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Green Manure and Cover Crops


It's nearing the Summer Solstice and the garlic is beginning to dry down and the bulbs are all quickly swelling. While we are waiting for the harvest season to begin, I decided it was time to tackle an area I get a lot of emails about: cover cropping and relatedly, green manure crops. This post will talk about what we do on our farm here in Northeast Washington but you should experiment and see what does best in your garden. We never stop trying new things, so I expect this post will evolve over the years.


What is a cover crop?

A cover crop is a plant that is planted for the sake of covering and protecting the soil, rather than being harvested for a cash or food crop. A green manure crop is specifically grown to build and improve soil fertility. The benefits include:

  • Improve soil fertility through nitrogen fixation and nutrient scavenging

  • Increase organic matter in the soil

  • Prevent erosion (from wind and/or water)

  • Improve soil microbial life

  • Conserve water

  • Suppress weeds

  • Increases biodiversity

  • Attract pollinators

  • Control pests and disease


The above picture shows 3 different 1-acre plots each in a different cover crop. Nearest to us on the left is newly germinated buckwheat that was planted into a winter-killed vetch crop as soon as the soil was workable. In the middle-right is over-wintered vetch beginning to grow after a long period of dormancy. In the distance we see a fresh planting of vetch frosting the field with incredible germination.


What plants can be grown as a cover crop?

There are number of options and what you'll need depends on your location, your season, and your goals. Here is seasonal list of commonly used cover crops:


Spring:

  • Spring Field Peas - Fixes nitrogen and provides biodiversity.

  • Oats - Biomass, ground cover, and suppresses weeds.

  • Red Clover - Fixes nitrogen, suppresses weeds, attracts pollinators.

  • Annual Rye Grass - Quick to germinate and provide ground cover.

  • Alfalfa - Fixes nitrogen, suppresses weeds, good perennial cover crop.

Summer:

  • Buckwheat - Lots of biomass, weed suppression, scavenges phosphorous.

  • Vetch - Fixes nitrogen and suppresses weeds.

  • Sudan Grass - Lots of biomass and good weed suppression.

  • Forage Radish - Breaks up the soil and increases drainage.

  • Many more including sun hemp, cow peas, soy beans, mustard, kale, turnip, rapeseed, and pearl millet.

Fall:

  • Forage Radish - Loosens up tight soils, provides soil cover through Winter.

  • Vetch - Fixes nitrogen, covers soil through winter, and continues to grow through the next Spring. This our "go-to" late Summer/early Fall cover crop.

  • Austrian Winter Peas- Fixes nitrogen and covers soil through the winter.

There are many more options available. Talk to your local farmers and organic gardeners and see what they're using. There should be a variety of options available through your local nursery or garden supply store. National seed suppliers like Johnny Seeds, High Mowing, and Peaceful Valley have small quantities that can be shipped. Our favorite commercial cover crop seed supplier is Welter Seed Co.


Our cover cropping routine is always changing and evolving based on what works for us (or doesn't work). Out of necessity we "dry crop" our cover crops (no irrigation is provided). We also live in a very arid region with a short growing season, so we have to take advantage of every bit of rain we get during the growing season to establish cover crops and keep improving our soil.


We choose not to use any cereal grasses such as oat or rye because we are farming on land that has been established pasture for the last century. We began with soil that had a lot of wire worm pressure (see link for more information) and so we try to avoid large plantings of anything they are attracted to. Wire worms can cause massive damage to garlic plants, if you ever find a little orange worm-like critter burrowing into your garlic bulb, it is likely a wire worm.

We tend to stick with alfalfa, peas, buckwheat, and vetch in our cover cropping.

  • We always have 1 plot that needs extra love and soil building. We take that plot and only plant a cash crop 1 in 6 years and spend the rest of that growing rotation growing and mowing alfalfa. This does an amazing job of building beautiful, fertile soil and by the time the next garden rotations starts we're ready to start building on a new plot.

  • We plant peas in the early Spring if we anticipate a cool enough Spring, but in our region, it often gets too warm for peas at the same time it is dry enough to prepare the soil.

  • Hairy Vetch does well for us and is more forgiving if we are hot and dry (most years), we are usually able to plant and germinate vetch into September. If it flowers during the season, it fixes an incredible amount of nitrogen and then dies off and forms a nice Winter cover. If it does not flower it will die off, form a nice cover, and often begin regrowing early the next season, putting on a full season's worth of growth in time to turn it into the soil before planting our warm weather crops.

  • Once the soil warms and there is no chance of frost, we begin planting buckwheat. Buckwheat germinates quickly and does a great job suppressing weeds, in 30-45 days our buckwheat is generally 2-3 feet tall and ready to turn into the soil. It helps us to get multiple cover crops planted in each plot, each year. Besides providing a lot of fast-growing biomass, Buckwheat also scavenges phosphorous which makes it an ideal cover crop for preparing ground for garlic or potatoes.

That wraps up the basics on cover cropping around our farm. If you have a question or something you'd like me to add, please email me at orders@mountainvalleyorganics.com and I'll do my best to address your questions and update this blog post.



This luscious buckwheat was waist high and starting to bloom 30 days after a mid-May planting.


Cover Cropping Questions:

Are cover crops easy to grow?

Cover crops require water and the right environmental conditions to grow but are otherwise pretty easy. Timing the right cover crop for the time of year is important.


What do you suggest for cover crop towards mid-August?

Our cover crop of choice in August is buckwheat because it is a warm weather crop that is easy to germinate when it's hot, it grows fast and usually within 30-45 days it is ready to turn into your soil, it also will die off with your first frost. We love it for its ability to quickly outcompete weeds, the loads of organic matter it brings to the soil, and its ability to scavenge phosphorous from deep in the soil and bring it to the surface.


We ran into a heavy moisture problem this year, which has caused some rotting of bulbs, have to pull about half of what we planted. I read that mustard is good to battle fusarium (sp.)? Any insight you could offer would be hugely appreciated.

Mustard can be used as a biofumigant, but it requires a specific variety of mustard and specific regiment for treatment. The variety of mustard goes by the brand name Mighty Mustard (there may be others, this is just the one I am familiar with). The process is to allow it to grow into full flower and then mow it (ideally with a flail mower where it can be chopped into lots of little pieces), incorporate the mustard into the soil using a tiller or disc, and then pack the surface tightly (usually with a cultipacker) to seal it all together and water it really well. This process releases the biofumigant properties into the soil and the water carries it through the soil. It's difficult to do on a small scale without specialized equipment.

We have struggled with fungal problems in the past (we have very heavy, sub irrigated soil). We played with mustard for a couple of seasons and really didn't find that it decreased our fungal problems. It was a beautiful cover crop that the bees and native pollinators loved and it provided a lot of organic matter, but it was a lot of work that didn't live up to the hype (in our opinion).


What we have found to work best for us, is to use T-22 (brand name Root Shield, I'll link it below) at planting time as well during the growth stage of the plant (from emergence to scape removal). The T-22 is a mix of fungi designed to target fusarium. We dust our cloves with it prior to planting, and then apply using water (we apply it through our drip irrigation system using a fertigation system, but you could also apply using a watering can on a small scale). This was our 3rd year using T-22 and even with our warm, wet conditions we have had very little fusarium show up. It's expensive, but has been very effective for us. The most important aspect in controlling fungal problems is crop rotation. We plant garlic 1 year out of every 5-6 years, but a more common rotation would be 1 in 3 years (we have a larger rotation largely because our soil is so prone to fungal problems). I hope this helps, please let me know if you have other questions!


Beautiful Mustard cover crop teaming with bees and pollinating insects.

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