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.