Eavesdropping on Plant-Fungi Chats: The Implications for Gardening

Almost all land plants are hooked into an underground network of fungi through which water, nutrients, and other signals travel. The network facilitates germination, sapling growth rates and plant survival, as well as the very composition of most terrestrial ecosystems. This is a fascinating new area of research, which we’ll explore below, but I’ve also been struggling to pull out the implication for gardening. Much of modern gardening lore advises spacing out plants. We’re advised to put plants far enough apart so that when they are fully mature their leaves would just brush neighbors. I learned in my RHS horticulture course that this ample spacing of plants is important so the plant can get sufficient water, nutrients, sunshine, and even air flow. Yet the fungal connectivity of plants underground suggests that having close neighbors can help a plant survive times of scarcity or other threats.

Not all gardeners adhere to the well-spaced-planting recommendations. The most exuberant of gardens are very densely planted, think Christopher Lloyd’s Great Dixter, pictured above, or Piet Oudolf’s High Line in New York. Oudolf explains that the key to a successful garden is having a matrix of dense planting, covering up to 70% of the ground surface, through which superstars emerge and shine (Oudolf Designing with Plants). The matrix may be called ground cover, but that’s a bit uninspiring as the matrix is meant to provide structure and beauty through out the growing season. So grasses, long blooming plants, and plants with beautiful foliage are good matrix choices. This matrix will be mostly absent if we follow plant-spacing guidelines.

Vegetable gardeners, too, are turning to dense planting. You may have heard of square foot gardening, or postage stamp gardens - they are all about intensive, densely planted plots. Many older gardening traditions have long used dense planting - the Chinese have underplanted rice crops with the water fern Azolla for at least a thousand years. The Azolla blocks taller weeds from growing and fixes nitrogen into the soil, thus feeding the rice. Sometimes this kind of planting, where plants provide a service to a neighbor, is called companion planting. The most famous example of which is probably the Native American’s three sisters which is the growing of beans, corn, and squash together. The beans fix nitrogen into the soil, the corn provides something for the beans to climb, and the squash provides shade and reduces water loss. And as all gardeners who prefer dense planting will tell you, putting plants closer together will reduce weeding and, counterintuitively, also reduces the need to water. So, what does science say is the best practice for planting distances?

Much of the answer lies underground, in mycorrhizal networks. Mycorrhizal networks are the fungal network that connects fungi (myco) and the roots of the plants themselves (rhizomes). Many fungi actually send thin fingers into their plant partner’s roots, but others just grow very near the root surfaces. In either case, the fungi grow through the soil into webs, and connect to other plants of various species. Mycoorhizal networks can form vast underground highways where water, phosphorus, nitrogen, and carbon are moved around. Typically, plants produce sugars, in the process of photosynthesis, and feed these to the fungi, who can not produce sugars. In turn, the fungi exude organic acids that frees up phosphorous and nitrogen, making these nutrients available to the plants who otherwise could not access these nutrients. You may know that nitrogen is essential for plants to grow leaves and phosphorus is needed to grow roots, seeds, and flowers. Because the fungal web is large and stringy, and fungi are small, the network has a giant surface area and can absorb, and hence transport, huge amounts of nutrients and water. Most land plants would not survive without the support of fungi.

The direction of the flow of water and nutrients in the mycorrhizal network goes from high to low concentrations, as any chemist will have guessed. Often the flow is towards an ailing plant. One example of resource flow is when a mature tree accesses water deep in the soil column. In times of drought, water will flow from the deep soil into the deep roots of the mature tree up towards the surface and into the tree’s surface roots. From there, the water will flow into the fungal network, and finally to smaller plants on the network that have a deficit of water.

While the idea of a gigantic fungal network transporting water and nutrients around ecosystems may sound like something out of science fiction, it is ubiquitous. Somewhere on the order of 80% of all land plants form beneficial associations with fungi. What’s more, fungi-plant relations have been around since the time land plants first emerged from the oceans. Because plants give 10-20% (even upwards of 50%) of their sugars to their fungi friends, some argue this process is fundamental to the evolution and functioning of land plants. There are even plants which don’t photosynthesis for themselves at all, but rather rely entirely on the mycorrhizal network for all their sugars, nutrients and water. A common example of this are orchid seedlings. In fact, stealing sugars from nearby plants is such a successful strategy that it has evolved independently at least 40 times. A common counter example are the brassicas: broccoli, kale and brussel sprouts - which are all actually the same species. Brassicas do not play with fungi.

Lest you think the mycorrhizal network is only about water, carbon, and nutrients, prepare to be more amazed. The network also helps plants to resist soil borne pathogens by exuding enzymes that are toxic to harmful soil organisms. Fungi even inoculate the plants in their network by triggering the plant’s defense system so the plant can withstand a subsequent attack. But what has received even more attention, is the signalling sent by a plant under attack via the mycorrhizal network. Plants respond to these warnings by producing toxic chemicals in their leaves that will repel munchers or by releasing pheromones that attract predatory insects which will eat the attackers. The evidence of this communication amoung plants has led to countless arguments about trees talking, plants having intention, and the abhorence of referring to this communication as communication at all. For my own self, yes I think the plants are communicating, and yes, I think we could call that language, but I have no idea if the sending planting had any intention of warning it’s neighbors. I can rarely unravel the intention of the other humans let alone that of a tree.

