the Cypress Knee Museum located in Palmdale).

  5. George Rogers

    January 4, 2015 at 2:03 pm

    I’ll bet he gave it some thought…you gotta know “the” real answer would embarrass us all if revealed…

  6. theshrubqueen

    January 4, 2015 at 5:12 pm

    Great post, the dino dinner abatement concept?!

    • George Rogers

      January 5, 2015 at 6:33 pm

      I was working in the swamp today and the knees sure abated me.

  7. Mike Yustin

    January 5, 2015 at 10:56 am

    One question though – isn’t it true that cypress knees only grow to a height that marks the high water line in an area?

    • George Rogers

      January 5, 2015 at 6:31 pm

      This high water idea seems to be out there. I see plenty of knees on “dry land” and sticking high up above any conceivable high water line in places. I Given that they grow at the rate of glaciers, from a physiological standpoint, I can’t even imagine how they would mark some fluctuating high water line. Being submerged from time to time probably has no effect on growth, and I’m very certain they have no need for “high water” to keep rising. You see knees rising from never-submerged lawns and other plantings when mowers don’t tangle with them. If that idea has merit, data would help. I’m beyond skeptical.

  8. Dane D'Avy

    November 27, 2016 at 11:22 pm

    Carrying only a gps to track my new hunting area, I followed a 70-foot long, 10-foot wide ridge, trying to get the most accurate reading. I turned around the point and headed up the sandy bank. I tripped and fell HARD! I did not notice the small bumps (knees) protruding. I did, however, feel my left quadriceps and tendon tear – also felt then next 3 knees deliver blunt force trauma to my abdomen and break one rib!. The final thing I saw was the fourth – a round-top 4-incher, which I was fortunate enough to catch with the palm of my right hand, directly in front of my throat!
    I realized I was injured very badly. No cel coverage. My safety would be my car, luckily only 350-yards away. I literally screamed with every step on my left leg. Once in my vehicle, I took 800mg ibuprofen. When I reached cellular signal coverage, I called my wife and told her to prepare for an E.R. visit. Hunting club members were able to swing my feet out, remove my boots, and replace my feet back into the vehicle. Left foot resting on a pad on left floor, my leg was virtually immobile, thus no flexion or extension of lower leg AND no pain. Following a 2 hour drive to E.R., it was determined I had sustained a very severe tear in my left Quadriceps (3 of the 4 muscles) and had only 10% of the common tendon remaining attached at kneecap(patella). I had surgery to reattach tendon to patella. Three weeks post op and tomorrow I will be starting 4 – 6 months of physical therapy.
    Primary insurance card was presented at E.R. When injury occurs on hunting lease property, you also have to notify the land owner. Their insurance adjustor contacted me. They are considered third-party insurance.
    Cypress knees can inflict severe injury, simply from tripping on one. There are many more waiting for your fall!

    • George Rogers

      November 28, 2016 at 11:51 am

      Ouch! Thank you for the fist hand account. I’ve tripped on them too, painfully, but not that painfully. Hope the recovery and insurance aftermath goes well. Cypress knees are the “root” of much evil.

  9. American MegaConifers

    April 28, 2017 at 9:50 pm

    There is a nature writer from the 1950’s named Rutherford Platt. He wrote several books but one in particular had a very interesting theory about the knees. Here is the link to the quote from “The Great American Forest” (out of print).
    I am looking for a bald cypress expert to talk to about a paper I am writing. Do you know anyone?

    • George Rogers

      April 28, 2017 at 9:57 pm

      Thanks. Afraid I do not know an answer however. Good luck.

      • George Rogers

        April 28, 2017 at 10:25 pm

        Took me a minute to find a link straight to the reference. This works:

        Interesting for darn sure, and see that R. Platt postulated a possible trigger to make root primordia launch but doesn’t that leave the adaptive significance of the knees still unexplained? That said, though, my personal leading unsupported thought is not that far beyond R. Platt’s…that maybe the knees don’t have as much oh my purpose as some hope, but rather are extreme reactions to a rare chance to expose themselves to air and gas exchange, maybe triggers by some combo of air exposure, light, and even abrasion. Who knows. I suspect the boring hypothesis is that the knees represent the roots “coming up for air” with a vengeance, creating a big surface area for gas exchange, not as snorkels to carry oxygen down below, but rather to get a big gulp of life giving air when possible and act as a phloem pumping station down into the root. If I’m missing an explanation of the adaptive significance in Platt, let me know. I miss things for sure. I don’t see is suggestion as at odds with too many notions of what the knees are all about.

        Platt talks about an oxygen transport system in the bald cypress, but honestly I don’t see that. Conifers are made of closed tracheids, which do not conduct air well, in contrast with the larger open-ended vessel elements in willow that could conceivably (?) carry oxygen. And Platt talks about enzymes moving from the knees into the deep roots. That seems to indicate a misunderstanding of enzymes. Hormones moving from the knees into the roots are possible. Hormones, sugars and other life sustaining benefits could be pumped in the fashion I just described for the pumping station.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

%d bloggers like this: