Are ‘dynamic accumulators’ a thing?

‘Dynamic accumulators’ are often promoted as an essential ingredient of organic gardening and forest garden design. I’ve planted them myself, but the longer I grow my forest garden the less I find there to be any point to them, so I’d like to try to answer three questions in this article. First, what is a dynamic accumulator anyway? Second, do they work as advertised? And third, do you need them in a forest garden?

What is a dynamic accumulator anyway?

Let’s start with Wikipedia. According to the entry current at the time of writing, “Dynamic accumulators are plants that gather certain micronutrients, macronutrients, or minerals and store them in their leaves.” The trouble is that there’s a shorter term for this kind of plant. It’s a “plant”. All plants do this so by this definition dynamic accumulators aren’t a thing.

Trying to get back to the source of the term, all roads seem to lead to Robert Kourik’s book Designing and Maintaining Your Edible Landscape—Naturally (1986). Kourik included a list of ‘dynamic accumulators’ – plants that he understood to be high in particular nutrients . The purpose of the list was to give some guidance as to what different plants might be contributing to the compost heap and which might be best for producing high-value compost. The trouble is that Kourik himself seems to have disowned both the term and his original list. For more detail, see The Facts About Dynamic Accumulators, an excellent article from the Permaculture Research Institute tracing the origins of the term.

Dynamic accumulators, then, rather resemble the monster in Mary Shelley’s Frankenstein. Rejected by their creator, they roam the countryside looking for purpose and acceptance. Have they become, as Frankenstein’s creation eventually did, embittered and malignant or have they, as the monster dreamed of doing, found their own way to becoming a useful and accepted member of society? Let’s move on to how those authors who still use the term, almost all of them in the field of permaculture, define it. Taking a survey of these, three key ideas seem to contribute: hyperaccumulators, nitrogen-fixers and deep-rooted plants.

Hyperaccumulators are used in the fairly well established field of biological remediation of polluted soils, in which plants are used to pull pollutants out of the soil, before being cut down and disposed of at a hazardous waste site. As a result there is a reasonable body of research into plants that are particularly good at taking up different nutrients and these have acquired the term ‘hyperaccumulators’. Incidentally, this is the only use of the term ‘dynamic accumulator’ that I can find in the scientific literature.

Nitrogen fixing is quite uncontroversial. Some plants form an association with bacteria that allow them to turn nitrogen gas in from the air into forms of nitrogen that plants can use as fertiliser. Most of these are in the massive legume family (Fabaceae), which runs from weedy climbers to great trees. A few, such as alder (Alnus) belong to other groups collectively known as the actinorhizal plants.

Finally, plants with deep roots are often proposed as dynamic accumulators, with the idea that they will take up nutrients from deep layers of the soil, otherwise at risk of being washed out of the system entirely, and return them to the surface layers where they will become available to other plants, either through leaf fall or through the gardener actively cutting them for mulch or compost.

Some definitions of dynamic accumulators take them to be identical to just one of these groups. Others, including the one on Wikipedia, attempt to ram them together into one concept, ignoring the fact that few plants belong to all three groups and the most popular ‘dynamic accumulators’ almost never do. Is this a case of grouping together ideas that would have much more clarity separately or is there a fuller picture that the all contribute to? Let’s find out in practice.

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Nettles are high in nitrogen, sulphur and magnesium

So do they work?

In some places dynamic accumulators seem to be assumed to have almost magical properties. In one blog I read the author expressed confusion at the fact that her crop plants were struggling despite the large and vigorous dynamic accumulators she had growing all through them. It’s almost like trickle-down theory for plants. Dynamic accumulators are imagined to create nutrients out of nowhere and share them freely with surrounding plants. In reality they are almost by definition plants that excel in grabbing nutrients and keeping them for themselves. Whisper it, but another name for dynamic accumulator might be ‘weed’. If we want to put them to good use it will have to more intelligently than this.

Using plants as a means of moving nutrients around requires us to ask some questions. Which nutrients do we want to move? From where? To where? What for? However you define them, dynamic accumulators are plants that take up space, light and water. Unless we can give good answers to these questions there is no point in using them.

The hyperaccumulator model

This is why the comparison to hyperaccumulators is misleading on several counts. Firstly, the hyperaccummulators that have been researched so far have been identified for their ability to accumulate problematic, toxic elements, not the ones we want in our gardens. There might be equivalents for the more useful nutrients or there might not be, but in any case the hyperaccumulators so far identified aren’t much use for productive gardening.

