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.
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.
Using this criteria comfrey is not a dynamic accumulator for NPK.
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.
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.
- Comfrey The Wonder Plant; aeronvale-allotments.org.uk/downloads/Comfrey_factsheet-01.pdf
- NPK Values of Organic Fertilizers; http://extension.oregonstate.edu/lane/sites/default/files/documents/lc437organicfertilizersvaluesrev.pdf
- Nutrient Values of organic Fertilizers; http://oces.okstate.edu/cleveland/horticulture/N-P-K%20rates%20of%20various%20organic%20fertilizers.pdf
- One Thing Lead to Another; https://portageperennials.wordpress.com/2013/02/21/dynamic-accumulators/
- Review of Understanding Roots; http://www.waldeneffect.org/blog/Understanding_Roots/
- Photo Source; Cornelia Kopp