Above ground, we see the impacts of the mycorrhizal network quite clearly. For instance, the survival rate of younger trees is much diminished if the largest tree in the area dies. And in turn, trees live longest and reproduce the most successfully when they are in a healthy stable forest. This mutual support of plants, from water to nutrients, to defense warning systems, to helping ailing plants, mean plants are better equipped to colonize barren soil, to resist toxicity, and to survive acidity and contaminated soils, as well as to live longer and reproduce more successfully. Mycorrhizal networks facilitate the establishment, growth, survival, and defense of plants in a wide range of ecosystems. At the community level, the network thus influences the ecosystem composition: which species survive and how many of each kind. In this age of mounting threats to natural systems, understanding the fungal network will be hugely important for conservation efforts. And because plants and soil play such a huge in the global carbon cycle, absorbing about 25% of the carbon we emit, the mycorrhizal network is extremely important for moderating global temperatures.

Before we leave fungi to return to the implications for gardeners, let me share three astounding, somewhat random, facts I’ve learned about them. 1. There are carnivorous fungi which lure in nematodes and kill them, then harvest the little worm bodies for nutrients. One control experiment found that eastern white pines get about 25% of their nitrogen from nematodes lovingly prepared by the fungi. 2. Fungi, like slime molds, can solve mazes - which is generally used as a test for intelligence in animals. Though I’d say the fungi cheat for they don’t have to choose just one path at any given intersection of the maze and indeed they don’t. They branch out and explore all routes. Then the path that gets them out of the maze, to a favored piece of food, is emphasized while the other paths are allowed to decay. A researcher at Cardiff University set up a puzzle by placing blocks of wood onto a map of Britain. The size and location of the wood blocks were in proportion to the size and location of cities. Then they introduced a wood eating fungus. After some time, the fungi had developed a network with main corridors that closely resembled the highway system in the UK, with the M1, the M4, the M5 and the M6 all clearly visible. (Sheldrake Entangled Life). 3. One species of fungi, Schizophyllum commune, has 23,000 different sex types. And some of us are baulking at adding an “other” gender category to forms. Each Schizophyllum commune can mate with most of the other 22,999 sexes and, apparently, it does so all the time. In case your mind is boggling at what this means, all I will say is that the sexual differences are in the genome and don’t really manifest as physical differences.

As enjoyable as our little “Gee Whiz Fungal Fact Time” was, let’s get back to the important business of the implications for gardening. Hopefully, you now have a better understanding of why a dense plant-fungal community can really help a plant survive. But what about growing that perfect specimen? Surely, it’ll get bigger if there’s no competition for resources. And indeed that is so. It turns out there are two ‘pure’ cases, end members if you like. On the one hand, if there are ample resources, a given plant will grow larger if there are no close neighbors. In this case competition with others is the driving factor. On the other hand, if there is a scarcity of resources, plants benefit from dense plantings. Neighbors will facilitate growth in this case. In summary, whether facilitation or competition dominate depends on the availability of plant resources. Typically, facilitation is more important at dry arid sites, and competition is stronger at wetter sets. But to make matters even more complicated, the relative importance of facilitation and competition doesn’t just vary from site to site, but also during the course of the day at any given site.

Blimey. What’s a gardener to do? As always, that depends on what you want. If you want a beautiful dogwood tree to ogle at - give it space. But if you want as many flowers as possible, or want to create a habitat for wildlife, go for dense planting. Alternatively, you could decide your plant spacings based on how you garden - if you are a gardener who waters and feeds the soil regularly, well spaced planting may suit you. However if you are a live and let live kind of gardener, dense planting is probably best for you. I have certainly found that the beds that I have planted thickly are far less work, and hence more successful.

How else can we gardeners promote the mycorrhizal networks in our gardens? By following the advice given to protect our soil: avoid walking on soil, especially when the soil is wet, and avoid over digging, good news for our backs. And, we need to be mindful of how we fertilize our garden beds. Perhaps the most commonly sold lie about the need for fertilizers is when we are told to apply phosphorous rich fertilizers (high P in NPK fertilizer labels). Lawns and gardens, as opposed to commercial farms, are very very rarely phosphorous deficient. Claims made by fertilizer companies that adding phosphorous to our soils will improve root growth are highly misleading if not downright lies. In fact, adding phosphorous rich fertilizers to our soils can damage plants directly by causing other essential nutrient deficiencies. But even more so, adding fertilizers with phosphate lead plants to forsake their fungi partners, which then die back. This is of course a disaster for the future phosphorous availability of plants, but also for the present as disconnection from the network causes the loss of all the other services the mycorrhizal network provides. Most commercially available fertilizers have an overabundance of phosphorous. The best way to fertilizer your garden, in sympathy with the ecosystem, it is to feed it is with slow release materials such as mulch, including leaf and grass cuttings left on the lawn. These have the hugely important advantage of also adding organic material to the soil which benefits all soil types, and mycorrhizal networks.

As I was writing this article a friend asked if adding mycorrhiza to her lawn was helpful. In short, no. There are already plenty of mycorrhiza in almost all soils. Adding more is a drop in the bucket. And if there aren’t any mycorrhiza present at all and the soil is completely barren, adding mycorrhiza will only lead to their deaths as there is no ecosystem to support them. Studies have shown that mycorrhizae additions to the landscape have no significant effect on plant growth so it is simply a waste of money and resources to carry on with this practice.

Writing about the mycorrhizal network has been surprisingly profound for me, perhaps emphasized by the surreal experience of sharing my biome with the Omicron variant this past month. Thinking about the inter-connectivity of plants and fungi, has given me a sense of connectivity and peace. It has had a similar reckoning to the shift I experienced when I learned that humans have more non-human cells in our bodies than Homo sapien cells. Of course I’ve know that we rely on the oxygen and food from plants, but the unerstanding of the connectivity of plants and fungi has increased the bluriness of the concept of an individual. I am amazed at how my consciousness has arisen out of this chemical soup and I find it very comforting to know “my” body will remain a part of this network of life after my consciousness has moved on. The interconnectivity of life is as wonderous as the fact that our bodies contain recycled star material. I know I’ve used this quote from the Bard before but I can’t refrain from concluding: “There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy”.