Techniques- Mulch

Starting in the early 20^th century various growers started experimenting with heavy mulching to grow vegetable crops. This was touted as a way to reduce work weeding and digging the soil and build soil organic matter at the same time. The technique basically works as advertised, though has the downsides of lowering the soil temperature (useful in hot climates but not in cold ones). It also somewhat reduces soil moisture loss, though this predominantly occurs through the plants, but can also reduce absorption of light showers. The trick of pushing back the mulch and seeing moist soil is a bit misleading. Peeling back the top 5 cm of exposed dry dirt will reveal the same with a bit more effort. Weed suppression also mostly works as advertised, but with a few important caveats. Perennial weeds that can spread vegetatively are often strong enough to grow through the mulch layer, in which case the mulch acts as a barrier that makes them more difficult to manage with techniques like hoeing. On top of this imported mulch is often contaminated with just these kinds of weeds, particularly the mulch made from municipal green waste that has the most balanced nutrient profile and best price point.

My biggest issue with mulch gardening is how it doesn’t necessarily reduce the total amount of work involved but instead shifts it into other forms that are often invisible. Most mulch dependent gardens rely on importing large quantities of biomass from elsewhere that has been produced and transported as a result of prodigious fossil fuel consumption. This explains why mulch dependent gardening is such a recent discovery despite its apparent simplicity. Pre-industrial humans would never devote so much precious time and energy to cutting and carrying vast amounts of plant matter just to save a little time hoeing weeds around their small vegetable gardens. Animal manure, urine, bones and ash represent much more concentrated forms of plant nutrients that can be moved more efficiently by hand. Instead the raw biomass can be fed to livestock, either laboriously stored as hay in climates with a severe cold season, or directly by the animals themselves to avoid all that labour. The modern practice of relying on paper or cardboard is especially amusing to me from a deeper historical context. These materials were extremely rare and expensive in pre-industrial societies since they are so energy intense to produce. In our profligate industrial age they are produced in vast quantities and then discarded. Using these waste streams to grow food is admirable in some ways, but only in the short term. At some point in the future they will likely become expensive or inaccessible so it to me it seems foolish to build your gardening techniques around them. People in the future may look back on paper and mulch gardens with as much wonder as if we had been using old silk sheets to smother our weeds.

One solution to this is to grow your own mulch and this is an admirable step. This gets closer to solving the problem but still presents some challenges. The first is that it dramatically increases the required growing area, with more than double the crop growing area needed to be put under biomass plants. It also requires a design decision. Either the mulch plants can be grown separately, requiring more labour moving the biomass to the cropping area, or they can be integrated into the growing areas, requiring more labour to manage the biomass growth to prevent unwanted competition with the crop (though done correctly in hot climates the biomass can provide the right level of shade). Another alternative is to rotate the cropping area through biomass plantings over time, however this either limits the biomass species to easily eliminated annuals or demands the labour of removing the perennial biomass plants before cropping. My feeling is that a combination of all these approaches would be the optimum, with long banks of manageable long lived biomass species grown in alternating rows with cropping spaces. The Inga Foundation (http://www.ingafoundation.org/) is developing a system of using the icecream bean tree in carefully spaced rows to produce a shading canopy that eliminates weeds and drops deep litter. This species fixes nitrogen and accumulates phosphorus. Before cropping the trees are cut down producing more biomass and firewood and a crop grown before the tree canopy closes again. I have established this system on my farm and will report on its cropping performance in the near future.

One difficulty I have with this Inga based system is the potential for the trees to grow to a great height if left untended long enough. This prevents me planting them under the powerlines that are over some of my best cropping land on my creek flats. Instead I think a similar system can be developed using some combination of Tithonia, pigeon pea, or long lived competitive field crops like Canna. In this system the cleared space would be cropped for a year then the longer lived species left for several years. Pigeon pea has the advantage of naturally dying off after a time and mature trees normally don’t come back when cut back to the ground. Tithonia is more persistent and prone to layering sideways if left untended for too long. I am in no rush to design the layout of this space and will instead keep weighing up different options (likely to be the focus of a future post).

Recently observations have changed my opinion on exactly what mulch is and how it works. The usual model is that the mulch needs to be run through a machine to make it easy to create a dense blanket at least 5 cm thick in order to “smother the weeds”. I do not believe this is the whole story. Weeds that do manage to establish in mulched ground often behave very differently to those growing on exposed ground. While they grow quite well, often better than their exposed counterparts, their root systems are often relatively under-developed, making them much easier to remove (one of the hidden benefits of mulching). I suspect the relationship between the weed and the soil microbiome is different. As mulch decomposes it releases a stream of soluble carbohydrates that percolate into the soil, favouring different soil microbes. Soil fauna and their effect on soil structure likely plays a part as well, but plants fundamentally rely on microbes to deliver water and nutrients to them from the soil. Weeds in mulch seem to have an easier time getting what they need, so invest less heavily in their root systems. They also seem to delay flowering and setting seed (also making controlling weeds easier on a longer time scale). When the soil microbiome changes enough then weed seeds may not even germinate, as we now know dormant seeds interact with the soil microbiome to decide if they should grow.

Thus the smothering effect of mulch is not the main reason to use mulch, and running plant biomass through mulching machines is mostly for the convenience of machines and people transporting it long distances and applying it around small seedlings. Instead the main advantage of mulch is in providing a steady source of energy for soil microbes. I have seen this effect in small piles of branches left around the farm after cutting the green tops for my goats. The piles have numerous gaps to exposed soil and yet the normal pasture weeds do not grow through them, or if they do they behave very differently just like in machine processed mulch. The soil under them doesn’t look that different, but then again what does one soil microbiome or another look like to the human eye? It should be possible to harness the advantages of mulch without relying on machines by linking piles of unprocessed woody waste with crops that are structurally compatible. I stacked up a large pile of stripped goat branches (pigeon pea and Tithonia) in my melon bed. I decided to put all the branches in one pile for a few reasons. By sowing the melons along the edge of the pile I can ensure they are sowed properly in small piles of goat manure (useful to give them a strong start, and imitating their wild ancestor’s habit of growing in piles of animal dung). The melons can climb over the branch piles and also send their roots under the pile to potentially access any nutrients from decomposition. The other side of the rows will be left exposed to make hoeing weeds easier in the two thirds of the beds still exposed. The piles should grow no weeds, leaving only the edges as somewhat problematic (possibly sites for limited hand weeding or sickling off the tops). I have also noticed that the quality and stability of the long term end products of decomposing organic matter increase when the material is more concentrated, plus any left over material can be dealt with more easily later if I need to move it. The concentrated piles also make better home for both rats and snakes, potentially keeping that pest within limits. I have several piles in the space, one large one with all the woody branches, another smaller one with mostly weeds, and one of unstripped Tithonia to see if they behave differently.

Hopefully all these observations, ideas and experiments will lead to a useful result. I will report back around new year on my rockmelon and watermelon trials in this space and see if my intuition was correct.

The mysterious absence of weeds in an old wattle branch pile.

The pigeon pea and Tithonia branch pile in the melon bed.

A self sown wild jap pumpkin seedling in an old goat branch pile, giving me a hint about what it likes in the bed, and also telling me to get cracking sowing other cucurbits.