Secondly, we should remember that hyperaccumulators are used for taking chemicals out of soils. Where there are toxic levels of, say, zinc in a soil it makes sense to use plants that can hoover it up into their tissues in order to remove it from the soil. In vegetable gardening we are trying to put nutrients in to the soil. If we get those nutrients from the soil in the first place then we are merely moving them from soil to plant and back again to no obvious purpose. A futher point here is that you generally want to add the elements that your soil is deficient in – but these are exactly the ones that you won’t be able to accumulate from your soil as they aren’t there. These might seem like rather obvious points but it’s not uncommon to see dynamic accumulators recommended as if they are some sort of fertiliser that simply needs to be added to the soil.

Finally, one more problem with the hyperaccumulator model. Most hyperaccumulators are good for just one or two specific elements, but what your plants need is a balance. Things could therefore get very complicated as you try growing a whole range of dynamic accumulators in the attempt to get a balanced nutrient profile. There might not be much room for crop plants. Mike H, on the blog One Thing Leads To Another has put together a list of plants that at least have higher levels of a broad range of plant nutrients in ther leaves than others. Unfortunately it consists entirely of plants – generally considered as weeds – that either very or relatively shallow rooted, meaning that they will only give you back nutrients that were already in the top layer of your soil.

Green manure and nitrogen fixers

The concept makes a little more sense when we consider ways of using plants to move nutrients from places where our crop plants can’t use them to places where they can. One is deep in the soil. The other is the atmosphere.

There is a long tradition in farming and gardening of growing ‘green manures’, plants that are grown not for eating but to be ploughed in to the soil like manure to feed a crop the next year. Most green manure plants are in the legume family as they can do what few other plants can – pull nitrogen directly out of the atmosphere and use it as food. The nitrogen then becomes available to other plants when the green manure dies or gets turned in.

Green manuring is a well established practice and definitely works. It’s possible to use the term ‘dynamic accumulation’ to describe what’s going on as the plants are definitely accumulating a crucial nutrient in an active way, but does the term actually add anything to the established idea of green manuring? I’d suggest that it simply confuses the matter, conflating nitrogen-fixers with hyperaccumulators and deep-rooted plants.

Deep-rooted plants

The other place that some plants can access nutrients that others can’t is from the deep soil horizons. I first came across this idea during my forestry degree, reading Forestry Commission research papers (see here, here and here) describing how silver birch improves poor soils by bringing up nutrients from deep soil layers and depositing them through leaf fall on the surface.

However, I also thought I’d have a quick look at a site I often find useful as a check on ‘everyone knows’-type facts: Robert Pavlis’s Garden Myths. Pavlis questions the idea that deep-rooted plants get significant amounts of nutrients from deep in the soil, quoting Robert Kourik as saying that “…. 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.”

There are two points here. The first is that absence of evidence is not evidence of absence. The area may indeed be grossly under-researched but that doesn’t mean that we can just dismiss it. I have found plant roots below 1.2m in my garden. They must be doing something down there after all and with herbaceous plants it can’t be about stability. The only explanation I can think of apart from nutrient capture is that they are storing nutrients down below the reach of most things that might eat them, ironically in the reverse of what dynamic accumulators are meant to do! The second point is that deep-rooted plants don’t have to be getting the majority of their nutrients (I doubt if they are) from the deep soil in order to be doing a useful service to the system as a whole.

Dynamic accumulators in practice

So, finally, are dynamic accumulators any use in the forest garden? Let’s break that down in to its three aspects.

First, plants that accumulate particularly high levels of some nutrients in their leaves are of no particular use. They simply take up growing space that would be better used productively.

Second, nitrogen-fixing green-manures are useful, but there is no good reason to call them dynamic accumulators. In the forest garden there is the opportunity to plant perennial green manures that are active the whole season round rather than just the usual annual ones. When I visited Graham Bell’s forest garden in the Borders he was growing Laburnum, one of the few nitrogen fixing trees to flourish in Scotland, for composting material.

Laburnum

Laburnum

Third, deep-rooted plants may well have a role to play in preventing nutrient loss from the system as a whole, but I no longer see much point in planting anything specifically for this property as so many of the crop plants that you can use do it anyway. This has been my experience many times over with the forest garden. I began by planting some plants for crops and others as hoverfly attractors, wildlife plants and ornamentals. As the garden matured and the range of crops expanded I realised that the crop plants were fufilling all these other roles for free. It is the same with deep rooted plants. Crops like sweet cicely, udo, horseradish, monk’s rhubarb and no doubt many more have very deep roots and are useful for harvesting biomass as well as the desired crop. In fact I have come to think of these species as dual-use plants, providing both an edible crop and compost for the hungrier species in the garden.

