Comfrey – Is it a Dynamic Accumulator?

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Robert Pavlis

Comfrey (Symphytum officinale) is probably the most popular dynamic accumulator. Permaculturists swear by it, and organic gardeners use it frequently. Thousands of web sites make all kinds of claims for it and if you believe the claims everyone should be growing comfrey to add nutrients to compost, mulch soil, and make plants grow better.

All of these benefits are derived from the fact that comfrey is one of the best dynamic accumulators – or so people claim. It is time to have a closer look at this miracle worker.

Comfrey dynamic accumulator
Comfrey – Is it a Dynamic Accumulator?

What is Comfrey?

Comfrey is the common name for a perennial called Symphytum officinale which grows quickly in zones 4-9, produces lots of large leaves and has pink or white flowers which are not overly ornamental. It has a tap root similar to dandelions.

The common variety is quite weedy and spreads around the garden. Since it is difficult to remove most people grow a sterile version called Bocking 14, known as the Russian comfrey, Symphytum × uplandicum. This  variety is a cross between Symphytum officinale and the rough comfrey, Symphytum asperum.

Microbe Science for Gardeners Book, by Robert Pavlis

Is Comfrey a Dynamic Accumulator?

I discussed the definition of dynamic accumulator in a previous post; Dynamic Accumulators – Do They Exist? If we use the common definition for dynamic accumulator all plants would qualify. In my previous post I refined the definition to the following:

A dynamic accumulator is a plant that will absorb and retain, in the leaf, at least one nutrient at levels that are at least 10 times higher than the average plant.

How does the nutrient content of comfrey compare to an average plant? A common claim is that comfrey contains high levels of nitrogen, phosphorus and potassium.

Stephen Legaree (see video below) had some comfrey analyzed and came up with the following dry weight value for total NPK, 3.5-1.2-8.4. The book, Comfrey, Past Present and Future, (as per ref 1) reports an NPK of 3-1-4.8 (converted to dry weight). These two values are fairly similar so for the purpose of this discussion I’ll round things off to 3-1-5.

What is the NPK of some common plants. I thought it would be easy to find such a list, but I couldn’t find a good one. If you know of one, please post the link in the comments. References 2 and 3 provide values for organic fertilizers which give us some values for NPK.

  • Alfalfa 2.5-1-2
  • Clover, crimson 2-0.5-2
  • Corn gluten meal 9-1-0
  • Cotton seed meal 6-0.4-1.5
  • Rye, annual 1-0-1
  • Seaweed 1-0.5-1
  • Soybean meal 7-2-1

Are these average plants? None of the above plants are dynamic accumulators (ref 4), so they are not considered to have high NPK values. It seems reasonable to consider them to be average plants.When you compare comfrey at 3-1-5, it is not much better than the average list – it is certainly not ten times better.

Using this criteria comfrey is not a dynamic accumulator for NPK.

 

YouTube video

If the above link does not work try this one: https://www.youtube.com/watch?v=HqjW4EtUCe8

What About Other Nutrients?

Maybe comfrey accumulates other important nutrients?

Mike H. in One Thing Leads to Another had a look at dynamic accumulators and used Dr. James Duke’s Phytochemical and Ethnobotanical Databases  to develop a spreadsheet of dynamic accumulators showing the amount of nutrients they accumulate. Comfrey accumulated such small amounts of nutrients that it was not considered to be a dynamic accumulator.

If you have a look at the above mentioned spreadsheet, comfrey does have higher levels of calcium, at 2,000 ppm, but this is far below the other accumulators who are up in the 10-20,000 ppm range. Besides, most soil has lots of calcium. When you look at the other nutrients, magnesium, iron and manganese for example, comfrey has very low amounts of these nutrients compared to other accumulators.

Comfrey is not a dynamic accumulator!

Does Comfrey Have Deep Roots?

Comfrey does not accumulate a lot of nutrients but that is not the only benefit attributed to this plant. It is claimed that comfrey has a very deep tap root system that is able to retrieve nutrients from deep in the soil. The theory is that comfrey mines nutrients for you that are not available to most plants.

Before looking  at the root system of comfrey, I’d like to point out one other mistake pro-comfrey people make. Even if a plant has deep roots, it does not mean that most of the nutrients in the plant are obtained from deep soil. Plants that have deep roots also have lots of shallow roots. It is quite possible that it is these shallow roots that are responsible for most of the accumulation of nutrients.  Robert Kourik, the author of Understanding Roots, had this to say in a recent discussion on The Garden Professor Facebook Group, “…. some plants are more efficient at absorbing some nutrients compared to others. Is this due, as many gardeners assume, to deep roots or is it due to more efficient accumulation at surface soils. This remains a grossly unresearched dynamic.”

