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.

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.

Microbe Science for Gardeners Book, by Robert Pavlis

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. 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.

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  2. 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?

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    • 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.

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  3. 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.

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      • 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.

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        • 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
  4. 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

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  5. 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.

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  6. My reason for using comfrey was always the higher potassium content for supporting blooming. That does seem to make sense.

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  7. Comfrey is easy to grow in a wide range of soils and conditions. Comfrey grows quickly. Comfrey makes good mulch. Comfrey thrives where others drown in couch grass, and forms a barrier which helps others survive. Other plants growing near comfrey seem to do well – it doesn’t inhibit them. Plants fed with comfrey tea seem to do well. Bees LOVE comfrey flowers. These are just observations of my own based on just a few years of using comfrey in the garden.

    So how to address the widespread and longstanding use of comfrey in the garden? When you find traditional practices, that implies many people, over a long time, observing effects. Perhaps you shouldn’t dismiss these so lightly.

    I appreciate that understandings of why these effects are seen may not be correct. With respect however, your analysis of comfrey as a ‘dynamic accumulator’ isn’t conclusive, and you don’t address other possible mechanisms for its value in the garden. For example, this could be by changing the conditions for certain organisms in the soil which help plants access nutrients more effectively – just one possibility of many. Proper understandings of ecosystems, plant interactions, soil biology etc. are really in their infancy: it wouldn’t hurt to acknowledge this.

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    • The goal of the article is not to look at all possible values of comfrey. The title is clear – is the claim of it being a dynamic accumulator true? It;s not based on all of the information i could find. If you have additional research on this – please post the links.

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      • I recently started gardening this year. Covid hobbies, am I right ;)? And as someone who has very limited space to work with I do appreciate the evidence based approach. I see Marianne’s point, an ecosystem is more complex and there could be a benefit from comfrey that isn’t being summed up through NPK. But at the same time, being a gardener who has very limited space to work with I think you hit the nail on the head. Do I want to use that space for comfrey? Given what you presented here, the answer for me is no. Specially that I’m also using a lot of containers.

        Thank you for the article.

        Reply
  8. The plant list used to compare to comfrey includes some above “average” plants. For instance alfalfa is a leguminous nitrogen fixer that is used as a Dry compost green Hence contains more N than your average plant and is extremely energy intensive to produce not unlike cotton seed meal which is also used as a fertilizer because of it high N content. Actually looking back all of these plants and plant products are used as fertilizer, that said the numbers that the comfrey offers are actually very good compared to these “average” industrial grade fertilizers. This article actually contradicts its own point due to the ease of growing and perennial nature of comfrey compared to the annual industrial nature and overall energy return on energy invested of the compared plant material.

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  9. Oh my gosh, there is nothing that irritates me more than someone trying to write a persuasive article based on other people’s research, rather than their own in-depth, personal research and experience. I’ve been growing comfrey for almost 10 years and I have close to a thousand plants on my permaculture farm. Comfrey is the absolute backbone of everything I grow, and for good reason.

    For starters, comparing NPK values of comfrey to products like cottonseed meal or corn gluten meal is useless at best. It’s downright deceptive at worst. The only useful comparison for the home gardener would be comparing the ratio of inputs (money, time, labor, physical products like fertilizer) to outputs. Comfrey has almost zero inputs, and MASSIVE outputs. Furthermore, breaking down any living organism, drying it, weighing it, analyzing it under a microscope, tells only a tiny portion of the story. What a living organism does as it interacts with other living organisms, the synergy and interdependence created in it’s natural environment, is something science has yet to figure out a way to measure.

    You said “Keep in mind that the space dedicated to comfrey can’t be used for other plants.” This is completely wrong. You have ignored one of the main benefits of comfrey which is its root exudates: https://ui.adsabs.harvard.edu/abs/2014EGUGA..16.5572W/abstract.

    What this means is that annuals can be planted right on top of comfrey beds. I’ve been doing this for years. In late spring, when I am ready to harvest my comfrey roots, I dig up the entire row of comfrey, then I plant my tomatoes, peppers, basil, parsley, etc. right on top of the comfrey that I just dug up. My annuals are the most healthy vibrant plants I’ve ever grown by doing it this way. By midsummer the comfrey has all started growing back, in and around the annuals, and I cut some of it back to use for mulch or fodder, but mostly I just let it grow. It acts as a living mulch that way, suppressing the weeds and giving my tomatoes a leafy cushion that protects them from the soil. The root exudates feed my annuals all summer, all the while attracting huge numbers of earthworms and other beneficial soil organisms. The soil here is black and fluffy and fertile, with zero inputs.

    …but…*sigh*… that’s just anecdotal evidence, MY personal, non-scientific research, based on years of dirty fingernails and gut instinct.

    I’m not sure what grudge you are holding against comfrey, what childhood harm it may have inflicted on you, but I hope you can find peace with it someday. And stop badmouthing this amazing plant.

    Reply
    • “there is nothing that irritates me more than someone trying to write a persuasive article based on other people’s research, rather than their own in-depth, personal research and experience.”

      If you don’t believe in science – you won’t like any of my writings.

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      • He didn’t say he didn’t believe in science, but the problem with only using other people’s research and not doing any research yourself is that it is easy to cherry-pick the papers that support your argument and ignore the ones that don’t, confirmation bias is human nature. Replicating experiments is a key part of science.

        I think you are also missing the point that many people who promote comfrey are making.

        For any given 1 sq ft of garden, what plant (without fertiliser being applied to that plant) provides the most total NPK and other nutrients when harvested over the entire growing season.

        Comfrey proponents are saying that comfrey is that plant (or at least ranks highly).

        And then you have the question of which plants are best for building soil structure and microbial life.

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        • It is naive to think that a gardener can do this kind of research.

          If you feel I cherry picked that research studies, then that is very easy to show. Just post the research studies that contradict what I said. We can then have a discussion.

          If you had taken the time to actually read my article, you would have found this statement, “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.” – but growing a lot of leaves was not the topic of this post.

          Reply
  10. Really interesting to see in the spreadsheet by Mike H/Dr James Duke that the best ‘accumulators’ are nettles, dandelions and horsetail…

    Reply

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