The truth about…Comfrey

Gardening isn’t difficult but its fair to say there is a lot to learn. There is more than a lifetime’s learning in fact and even if we could know everything, some of the things we used to know will turn out to be wrong. Take all the commotion about re-naming Rosemary for instance (see our previous post here). But unless you are a horticultural scientist employed in a lab, what gardeners really want is to know what works. Regrettably, things aren’t so easy; it’s often difficult to separate truth from myth in the horticultural world. Some of these myths we call ‘merry-go-round facts’.

Merry-go-round facts are ones that everyone has heard. They get passed from person to person and become ‘common knowledge’. Round and round they go until we are all singing the same tune. But stop the ride and no-one seems to know how they got on it in the first place.

Take Comfrey (Symphytum officinale) as an example. Doesn’t every good organic gardener know that comfrey ‘mines’ nutrients from lower down in the soil than other plants can access, on account of its long tap root? I’ve heard it from many a gardener and it even slipped into an RHS promotional video. Unfortunately, this ‘fact’ is actually a fiction.

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The truth about comfrey

Comfrey, particularly the sterile hybrid ‘Bocking 14’, is a wonderful plant in many ways. Its leaves rotted in water make a wonderful liquid fertiliser and it is a profuse flowerer that bees adore. It will grow in many unpromising situations and produces a copious quantity of foliage that is a boon for the compost heap. You can even add it in layers to your leaf-mould pile, yielding just enough of a nutritious boost to use it as a sowing compost (see our post on leafmould here).

What about the supposed capacity to collect (‘mine’) nutrients from lower layers of soil, sometimes known as ‘dynamic accumulation’? That is the myth.

The idea originates from a book called Designing and Maintaining Your Edible Landscape—Naturally (1986) by Robert Kourik (for a detailed background of the term’s history you can read this article on The short story is that there was no evidence at the time of writing that warranted such a claim for herbaceous plants, and no scientific research at all has been done on the subject since, although there are general databases looking at nutrient levels in plant tissues like this one. If you know of any research, we would be genuinely happy to hear it, but we have scoured the horticultural journals to no avail.

For a description of why the ‘long tap-root’ hypothesis doesn’t hold up, see this article on  which can be condensed into:

  1. Most of the fine feeding roots that absorb nutrients aren’t present along the taproot
  2. You can’t dynamically accumulate nutrients if they aren’t there in the first place
  3. Even if we agreed a standard for dynamic accumulators, comfrey doesn’t necessarily make the best candidate

Amazingly, despite just a one page mention in a single book, the idea has spread like non-Bocking 14 comfrey. It is everywhere. Hundreds of articles have been published, each reinforcing the next by virtue of no more evidence than ‘it’s what everyone else is saying’. Only this year has the subject begun to pique the interest of scientists, with the first small scale field trial of ‘dynamic accumulators’ being conducted by Cornell University and partners in the USA.


Some of the misunderstanding around ‘dynamic accumulation’ comes from its similarity to another term ‘hyperaccumulation’, which is the study of plants that take up above average levels of heavy metals into their tissues. Hyperaccumulators are a valuable for several reasons; their use in Phytoremediation which is the in-situ improvement of pollution on contaminated sites, and even Agromining or Phytomining which is the harvesting of plants on land (possibly contaminated), to extract heavy metals for processing and sale.

There is a great deal of work on hyperaccumulators, but the research is generally focused on heavy metals and does not tend to look at Nitrogen, Phosphorus and Potassium values (NPK), which are the nutrients of interest to gardeners. Evidence for hyperaccumulation is therefore not evidence of ‘dynamic accumulation’ and even if the two were linked, there is still no research to show Comfrey is a particularly good candidate.