In his book, Robert Kourik makes the point that “the vast majority of comfrey roots are found in the top foot of soil just like the roots of most other plants”, (ref 5). Comfrey may have deep roots, but the deep roots are not used to absorb nutrients. The deep root is used for food storage and for gathering water in times of drought. Nutrients are gathered using shallow roots.

Does comfrey have a deep tap root?

Root depth depends very much on the soil. Just because a plant has 5 foot roots in one place does not mean it will have 5 foot roots in your garden. Bocking 14 is reported to have up to 6 foot roots and that is possible in some places.

The important question to ask is, how deep are comfrey roots in your soil? The reality is that this is hard to measure. You can be quite sure that most reports of deep roots found on the internet are not the result of actual measurements.

The root system of one comfrey plant has been excavated by scientists and they found a fairly shallow root system, at about 30 cm deep, and no tap root.

Comfrey root system
Comfrey root system, source: Wurzelatlas (publisher DLG-Verlag)

Is Comfrey a Dynamic Accumulator?

The data indicates it is not a dynamic accumulator. It is just an average plant.

It may have deep roots in your soil, but there is no easy way to know this. Even if it does have deep roots, the nutrients it stores in its leaves are most likely collected using roots close to the surface of the soil.

One thing it does do is produce a lot of leaves quickly. If you are looking to grow a source of greens for the compost pile it might be a good choice. Keep in mind that the space dedicated to comfrey can’t be used for other plants. Harvesting and composting comfrey also adds extra work. Is it worth it?

Personally, I don’t see the attraction for growing comfrey and the science does not support its use as a dynamic accumulator.

References:

  1. Comfrey The Wonder Plant; aeronvale-allotments.org.uk/downloads/Comfrey_factsheet-01.pdf
  2. NPK Values of Organic Fertilizers; http://extension.oregonstate.edu/lane/sites/default/files/documents/lc437organicfertilizersvaluesrev.pdf
  3. Nutrient Values of organic Fertilizers; http://oces.okstate.edu/cleveland/horticulture/N-P-K%20rates%20of%20various%20organic%20fertilizers.pdf
  4. One Thing Lead to Another; https://portageperennials.wordpress.com/2013/02/21/dynamic-accumulators/
  5. Review of Understanding Roots; http://www.waldeneffect.org/blog/Understanding_Roots/
  6. Photo Source;  Cornelia Kopp

 

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

43 thoughts on “Comfrey – Is it a Dynamic Accumulator?”

  1. I would just say that comfrey has a higher potassium content, and this with the fact that its leaves are large would suggest that growing it for fruit growing plants makes sense.

    Reply
  2. I understand your article is about comfrey being a “dynamic” accumulator, but I am wondering if it accumulates at all. I liked the second part about the root depths which is also what I am interested in. In my mind the question isn’t the specific depth but to what horizon A,B, and C does it penetrate. I have a sandy loam 3-8 deep then a clay pan. I am searching for what plants will grow thru that and start to break it up and let water infiltrate. I’ve read all these claims about diakon radishes but talked to locals and it hits the clay and stops.According to my Ag univ I have 0-6 lbs of P an acre. I find this incredulous since I have 10yo 20ft trees, bushes, weeds etc. I’ve found they only test for the bio available form of P not total P. So what plants can change the form? Does comfrey? Idk. I have found a plant, and read scientific papers that it scavenges P from the soil, and can change its form, sweet white Lupin. After a long search I have a few seeds and am growing it. I also understand your article is aimed at gardening but I am wondering its effects as animal feed which would be a plus to the plant. But it regenerating from the root? I have had way too many weed problems already to deal with another rhizome type plant.

    Reply
  3. Well, my comfrey struggles in zone 10b so maybe it’s time to stop coddling it. I heard it is supposed to have few pest issues and mine gets HAMMERED by (I think) pincher bugs (I have 4 inches of rough chopped wood mulch, so a perfect hiding place for the little buggers). If I douse it almost daily with neem it throws up some leaves that survive. But it’s never created an attractive plant in my garden that I see in books.

    Reply
  4. Comfrey is normally applied where I live after several weeks, if not months, spent rotting (fermenting?) in water. Will this effect its usefulness and the availability of its nutrients to plants, or is it just a lot of time and smell for little purpose?

