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.


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.

7 thoughts on “Feeding the forest garden – part 1

  1. Marlyn Turbitt

    Thanks Alan, very clear and informative info, I have just grown some lathyrus so its good to know they will be good amongst the raspberries. I am trying to find out what is best to grow under asparagus-any ideas?

  2. Alan Carter Post author

    Hi Marlyn. The natural habitat of asparagus is on disturbed soils on the sea shore and river banks, where it can hang on due to its dense root system. Like a lot of things that live in those conditions, it relies on the competition being less well adapted and isn’t very tolerant of competition. There is often a succession in such habitats, where plants like asparagus stabilise the soil and then other plants come in and outcompete them.
    In a garden, this succession would begin immediately if the asparagus weren’t weeded vigorously. When you’ve put in the effort of getting asparagus established and are looking forward to 20 years of cutting spears, that’s probably not what you want, so the traditional way of growing asparagus is to keep the soil around them clear and feed them well. In this case I think the traditional practice is well based on the ecology of the plant so I would go with it.
    On the other hand, if you want to experiment, I’d try something low growing and deep rooting in the hope that it wouldn’t compete too much with the asparagus. WIld rocket (Diplotaxis tenuifolia) might be a good one.

  3. MikeH

    Comfrey is not the king of them all. The only science based research that exists is that of Dr. James Duke – http://www.ars-grin.gov/duke/, Dr. Mark Pederson – http://www.amazon.com/Nutritional-Herbology-Reference-Guide-Herbs/dp/1885653077, and Jerry Brunetti at http://www.agri-dynamics.com (See page 28, 29 where he shows lab results for a number of plants including lambs quarters, dandelion, plantain, chicory, and comfrey.). Search for comfrey in Pederson’s book at Amazon, click on the hit that starts “Comfrey was strictly used externally…”, and then move up a page where you will find a nutritional profile.
    So what is the king of them all? According to Duke’s database, Lambsquarter, Pigweed, Stinging Nettle, Dandelion, and Red Clover are the best nutrient accumulators with Urtica dioica being the best overall, ie, the king.
    Most of the dynamic accumulator information in circulation is very similar to this – http://oregonbd.org/Class/accum.htm which has no disclosed research supporting it.

  4. Alan Carter Post author

    Hi Mike. I’ve been thinking about this since our exchange in the comments section on one of Deano Martin’s posts on the Sustainable Smallholding. I don’t think that the value of a plant for providing nutrients to the rest of the system can be reduced to the relative concentrations of minerals in its leaves. Firstly there is the question of absolute rather than relative fixation rates: a plant that has twice the concentration is no more use if it grows at half the rate – the production per unit area is the same. Secondly there is the question of where the plant is getting the nutrients from. If it is taking them from the same layer of the soil that the food plants in a perennial system are exploiting then it isn’t so much scavenging nutrients for the other plants as from them.
    The plants you mention generally have shallow, fibrous root systems. While this would make them efficient at preventing nutrient loss in otherwise bare soil, a perennial system is completely different. There the perennial roots, mycorrhizae and soil bacteria should already be fairly good at taking up surface nutrients, so a shallow-rooted plant planted for that task will largely be taking nutrients (not to mention water and light) away from the productive species. The only uses I can see for dynamic accumulators are (1) as a green manure on bare soil that isn’t ready to be planted with a productive crop and (2) where they are sufficiently deep-rooted that they are mining nutrients from deeper in the soil than their neighbours and making those available once they are cut.
    I take your point (made on Deano’s blog) that the research base for all this could be much better. I’ve just been searching Garden Organic’s site (they are the successors of the Henry Doubleday Research Association, which originally popularised the use of comfrey) for actual research data and came up with nothing. However, I can say from my own observation that comfrey is undoubtedly very deep rooted and almost ludicrously productive, without noticeably depleting the soil for plants next to it. I mostly use my patch to convert urine into a more culturally acceptable input for the rest of the garden.
    Thanks for your comments


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