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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While the gardener’s away…

I’ve discovered another advantage to forest gardening. If you’re planning to do yourself an injury, it’s much better to have a forest garden than an annual vegetable patch. I wasn’t actually planning to incapacitate myself, but I made a pretty good job of it all the same. A couple of months ago I was hospitalised with what an MRI scan revealed to be a herniated (‘slipped’) disc in my spine. Since then my ability to garden has been limited.

I have two parts to my allotment, and the difference between them has been striking. One half is annual veg beds. Most of the beds had been dug and I had sown a few things by the time of my injury. By the time I got back down to the garden, the weeds had utterly run riot. The sown vegetables where nowhere to be seen. I have had a few pickings, but only because years of allowing self seeding has meant that a lot of my ‘weeds’ are actually edible, such as mustard greens, coriander, leef beet and mallow leaves. Another major component of my weed bank is Phacelia tanacetifolia, a green manure with beautiful lilac flowers. The bees were having a ball on these so I have simply left them to it.

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The annual beds

The other half of my allotment is the forest garden, which was a completely different story. It’s like the difference between having a dog and a cat. Where the annual veg had major abandonment issues, the forest garden barely seemed to have noticed that I had been gone. The lack of space for weeds meant that there hardly were any. The crop plants had mostly just grown bigger. I had missed a few harvests, such as the hostas, the udo, the Turkish rocket and the remainder of the wild garlic, but of course this only left the plants stronger.

The forest garden

The forest garden

As I’ve written before, forest gardening doesn’t necessarily save work, but it does shift the burden from time-critical tasks to ones that can be done at any time. Just now, I’m really appreciating that.

Seed-time

Oìdhche Shàmhna is upon us, summer’s-end eve, the great turning of the year from growth to rest. In the pastoral calendar of the ancient Celts, it was the time when the grass stopped growing and you slaughtered and salted your surplus livestock, feasted, then hunkered down along with as many beasts as you judged you could bring through the winter to thole the dark months. From these great feasts came the idea of a pivotal day in the year when the Otherworld was particularly close and, by long and convoluted paths, the pumpkins, parties and pyrotechnics of modern Hallowe’en and Bonfire Night.

In my more forest-based calendar, it is time for thoughts to turn from growth and harvest to seed stratification and bare-root planting. In other words, it’s time to stop managing the plants I’ve got and turn my thoughts to new ones. After spring, it’s the most exciting time of year, as little packets of seeds start winging their way to my door from all over the globe and I start dreaming of future possibilities. I’m already looking forward to American spikenard, udo, creeping dogwood, black huckleberry, sourtop blueberry, creeping snowberry, American bladdernut, alexanders, scorzonera and northern bayberry, amongst others. In celebration I’ve put together a page on the best suppliers of forest garden plants and perennial veg for the UK.

A lot of forest garden plants need stratification or winter cold in order to get them to sprout. Many of them also need a short warm period before the cold or they will go into a state of deep hibernation which it may take a couple of winters to awake them from. Therefore, if you can get seeds as soon as possible after collection you can save yourself a lot of time and disappointment.

A few suppliers will sell seeds moist-packed nowadays – packed with vermiculite in little plastic sachets and stored in the warm – so that they are in the best possible condition to sprout after the winter. Another possibility is to put your new seeds in a heated propagator for a couple of months before putting them outside or in the fridge for their cold treatment. If you can get your seeds early this fits fairly neatly with the natural cold spell; if you don’t start until later in the winter it all gets very complicated.

If you buy plants rather than seeds, it’s a good idea to try for bare-rooted stock, which you can only get while plants are dormant and not busy using their roots. The advantages are that you generally get larger stock that and you don’t move loads of soil around unnecessarily with the plants, keeping down weight, cost and the chance of moving plant diseases and New Zealand flatworms around. News is breaking as I write this of the discovery of ash dieback fungus in England, which has the potential to be a disaster on the scale of Dutch elm disease. We all need to get better at moving genes around without moving pests and diseases, which means local production first, seeds second, bare-root third and containerised stock last.

Incidentally, it is thought by some that the true derivation of Samhainn is not from Old Irish sam fuin (summer’s end) but from from samoni, an old Celtic word meaning ‘assembly’. It may not be an assembly in the traditional sense, but the connections I’ve made with others in my year and a half in the blogosphere have felt like a gathering together of sorts and are important to me. So Oìdhche Shàmhna math dhuibh and may all your seeds grow strong.

Off foraging, back soon

Work (and blogging) in the forest garden has had to take a back seat for a while as I’ve been overwhelmed by the amount of fruit and fungi to be picked out and about. There’s a close relationship between foraging and forest gardening in any case: a lot of the plants I grow in my allotment are ones that I could forage from the wild, given an infinite time and travel budget. Off the top of my head, the native wild plants growing in my forest garden include hogweed, sweet cicely, wild garlic, dittander, garlic mustard, sea beet, Scots lovage, buck’s horn plantain, common and musk mallows, Babington’s leek, Good King Henry, pignut, wild strawberry, various sorrels and wood violet. Oh yes, and raspberries, currants and small-leaved lime. A meal containing all of these would involve a week-long expedition taking in woods, heaths and coast – or five minutes in my allotment.

