Nematodes, marigolds, and crop rotation

Marigolds as a cover crop: they are not only useful, but also beautiful and
attract butterflies and bees.

This summer, for several reasons, I skipped most of the summer crops and planted a cover crop of marigolds. I'd also planted a lot of marigolds around the tomatoes earlier in the season--about 3 marigolds for every tomato plant.

This is my experiment to see if I can reduce the root-knot nematode populations in my gardens.

First, as I've been preaching in my talks, just planting marigolds around the tomatoes does not prevent nematode damage. Here is the result...


Tomato roots are stunted and filled with nematode damage even though the plants were surrounded by marigolds.	The marigold roots have no nematode damage and even have some mycorrhizae fungus intertwined.

While the marigolds can shun the nematodes, they need to be a cover crop that is turned under to reduce the nematode population in the soil. Having a cover crop also means that the soil gets a rest from the stress of growing a crop.

So here's my scheme: After pulling the marigolds and weeds, I dig out the garden soil about six inches to form shallow rows, then I add layers of fresh grass clippings, marigolds that were pulled last week, freshly clipped or pulled marigolds and then enough compost to bring the level back up to the original level. Then I chop the rows repeatedly with the shovel to break up the marigolds into smaller pieces. After that I add the original garden soil on top of the mixture.

We have an abundance of grass clippings right now because we were out of town for a few weeks. We had someone mow the front lawn while we were gone, but the back yard was a jungle.


The original garden soil has plenty of earth worms.	After smoothing the soil I sprinkle a light layer of grass clippings and then cover with pine needles and then irrigate with rain barrel water.

There's a very good reason why rain barrel water is important for this process: it doesn't contain chlorine that would impede the microbes that are needed to break down the grass clippings and marigolds. This is a form of in-the-ground composting. I will give these beds a few weeks before I plant the cool weather crops.

The walking onions need to be walked to another spot!
The onions are in the pot next to the garlic chives.

Crop Rotation

When you plant the same crop in one place year after year, it causes several problems, but here are the two biggest problems:
- It depletes the soil of the same nutrients over and over.
- Its soil-borne enemies, such as root borers, are already in place to attack it the next time.

Crop rotation by plant family helps to combat these problems, especially in organically managed gardens. For instance, if you plant parsley in one bed, plant a crop that is not in the carrot family there for at least the next two seasons.

This is easy enough to do if your crop lasts only one season, but with a perennial crop like walking onions or garlic chives, it's a different strategy. After my walking onions had been in place for at least six years, this year, they started to deteriorate. I usually use them for only for their greens, but when I dug them up, something had been eating their bulbs. So it's time to clean them up and walk them to an entirely new bed. I think I'll move them to the herb garden behind the chives. I need to plant some more chives there as well.

The garlic chives have only been in place for two years, so they are still looking good, but I'll thin them out to start some in a different location--maybe even a pot.

This year I grew lime basil and at this point in the season, the leaves are extremely citrusy and will probably not be useful for "normal" pesto. We'll see what we come up with maybe we'll think of something interesting to do with it.

How does your garden grow?

Green Gardening Matters,
Ginny Stibolt

Nematodes, marigolds, and crop rotation

Crop Rotation

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