I do still have a few comfrey plants around, including a clump planted next to my ‘toxics’ compost bin. This bin takes the tattie haulms, brassica roots and any diseased materials. Rather than returning directly them to the soil I just let them break down and be absorbed by the comfrey, which grows at a phenomenal rate and can be cut several times a year. In the future, however, I might replace even this with a crop plant.

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comfrey-triffid

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bee enjoying the comfrey flowers

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Paths

There is one kind of plant that really doesn’t belong in a forest garden, and that is grass. When I got my allotment, it had the traditional layout of bare soil beds with raised grass paths between them. The old boys actually seem to enjoy maintaining these structures, mowing the tops to perfection and deploying a range of implements that a hairdresser would be proud of to keep the sides in trim. If this is neglected for even a short while, the grass flops over the sides, first making a perfect habitat for slugs then starting to grow into the beds.

I decided that no matter how much work it took to replace the paths with something more sensible, it couldn’t be more work than keeping the things. The turves were dug up and piled, upside-down, to compost. I now have three main walking surfaces in the garden. The short stretch that bears the most traffic is slabbed. The other main paths are all woodchip and across some of the beds I have walking boards.

Woodchip is a great path material for several reasons. It is particularly high in carbon which makes it a very infertile medium. Woodchip should never be dug into the soil or, in my opinion, used as a mulch on top of beds, since the soil bacteria pull nitrogen out of the soil to help them break down the carbon, robbing your plants of essential fertiliser. Using it on paths turns its downside into a virtue. If plants do seed into a bed of woodchip, its soft, granular nature means that it is very easy to hoe them off. If you have ever tried to hoe gravel you will understand what a benefit this is. Finally, it is usually a cheap, easily available material. We just get the City Council, who produce mounds of the stuff and are delighted to get rid of it, to drop us off a heap every year. This year’s load turned up last week.

woodchip pile

To make the woodchip paths, I dug out about a foot of soil and used it to build up the beds. I lined the bottom of the trenches with any cardboard and bits of old board that I could scavenge, but didn’t worry about this where nothing came to hand. Then I simply filled them with as much woodchip as I could lay my hands on. They have to be hoed in the spring and topped up about once a year, but neither job is urgent and they can be done whenever there is time.

As well as being much less work, woodchip paths are far more robust than grass ones, something I’m appreciating in the current wet weather as the grass paths are trampled to mud while the woodchip ones show no ill effects at all.

Walking boards are even simpler. I only really use them in the annual parts of the garden as there is no problem in stepping on the soil in the perennial parts. They are simply boards laid down on top of the soil to make a path. They serve two main purposes: stopping soil compaction and acting as slug traps. If you turn over any piece of wood lying on the ground you’ll see what I mean: the slugs retire there during the day and can be collected easily. The boards also provide habitat for beetles and centipedes which eat slugs and their eggs, so the slimy molluscs get a double whammy.

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Here’s the end result. The forest garden has been thoroughly put to bed for the winter, with a mulch of leaves on the growing areas and a new layer of woodchip on the paths.

Four forest gardening myths

I’ve learned a lot of things in the course of trying out permaculture and forest gardening. Some of them I’ve then had to unlearn again. Here are my top four forest gardening myths.

Ground covers
Practically every forest gardening book you can find will tell you that you need ground covers: plants whose main function is to cover the soil. They are usually aggressive spreaders like mint and tansy that also attract insects and are said to benefit the garden in some rather intangible way through the aromatic substances they release. Sometimes they can be plants like Rubus pentalobus, a bramble relative that carpets the ground thickly and produces a token yield of small fruits.

I can see where the ground cover idea came from. If you have a large area and are mostly interested in the tree and shrub products, then perhaps all you want from a ground layer plant is that it will outcompete the weeds and save you some work (raising some interesting questions about how you define a ‘weed’). But in a smaller area there is absolutely no point to ground covers: you want to fill the whole ground layer with productive plants and fertility-builders.

In my experience ground covers are completely unnecessary for attracting wildlife: the whole point of a forest garden is that the productive plants get to go through their entire annual cycle of growing and flowering and most of them are excellent wildlife plants in their own right. I am also a firm unbeliever in magic ingredients like aromatics in ecosystems: the only magic ingredient is diversity.

