Garden information comes and goes in waves, it seems. There’s always a trendy suggestion or two each season that gets a lot of coverage. Considering gardening is such an expansive subject, that probably gives us a hundred years’ worth of trends to go through before we start to repeat again.
This past spring, I noticed a lot of talk about using mustards as a soil fumigant, particularly the Kodiak mustard. I thought I’d better add a few ‘notes’ to the conversation. There’s more to it than simply planting Kodiak mustard, letting it grow a few weeks, chopping it down and working it into your soil.
First, the question: Should you even use mustards to fumigate your soil?
Only if you need to.
The talk is all about how to do it, but let’s just cool our heels for a second and talk about the full effects of what happens when you use mustards to clean your soil.
To be a truly good gardener, you need to understand the soil, the critters & animals in your area, the weather & environment, composting, etc. Rather than pulling out the chemicals — whether it’s pesticides, fertilizers, fungicides, etc. — every time something isn’t going right in the garden, a gardener should be looking for and solving the underlying problem.
We highly support the use of green manures (cover crops used to protect and then enhance the soil). Green manures come in all shapes and sizes: clovers for nitrogen, buckwheat or other grass grains for organic matter, or mustards for organic matter & soil cleansing. As with everything, there’s a time and a place for each of these.
Mustards, and the rest of brassicas, naturally contain glucosinolates which are responsible for the spiciness, or pungent heat, of brassicas. Some brassicas contain more than others. Most mustards contain a greater proportion of glucosinolates per gram of plant matter than any of the other brassicas (except for cress), and they grow faster than any other brassica, which makes them ideal cover crops.
You can use mustards to fumigate the soil by chopping or mowing them down just after the flower buds have formed, but before they go to seed, and working the plant matter into the soil, where it will break down. The breakdown of the cells of the plant releases the glucosinolates which, in the presence of an enzyme & water, break down into different forms of isothiocyanate, a fumigant which will seep through the soil and kill or prevent the growth of nematodes and some other soil-borne diseases.
This is just another example of the potency and effectiveness of plants. Plants DO things. They contain medicines and poisons — for people, plants, soil, animals, insects, birds, etc.
(Ever heard of mustard gas?)
So our next question is: When (if) should you use mustards, or any of the other brassicas, as a fumigant?
Not all nematodes are pestilential. Many nematodes are beneficial – feeding on organic matter, insects, and bacteria.
If you use mustards to fumigate your soil without regard to what’s really going on in the garden, you may end up killing many of your beneficial nematodes and other organisms that make up the healthy living soil system, allowing space for pests to move in.
This means, you should only use mustards to fumigate your soil if you have a nematode or other soil-borne disease problem that is currently affecting your plants. And preferably only on the area of your garden with the problem.
Nematodes are so tiny that most are microscopic and cannot be seen with the naked eye. As such, they don’t travel very fast or very far in a season. Unless you start mixing the soil around your garden, most won’t spread further than three feet from the affected area in a single year.
Just like any good gardening principle, you need to know what’s going on in your garden before you jump in and do anything, or you may end up causing damage rather than doing good.