    Reply
    • All nutrients except for nitrogen and sulfur will remain in the pail. N and S can be lost to air. Fermenting does not really add anything, but it might speed up the decomposition – not sure.

      Reply
  5. I have previously responded to the unfortunate misallocation of number values used in this “research”.

    In addition, your functional premise is, and I quote “All of these benefits are derived from the fact that comfrey is one of the best dynamic accumulators”. No, they are not. This comment section, even the section of your article talking about root growth, belies that premise. When you say “all of these benefits” you functionally imply that the “all kinds of claims” are affected by your “debunking”.

    Is it outside of the scope of this articles relevance to recognize that large quantities of leaf matter are produced per plant per year are part of its total nutritional value? No it isn’t, Your conclusion is that it isn’t worth growing this plant because it, per gram, doesn’t offer nutritional value. But your own numbers don’t support that conclusion. Even if they did, if you are valuing a plant (which you are), not just a leaf, then your value must include the relevant variables of the whole plant, not just the leaf

    Reply
    • “When you say “all of these benefits” you functionally imply that the “all kinds of claims” are affected by your “debunking”.” – that is not correct.

      The listed benefits are the benefits others have claimed because comfrey is a dynamic accumulator. I am simply presenting the list proponents have compiled.

      The numbers are not mine. I am reporting the numbers of others. If you have better data to show that comfrey is a dynamic accumulator – then link to them here and we can discuss them.

      Reply
      • I observe that my original comment, describing in detail how the numbers you provide do not support your conclusion does not appear to be here.

        Brief recap – you functionally took the lower of the two sets of values you could find for comfrey, and unfavorably contrasted it with plant substances that were incompatible from a research context. Where the alternate plant parts were relatable, comfreys values were very competitive, surpassing crimson clover – another that is known as a dynamic accumulator.

        The reason I say they are not comparable is not so much that their nutrient values are different across the categories (although they most certainly are) it’s because nobody will be using seed to mulch or compost. The dry weight values have very different overall volumes to make an equivalence, and seeds are unsuitable for mulching, both because they germinate unless ground down, and because a full growth cycle is generally required to produce its seed, which in volume is far lower than leaves for the same period of time. It is simply not cost, energy, or land use effective to use seed for these purposes – making a comparison with it in the context of mulching/composting irrelevant.

        Reply
        • Your original comment was deleted because it made no sense to me. You quoted numbers, telling me they were wrong, even though they were not even in the article.

          Much like this comment, “The reason I say they are not comparable is not so much that their nutrient values are different across the categories it’s because nobody will be using seed to mulch or compost.” What?? I never said anything about using seeds as mulch.

          If you think comfrey is a dynamic accumulator – give me a link to a study that shows it is.

          Reply
  6. I was intrigued to find this article – cudeaux

    However, I am interested to see what amount of ‘accumulated’ growth the plant does per year in any given soil properties per gram / growth per year / m2

    That is, before making any judgement as to whether it is capable of being called dynamic

    If it hasn’t got the same values as a common plant – does it then grow more than other plants..? (Dry weight)

    One thing that is really interesting in my view is the ability of a plant to storage carbon in the soil to the help of increasing soil life – because carbon will help a bed to maintain water, create chemical reactions and thus nutrient exchange

    Those effects are what I am learning around the web and in permaculture courses as to why it is called a superplant (it works to amend soil so that others might thrive)

    That’s why it gets its reputation – that was before I read your article – it’s the first of yours I read and I’d be happy to read more

    I’ll give you a link to a spreadsheet I found years ago which lists all the elements of the periodic table accumulated by each plant

    … because “sharing is caring”

    I would be happy see you perhaps use this source in an article and analyze the shit out of it

    Reply
  7. One way to judge if it has deep roots is to see how it handles drought. At least at the onset of droughty period. I planted comfrey all over my garden in relatively deep straw mulch but with hardpan (man made – degraded yard). In the summer with intense sun radiation (xeric landscape – steppe) some confrey plants wilt rather quickly without irrigation (2 days, hope to improve as years go by). I still grow it for the large amount of green growth it puts up which i harvest all summer long to make fermented liquid. The growth hormone which is proven to be producing (based on its fast growth and fast bruise healling propertises, which i can see with my eyes) seems to gentle stimulate (vs synthetic N fertiliser which promotes aphids) fruit tree growth (conclusion based on comparing trees irrigated with fermented comfrey vs “neglected” ones, same age). All in all i think it is a good plant to have around but it’s a thirsty one. I also appreciate your cold facts.

    Reply

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