With every wild plant I have to weigh up whether or not it is worth giving it a place in the forest garden. Pluses are given for plants that I like and that are particularly productive. Minuses are for being too ‘spready’ or too big or for attacking me when I’m minding my own business, as with nettle. There is also the question of whether I have ready access to the plant on my foraging rounds. All these considerations are fairly individual, so the decision will be different for each person. I’m very much given to changing my mind: the latest one that I’m reconsidering is nettle, after talking to Fi Martynoga of the Scottish Wild Harvests Association, who was serving up out-of-season nettle brose at Wooplaw Community Woodland‘s 25th-anniversary bash. Fi has a patch of nettles is her garden that she cuts down several times a year to keep a steady supply of fresh new growth.

One species I’m still definitely leaving for wild foraging is the bramble (Rubus fruticosus agg.), a thorny, rumbustious plant that loves to romp around an area, pining dreadfully if it is restricted. I once saw some speeded-up footage of bramble growth on a David Attenborough programme. The briars thrashed around like groping hands; then, finding a purchase with their thorns, they surged forward. Take a look on YouTube and you’ll see why I don’t want them in my garden! We’ve just had the first flush of blackberries in Aberdeen. They are always the nicest so we’ve frozen what we didn’t eat and will make jam with a later batch.

cherry plum

Another fruit I have been picking, literally by the bucketload, is Prunus cerasifera, the cherry plum or myrobalan/mirabelle. I’ve raved about cherry plum before but well, I’m going to do it again. It is a mystery to me how neglected mirabelles are, seeing as how they produce curtains of tasty, juicy fruit and never suffer any disease problems that I have seen. True, any given cherry plum tree can produce fruits that are small, tasteless, sparse, unreliable, perishable or quick to fall from the tree, but equally I have found trees that carry fruit that is large, tasty and lasting, ones which crop reliably and ones which don’t drop their fruit at the first breath of wind. I’m sure it can’t be beyond the efforts of plant breeders to combine all these characteristics in one tree. Indeed there are some named varieties of P. cerasifera, available in the UK from Orange Pippin Trees. Has anyone out there had any experience with any of them?

There is an impromptu breeding experiment going on on a bank near my house, where there are perhaps a hundred cherry plum trees, probably planted with their blossom in mind more than their fruit. Their qualities vary wildly but some are very good indeed. I discovered one this year that has incredibly sweet fruits, even when still partly green. It is yielding so heavily that I picked a bucketful in less than half an hour. Right next to it is a purple variety that has proven itself to be an excellent keeper. I have been growing on seeds from the best varieties that I have picked for a few years now, so if anyone has a field that they aren’t using and would like to do a cherry plum trial orchard, I’m waiting to hear from you.

To add to the plum orgy clearly going on in these parts, cherry plums have evidently been crossing with my Japanese plum tree, Prunus salicina. I’ve been growing on seeds from it and some of them obviously have a variety of cerasifera called Atropurpurea as their pollen parent. Atropurpurea has been bred for deep purple bark and fruit and pink flowers and is unmistakeable. It is a rubbish fruiter unfortunately, but it suggests that other cherry plums will have crossed with the Japanese one too. Since the domestic plum arose as a cross between P. cerasifera and P. spinosa, the native sloe, who knows what will result?

Japanese plums ripening on a window sill

Leycesteria

Some fruits are just meant for picking and eating, there and then, in the garden. Alpine strawberry (Fragaria vesca) is one, with its tiny, sweet, ever so slightly vanilla flavoured fruit that are doled out carefully throughout the summer. But the king of instant consumption has to be Leycesteria formosa, known to gardeners as Himalayan honeysuckle and to connoisseurs as the ‘treacle tree’.

No plant in the forest garden divides opinion like leycesteria: you either love or hate its startling mixture of molasses sweetness and bitter aftertaste. But however much you like it, don’t expect to take any home – the berries burst and splat so easily that storage is practically impossible.

Foods that you’ll never see on a plate have a special allure, but even some more common fruits are best eaten one by one, on the go. Blackcurrants and gooseberries, for instance, are at their best when they are far too soft and squishy to be picked and stored easily.

My treacleberries kicked off a conversation recently about planting food plants for children. Instead of coaxing kids to eat their five-a-day at the table, how much more effective to just plant a tangle of fruit in the garden and leave them to it, play and feeding all in one. I’m sure my love of fruit and foraging came from grazing on the yellow raspberries that lined the half-mile walk home from school. For maximum effect it is probably best to strictly forbid the kids to eat it.

