Green Manures and Raised-Bed Gardening

I’ll always remember the first time I heard the expression “green manure”. I had just started a course in horticultural production at the Institut de Technologie agroalimentaire, in La Pocatière. Arriving just after Christmas, I was straddling 2 cohorts. To be honest, I didn’t know much about the world of agriculture. I was catapulted into this world, a true neophyte.

I had to produce a piece of work on a theme of my choice. A classmate suggested that I talk about cover crops. Perhaps out of pride or surprise, I simply agreed. In my heart of hearts, however, I had no idea what she was talking about. Green manure, like the color of fertilizer? I rushed to the library in search of answers. Needless to say, I was nowhere near the real definition. For those of you like me who are new to these terms as you read this article, here’s a simple definition. It comes from an article published on the Laidback Gardener blog. It appeared at the end of August 2023:

“A green manure is a plant that is used to protect and amend the soil. Traditionally, these plants are sown in a vegetable garden or farmland when these are not being used by a crop, either before, after or even in between. You can even, as part of a crop rotation plan, grow a cover crop for an entire season to allow the soil to regenerate.”

Green Manure and Urban Agriculture

I’m well aware that Mathieu recently wrote an article on this theme. With mine, I’d like to add a little touch of urban agriculture. When I started working at Urbainculteurs, the use of green manure was not customary. However, we were experiencing certain difficulties, such as wind erosion of topsoil for rooftop installations. After a few years of production, we noticed a gradual compaction and impoverishment of the soil. There was a lot of talk about the benefits of green manures in the open soil, but what about in the soilless substrates of container gardens?

Since there was little or no literature on the subject, we decided to give it a try on an urban farm. Would it be a success? Would we regret it? We had to try it before we could draw any conclusions.

Try It on for Size

We did our first tests with a mixture of oats and peas. The oats create organic matter to nourish the soil, while rooting so that the soil stays in place. The peas fix nitrogen. We broadcast seeded the whole thing with shallow ploughing. First thing we noticed: birds love oats! To ensure a sufficient germination rate, we decided to cover the seedlings with anti-insect netting. We waited until the green manure had grown sufficiently, then removed it. When we judged that it was high enough (the cover crop must not flower), we mowed it down with small sickles. Obviously, this takes more time, but that’s the reality of soil-less production. At the urban farm, almost everything is done by hand!

We were satisfied with these first trials, but the step that stressed me the most was the next one. What would the spring burial be like? Oats and peas are not frost-resistant, but the plants had been left in place all winter. As I just mentioned, we have to do everything by hand. Would the green manure dislodge easily? Would decomposition be nice and quick despite the more limited soil life?

Our Findings

As soon as we were able to work the soil in the spring, we set to work. It certainly took a bit more effort than going through with machinery. We were pleasantly surprised by the results. The plants turned over well with several tools (hoe, rotary claw, rake). We then loosened the rootballs as best we could. After a few weeks, the soil became loose again, with small residues of course. So yes, it’s possible to use green manure even if we’re growing above ground.

For the moment, it’s difficult to measure the real impact of these green manures on soil structure and life. However, we really enjoyed the experiment and applied it on a large scale on the farm this season. To extend the sowing season, we even sowed autumn rye, which has the ability to germinate in colder temperatures. It was so popular that several volunteers even adopted the practice at home. And what about the look! It’s so beautiful! Our intention for next year is to extend this practice more widely to our activities. If possible, to sow cover crops at the start of the season, wherever possible. The experiment continues!