Comfrey (Symphytum officinale) is probably the most popular dynamic accumulator. Permaculturists swear by it, and organic gardeners use it frequently. Thousands of websites 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 a 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, reports an NPK of 3-1-4.8 (converted to dry weight). These two values are fairly similar, so for 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. I did find values for organic fertilizers.
- Alfalfa 2.5-1-2
- Clover, crimson 2-0.5-2
- Corn gluten meal 9-1-0
- Cottonseed 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, 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, and it’s certainly not ten times better.
Using this criterion, comfrey is not a dynamic accumulator for NPK.
Update: there is some new science related to dynamic accumulators, as well as a database of several hundred of them. Comfrey is on the list, but Russian comfrey is not, because there have been no scientific studies looking at its nutrient content. That is odd since so many gardeners are convinced it is one of the best accumulators.
Some additional research on Russian comfrey has been done, and it shows it is an accumulator for potassium and silicon.
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, which are up in the 10-20,000 ppm range. Besides, soil is not usually calcium-deficient. 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 according to my definition, but does qualify as one for the new definition proposed by the USDA.
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, “…. 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 in 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”. 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 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.
Soybean meal 7-2-1, Corn gluten meal 9-1-0
Cotton seed meal 6-0.4-1.5,they are seeds! you compere rate of a plant to seeds! the plants on all have lower value than comfrey!
There are four points I’d like to make:
1) Much of the benefits of comfrey will be lost if you let the plants flower.
2) Comfrey leaves contain little cellulose, so decompose far quicker than your average plant leaf, making the nutrients available far more readily. When applied as a brewed extract, these nutrients are immediately available, and have twice the potassium levels of diluted commercial tomato feed.
3) I have seen the npk values quoted elsewhere as up to 4-1.8-10, so perhaps its concentraition is dependant upon the soil it is grown in.
4) NPK numbers are not, by themselves a great indication of availability to plants. How different forms interact with soil microbiotics is also important, For example, high phosphate levels can deter mycorrhizal formation, even if that phosphate is not available to plants.
Why would #1 be true? Why would a plant suddenly eject all its nutrients?
Re:”When applied as a brewed extract” – that depends on how you brew it. Most of the nutrients will still be in the form of large molecules after most colder brewing processes.
Re: “high phosphate levels can deter mycorrhizal formation, even if that phosphate is not available to plants” – Only the phosphate in solution is going to inhibit fungi, and that is the same form that plants use.
Hi Robert.
I should have been clearer; the brewing time for the extract is a couple of weeks whilst the leaves, apart from the midribs, break down.
“Brewing,” is meant more in the sense of tea, than enzymes brewing as in alcohol production.
I hope I have not misled you, or others, as that is as far from my aim as is possible.
One must take into consideration the fact that the soy, cotton seed and corn gluten all take large amounts of energy to produce and are most likely gmo plant biproducts, that said the numbers supplied by the comfrey are very good when considering the low energy input. I’ve been using comfrey for years and as far as green biomass production only Tithonia diversiflora/ perennial sunflower, another “dynamic accumulatior” comes close but takes up far more space. I’d like to see a NPK analysis on that plant because I’ve seen gardens fertilized with nothing but leaf mulch of it with grand results.
A very nice article, one of many I must say!
This particular article deserves a critical comment, though. I don’t doubt the numbers presented are correct, but I do doubt their relevance for the average gardener/ homesteader.
1) The quest to find the plant with the best nutrient accumulation in dry weight leaf material is academically interesting, but for a gardener the focus is rather to maximize the soil health improving effect to be harvested from a limited “wasted spot” within, or close to, the garden.
2) Because the spot is situated in said location, many potential plants are disqualified. The plant should not spread, not sting, not be too high (preferably knee high), not be poisonous, be perennial, suppress weeds effectively, tolerate drought without watering, tolerate heavy harvesting/ cutback, be productive in both sun and shadow, produce large amounts (leafy bulk mass) and decompose quickly and completely to be useful as green manure.
I dare you to find any plant that can meet all those criteria anywhere near as good as russian comfrey, at least for the typical garden type of soil that is not too shallow, in a temperate climate. It has it’s flaws, of course, most importantly IMO: it attracts copious amounts of slugs.
Comfrey may be a good choice for growing green material, as mentioned in the post. It is just not any better at accumulating nutrients as other plants, although it does make a lot of leaves.
Yes, it is not particularly good if you examine equal amounts dry weight of leaves. Then there are better alternatives.
When counting how much (soil) nutrients you can harvest from a limited spot, it performs better, and when you add restrictions like I did, it is probably the best choice, in many parts of the world.
It would be interesting to investigate further.
You might well be correct. If you find some hard data on this I would be interested in seeing it.
Brilliant article. I read all kinds of claims in gardening/agriculture books and it’s great to find some science based critiques out there (even if it partly points to a lack of nutrient benchmark data for comparisons!)
What about comfrey as a compost activator, any truth to this claim?
A compost activator is anything that is missing in the compost pile that would speed it up. This is normally nitrogen. Adding any green material will work, so yes green comfrey leaves will work, just as well as green grass clippings.
Compost Accelerators, and Activators
Thanks for the article. It cleared up some of the misconceptions of root systems especially in this plant. I do however think that Comfrey can be useful to a gardener, like you have mentioned in your article. If you want a steady source of green material for your garden, comfrey would be a good source for just that. I also know a few backyard farmers that use it to feed their chickens.
I like that you are making comparisons with other plants, but you said you had a hard time finding the nutritional data. Still, I don’t think its a fair comparison to put the nutritional aspects of comfrey in the same category as the cotton, soy and corn meals because the meal is a product of the plant’s seed and not of the leaves themselves.
You might have a point there, but how does the nutrient values of seed compare to leaves? I am not sure.
I’d love to find better data.
I agree with Mark. Comfrey produces lots of green material from waste lands like ditches.
I also think that comparing merely mineral dry weight values doesn’t give the right results. The results may be quite different if you count dry weight values from plants in certain area in one growing season.
“pink or white flowers which are not overly ornamental.” – they are to bees, with a long flowering season. What makes comfrey particularly good compared to other plants is the fact you can harvest almost the entire plant for leaves in great quantities and it regrows quickly giving you a lot of material.
Even if the nutrient amounts are poor, weight for weight, this can be made up by the sheer amount that is easy and fast to grow. I think it’s convenience is this plant’s saving point when adding to teas, compost and mulch….despite being a busted myth as a special nutrient accumulator.
Nice post though.
Another good article. It will be interesting to see if others are able to provide additional documented information.
On a somewhat related area, I found the following resource to be excellent in understanding more about what the physical root system looks like of some common plants.
http://soilandhealth.org/wp-content/uploads/01aglibrary/010137veg.roots/010137toc.html
Excellent article!