Straight lines are bad
Funky, curvy lines almost seem to be a defining feature and article of faith of permaculture, which is the route by which a lot of people come into forest gardening. However, at the risk of sounding very… well… straight, I have to say that there’s a lot to be said for planting in straight lines. One reason is that it makes hoeing much easier. There isn’t a lot of weeding to be done in a forest garden, but there’s still some, and running a hoe down between rows is by far the easiest way to do it. It also helps you to avoid hoeing down your crop plants. A lot of perennial vegetables die down during winter and it is very disheartening to slice the tops off them just as they should be bursting into new growth. Having them in straight lines helps you remember at least generally where they should be.

Another reason for hoeing is that sometimes you want to thin out your plants, particularly with the self-seeders. I neglected to hoe lines through my leaf beet this year and was rewarded with a patch of scrawny plants that were more interested in producing stem and racing each other for the light than they were in producing fat, juicy leaves for me to eat.

Finally, straight lines make for good access. Mandala beds may look great when they are first laid out, but regrettably a New-York-style grid is probably the best compromise between getting round the garden, getting access to individual beds and not taking up too much space that you can get.

No work
A mature forest garden is often said to be a ‘no-work’ system. If only! However, there is a kernel of truth in this one. There is far, far less work involved in digging and weeding a well-planned forest garden than there is in an annual veg plot. However, there tends to be more work in the harvesting. With annual vegetables you generally lift the whole plant; in a forest garden you pull bits off it, which takes more time.

I would change ‘no work’ to ‘better work’, as I don’t know many people who prefer weeding to harvesting. Perhaps the biggest advantage in work terms is that the the work schedule in a forest garden is much more forgiving. In the veg plot, you generally have to weed those seedlings NOW or you’ll lose them. In the forest garden you can usually leave a task til mañana, or indeed next month, with little harm done.

Bare soil is the devil’s work
A quick walk round any forest should be enough to dispel this one. You certainly want to keep soil disturbance to a minimum and only those who like thankless work would try to maintain areas of bare soil for the sake of it, but there is no need to obsessively try to cover every square inch with ground covers and mulch. Woodchip is for the paths and the paths only and other, more nitrogen-rich mulches are a useful bonus if you can get them – a way of adding fertility to the soil – not a requirement.

Shady characters

Forest gardening is all about growing plants in some degree of shade, and plant books and websites will usually give you a helpful indication of whether a plant prefers sun, light shade or deep shade. Less obviously, the same plant can often tolerate a wide range of shade conditions, often becoming almost a different plant in the process. When I get a new plant to experiment with, if at all possible I plant it out in a wide range of shade conditions to see how it fares. Not all shade is the same: morning shade is different from evening shade for instance, so it’s worth experimenting a bit to see what your plant really likes.

I was reminded of another benefit of this approach recently when I found this wild garlic, growing in the most shady part of the garden under the privet hedge that forms my border with one of the gardens that back on to the allotments.

Wild garlic really hates hot sun and most wild garlic round here curled up and died a couple of months ago (which come to think of it, was about the last time we had hot sun). The one in really deep shade, however, has remained in leaf, despite the fact that it has flowered and set seed (you can see the seed heads) on about the same schedule as all the others in the garden. This is quite a common effect of differing shade and you can use it to extend the season of all kinds of plants.

Feeding the forest garden – part 1

Not all the plants in the forest garden are there to feed me. Some are there to feed the garden itself.

The main two types of plant I have planted for this purpose are the nitrogen fixers and the dynamic accumulators. There’s a third kind too, but if it’s there it wasn’t put there by me.

Nitrogen fixers are the simplest. They are plants that, unlike most, can pull nitrogen, the main component of fertiliser, directly out of the air rather than needing it supplied as soluble nitrates by other organisms, such as gardeners. To be completely accurate, they don’t do it themselves either, but co-operate with bacteria in their roots which do the job for them. Most of these plants are legumes, in the pea and bean family. Worldwide, legumes come in all shapes and sizes, from tiny herbs to mighty trees. Native British legumes are a bit deficient at the mighty-tree end of the scale, but we have a wealth of smaller ones.

lupins

The largest legume you’ll usually see in Scotland is Laburnum, better known as a bright yellow street tree than as a nitrogen-fixing powerhouse. I haven’t got one in my own forest garden but Graham Bell has several in his rather larger one, feeding the rest of the system through a steady stream of cuttings. The only leguminous tree I have myself is a Siberian pea tree (Caragana arborescens), which is a relatively new addition that is yet to seed. When it does, I am fascinated by the idea of seed described as being very similar to lentils growing on a tree.