The merry-go-round turns

How many people took the RHS at face value in that video? How many thousands of people have planted comfrey believing that it will draw up trace nutrients from the sub-soil? How many times have we repeated this ‘fact’ ourselves over the years? We can’t be sure. One thing we can be certain of is that the merry-go-round continues to turn. Notwithstanding writing a lengthy article about exactly why ‘dynamic accumulators’ are a horticultural imaginary, the author of the Permaculture News article linked earlier, ends the piece with this:

“Personally, I will continue to use dynamic accumulators in a holistic approach to soil improvement. It may help our soils for our intended purposes in exactly the way that we think, or it may help for entirely another reason. If it works, I don’t really need to know why.”

Astonishingly, this myth is so ingrained that even writers exposing the myth continue to espouse it!

Must we say nothing?

One challenge to requiring peer-reviewed proof for any claim is that there simply aren’t enough horticultural scientists to study everything we need to know, as quickly as we would like to know it. It will remain the case for many generations that we will have to make decisions about what to do in the garden based on advice, rather than fact. The difference can be posed like this.

Gardener A

“I use comfrey as a liquid feed and my plants grow fantastically well. It’s always worked for me and it makes good compost. The bees seem to like it too!”

Gardener B

“I use comfrey because it accumulates nutrients from deep in the soil and has super-high levels of nutrients in the leaves. You should use it too, it’s the best plant for the job!”

The first gardener is giving advice, the second is telling you a fact. Let’s assume that gardeners are a friendly bunch and aren’t trying to maliciously diminish your treasured crop of carrots. In that case, it’s worth taking the advice of your allotment neighbour who might also be able to add some recommendations for good varieties that have worked for them. Assuming they have the same growing conditions as you, that is a great place to start!

The issue with facts-that-aren’t is that they tend not to be challenged when they should, whereas good advice is just advice, you can take it or leave it. In the case of comfrey, these ‘facts’ should be challenged because there might be something better! If the tentative research now being conducted by Cornell was completed 20 years ago, perhaps we would all be growing something different.

What if another plant altogether was higher in nutrition, more beneficial for wildlife, more effectively suppressed weeds, spread less easily and produced an even greater crop of leaves? What if an annual sowing of green manures like Phacelia were just as effective at nutrient cycling and just as beneficial for pollinators, but didn’t take up valuable bed space? What if you already have enough nettles nearby, which make an equally useful liquid feed? Perhaps some plants better suit particular conditions, soils and temperatures. And if ‘dynamic accumulation’ turns out to be nonsense, maybe we would prefer an apple tree or some asparagus in our comfrey corner, assuming we are getting enough material for the compost heap elsewhere. Or even better for the bees, a mini meadow!

This doesn’t mean we shouldn’t give advice if there hasn’t been a scientific paper. Good advice will continue to be shared as freely as the annual glut of courgettes. But we should also be prepared to stop the merry-go-round from time to time and ask the facts on it where they came from.

A golden age of organics

With all that said, we shouldn’t dismiss the truly impressive qualities of Comfrey. It is a good candidate for the production of vegan fertilisers, which are less troublesome to the environment than the ubiquitous chicken manure pellets. It deals with poor conditions well, where other plants may struggle. It’s hard-as-nails, marching on whatever the weather can throw at it and is perennial whereas green manures need to be re-sown every time. So if those are the qualities you need, Comfrey is the plant for you! Just don’t be misled by the idea that it will deliver nutrients from the depths.

For our part, we hope good research will contribute to a golden age of organic gardening, where we can produce the best blooms and bumper harvests with the minimum level of input and the lightest tread on the planet’s resources. There is a tendency to assume that the ‘old ways’ are best and certainly many ‘modern methods’ in the age of chemical gardening have been untold disasters for nature. But whether we are searching for disease resistant cultivars, new biological controls, the best mulches for increasingly droughted soils, or the perfect plants for pollinators, it is detailed research that will truly tell us what’s best. That may be the old ways, it may be the new, but we won’t know unless we ask.

That, in the end, that is the point. If you really love Comfrey, that’s great! But if someone tries to convince you to give over precious space in your garden or allotment for this supposed wonder plant, it is worth asking if that is the best thing to do. If you already have plenty of greens for the compost, nettles to make liquid feed and a plot full of flowers, why not treat yourself to that new apple tree instead.

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