When I first started in my allotment, my neighbour’s daughter used to beg to be allowed to come down and eat the sprouting broccoli. I think that’s when the full extent of how much more appealing self-picked food is to kids dawned on me. I’ve taken this insight into the park that I manage, which is stuffed with as much fruit as I can fit in. Leycesteria is an excellent option for a public food plant. It ripens its berries four at a time down the flower head, so it produces a regular supply rather than a glut that can be stripped. It is a very attractive, structural plant, sometimes known as ‘shrimp flower’ because of the look of their flowers, and any that don’t get eaten by people are made very welcome by the birds.

treacle tree

shrimp flower

Forest garden wildlife

Cuteness overload in the allotment today. The baby wrens have just fledged and are bombing about the place like tiny balls of fluff, each with a tiny stub of a tail sticking bolt upright at the back in the way that wrens have. They are unfortunately far too small and fast for me to get a photo of, but it set me thinking of some of the other wildlife that has been seen in the forest garden over the years.

Birds easily top the vertebrate list, such as the robin that follows me around hoping for worms* or the crowds of starlings that hang out in a nearby cypress, swooping down in a continuous feed to take a turn bathing in the pond. They flit down lightly, splash around madly creating a mini fountain in the pond and then flap back up labouriously on wet wings. Sometimes however, all the birds fall silent and scurry for cover. That is when you look around for the sparrowhawk. One time when I was working in the garden I heard what can only be described as a feathery thump behind me and looked around just in time to see the hawk heading off with a small bird in its claws.

(*Supposedly robins first developed the habit of following large mammals about due to pigs, which dig up large numbers of worms and grubs as they rootle around. To them, a gardener is just a pig standing up.)

Mammals are usually harder to see. We had a young fox coming into the allotments earlier in the year. Andy, our local photographer, eventually got a picture of it but it had us guessing for weeks. One of the most noticeable signs of its presence was the fact that it really loved digging up my woodchip paths, presumably looking for grubs underneath to eat. It also left tooth marks on the floor of the beehive. Other mammals are more obvious, such as the young hedgehog that I found ambling down the path one day.

I pile twigs and branches up in a ‘habitat pile’ for hedgehogs to hibernate in, so it’s possible that this one came from there. Other mammals that can be seen in the allotment include the occasional squirrel and bats flitting overhead at night. While a lot of the wildlife is obviously coming in from the surroundings, it is very noticeable that the forest garden always has a lot more bird and insect life than the surrounding allotments. Wildlife seems to be another of the things that come for free with a forest garden.

Tomorrow the world!

By now, the number of forest garden species I have has seriously outgrown my allotment and space is becoming a real issue, particularly in the canopy layer.

Luckily this threat is also an opportunity and I am finding lots of places to plant what I think of as an ‘extended forest garden’. Closest to home, I am fortunate enough to live on a housing estate where there is lots of green open space. We have a lot of mature trees, the legacy of the houses having been built in the grounds of what was once a country estate on the edge of Aberdeen. Over the last two years a series of storms have taken down a whole lot of these trees, creating a need to plant the next generation. I have been out there with my spade, making sure that the new trees are productive ones.

I have been trying to plant trees on this estate for some time, but finding that the combination of vandals and Council grass-cutters is a difficult one. The solution that now seems to be working is to put a serious stake and a wire tree guard on the trees, then a tree shelter inside that. Council workers work round them like any other, official, tree and vandals seem content to put a few dents in the shelters, which I can always straighten out. The tree shelter protects the tree from Council sprayers.

So far I’ve put in 25 trees, including small-leaved lime (for salad leaves), hazel, cherry, domestic plum, Japanese plum, cherry plum, apple, juneberry (Amelanchier), handkerchief tree (Cornus kousa) and Pinus cembra, the tree that pine kernels come from.

Most of them I have grown from seed, including the cherries, which came from seed gathered from prolifically fruiting cherries on the Black Isle by a member of Reforesting Scotland. Similarly, the cherry plums come from particularly nice selections from the many trees growing around this area.

I’ve also managed to spread trees by donating them. The local Botanic Gardens were delighted to take a Japanese plum (Prunus salicina) and a rare intergeneric cross x Sorbocrataegus ‘Ivan’s Belle’, and the local park took half a dozen of my seed-grown trees.

For the shrub layer, I have pioneered the fruiting hedge on my housing estate. I find that a row or currants (red, white, black and buffalo), gooseberries and wild raspberries can be cut as an urban hedge and still fruit quite prolifically. The raspberries have to be wild raspberries as cultivated ones grow too tall and don’t respond well to being cut back. It’s not the traditional way of pruning these fruit and I’m sure yields aren’t as high as they might be, but it gets them into the city landscape. I’ve also managed to sneak a few into shrub beds where they can express themselves more fully. Several other people have taken up the idea and fruiting hedges are slowly spreading around the whole estate.

The ground layer is the hardest to guerilla garden as you have to find somewhere out of the way of Council sprayers and strimmers. Fortunately the local park is going a little wild in places, so I’ve managed to plant out wild garlic, wild strawberries wood violets and a selection of others in the shady parts.