Down at shrub level, legumes are an essential component of the Scottish landscape: the furze and whins that turn whole hillsides a mad yellow and fill the air with intoxicating coconut-vanilla scent. In the forest garden, I draw the line at gorse (Ulex europaea) due to its extreme spikiness, but make a place for broom (Cytisus scoparius).

In the herbaceous range, the familiar peas and beans give a yield at the same time as fixing nitrogen. This year, I’m also trying a variety of white lupin (Lupinus albus). The drawback is that none of them are shade tolerant so they cannot be stacked in with other species. This niche is taken by various other, mostly smaller members of the family. I have an assortment, including an everlasting sweet pea (Lathyrus latifolius) growing in about my raspberries. It provides occasional pea shoots for a stir fry as well as fixing nitrogen. In the ground layer under the fruit trees I have wood vetch (Vicia sylvatica), which is there purely as a nitrogen fixer.

Dynamic accumulators don’t have any supernatural nutrient-fixing powers, but move nutrients around in ways extremely useful to the gardener. The king of them all is undoubtedly Russian comfrey (Symphytum x uplandicum), which is also widely used by organic gardeners to provide organic feeds. The beauty of comfrey is that it is extremely deep-rooting. Not only does this mean that it can safely be planted under fruit trees without competing too much with their roots, but it allows comfrey to scavenge nutrients, especially potassium, from deep in the soil where they would otherwise slowly be leached out and lost to the system. This really illustrates the beauty of forest gardening for me: the way the system really fills the available space.

The third kind of organism that feeds the forest garden is not a plant at all but a class of fungi called mycorrhizae. Mycorrhizae have a symbiosis with tree roots, exchanging nutrients like nitrogen for sugars made by the tree. In a typical forest, one tree will have several species of mycorrhizae and each fungus will have several client trees, meaning that the entire ecosystem is interlinked in a global trading network. Fungi are the dark matter of ecosystems – they influence everything but it is very difficult to detect them – so it is unknown whether or how quickly a mycorrhizal net gets established in a forest garden, but is a real possibility that a mature forest garden has this invisible trading web moving nutrients around to where they are needed most.

Permaculture

I can’t believe I’ve been blogging about forest gardening for two months now and haven’t mentioned the p-word yet. In the unlikely event that you’re interested in forest gardening and haven’t heard of permaculture already, it’s a design system that can be used for designing forest gardens, amongst many other things.

If forest gardening and permaculture go together well, it’s no surprise, as forest gardening was the original inspiration for the whole permaculture system. Its founder, an Australian lecturer called Bill Mollison, had become disillusioned with the mainstream agriculture that he taught and was looking for more sustainable, less dehumanising possibilities. The best example he found was the Indonesian home garden (or forest garden), with its recreation of the natural forest structure using edible, medicinal and otherwise useful plants and animals. The genius of permaculture is that Mollison didn’t just become a forest gardening teacher but generalised the lessons of the forest garden into a set of design principles that capture the ways in which forest gardens mimic natural ecosystems but can be applied far more widely to all sorts of design situations.

The best source in the UK for permaculture information is the Permaculture Association. In Scotland, there is a Yahoo group for Scottish permaculture and there in the north east there is a Google group for ASPeN (Aberdeen & Shire Permaculture Network).

The Permaculture Association now focus on the design principles of Mollison’s student, David Holmgren. Personally, I find these an unhelpful rag-bag, mixing up tips about the design process with principles to be applied to the thing you are designing. If you’re finding out about permaculture, I’d recommend digging around for something that uses Mollison’s original principles, which are a much more coherent set, focusing on the ways in which a permaculture design should draw on the strengths of nature’s own systems.

Frogs

The summer seems to be running backwards this year. After a scorcher of an April and a pleasant May, June was a bit dull and July is now getting its wellies on. Still, at least one member of the forest garden staff is well pleased with the weather.

frog

A pond is well worth making in almost any garden. Mine is tiny – no more than a metre across – but is still stuffed with frogs. I counted 6 pairs of them during the spawning season and they practically turned the whole pond into one wobbly mass of spawn. A few months later, I constantly have to be careful where I put my feet due to the little penny-sized froglets hopping all over the place. As well as the sheer delight of it, this makes for an unpaid army on full time slug patrol – just as well with all the rain.

The ground all around the pond is shaped to run into it and I have an overflow from the water butt that collects rain off my shed roof that runs down into it too. This means that I hardly ever have to add water, a great improvement on my previous attempt which required regular topping up. If you do have to add mains water to a pond, put it in an open container for a day or more first for the chlorine or other treatment chemicals to come out of it.