Understanding Cover Crops |The Basics and Beyond

This week, I continue my conversation with Jack Algiere and we shift our focus to cover crops. If you missed last week’s episode on crop rotation, be sure to check that out too. Jack is the Farm Director of Stone Barns Center, a highly-regarded expert on sustainable and regenerative farming, and a wealth of information on all of these important aspects of soil health.

Stone Barns Center is located just about 30 minutes from downtown New York City. This spectacular landscape is expertly managed by Jack and his team, providing much of the food for renowned chef Dan Barber of nearby Blue Hill restaurant.

In 2017, we filmed an episode of my show Growing a Greener World® at Stone Barns Center and Blue Hill. While an in-person visit can’t be beat, you can tour these leading edge operations by watching our Season Nine episode.

I have been looking forward to this conversation with Jack for a long time, so let’s dive in to the rich and diverse world of cover crops in the garden.

Jack Algiere is renowned in the world of agriculture. (photo: Ben Hider)

Cover Crops

The most effective cover crops don’t produce something you can eat. While that can feel like a waste of space for some gardeners, the benefits to your soil are well worth it.

There are many reasons to grow a cover crop and hundreds of varieties to choose from, so how do you know which is right for your garden? Well, that depends on what you are trying to achieve and the effort you are willing to invest. Even the weeds that grow in your garden can be considered a cover crop if you manage them properly.

Cover cropping is all about thinking ahead and managing as you go. Jack calls it choosing your own adventure. The primary function of a cover crop is to protect the soil surface, but in addition, it can provide your soil environment rest, nutrition, aeration, or “exercise” – or a combination of those benefits.

As a general rule, cover crops are sown in a garden space after the season for edible crops has ended. Once your summer crops are spent and removed from the space, the cover crop is grown within the space during the fall and winter season.

There are three basic families of cover crops, and they each offer specific benefits and challenges:

Grains – like annual grasses, rye, oats, and wheat These crops build biomass and break up soil compaction with extensive root systems. Their leaves also improve water infiltration by slowing down the movement of water from rain or overhead irrigation.
Legumes – like peas, soybeans, clover, and vetch These are commonly known as nitrogen-fixers. The varieties within this family each do slightly different things.
Broadleaves – like buckwheat, mustard, and alyssum These germinate quickly to shade out undesirables, like weeds, and they are easy to turn in for nutrient benefits.

Some of the varieties within each family are perennial and some are annual. Like other plants, their life cycle will often depend on your hardiness zone.

Seen here are cover crops in two of my raised beds at the GardenFarm. The bed in the foreground has been cut down and is ready to turn in to the soil. The cover crop in the background is still growing and ready to be cut down.

Let’s explore some examples.

If you want to add to the nutrient bank in your soil, plan ahead to incorporate legumes or grasses as a cover crop at the end of the summer season. Both these families of cover crops draw more photosynthesis than others, so when you allow them to grow to the proper stage and incorporate the plant material back into the soil, the nutrients held in the plants are released into the soil food web to feed your future edible crops.

If your soil is compacted, the deep roots of grain cover crops will break up and aerate the soil naturally, while also adding organic material and nutrients.

You can watch the episode of Growing a Greener World where I demonstrated cover crops in my raised beds – what they look like and how I incorporated them back into the soil. That episode focused on traditional cover cropping, but there are many variations you can use in your garden.

Some cover crops can require time and a little brute force to work in. Winter rye is a common option, in part because it will grow through even harsh winters. As a grain, its deep roots aerate soil, and it’s a great source for feeding your soil with nutrients.

That said, the deep roots of winter rye can require a lot of work on your part to turn over into the soil, so if that’s not realistic for you, consider some gentler options. Austrian winter peas can be much easier to work in, but they provide many of the same nutrient and soil protection benefits you may need from a cover crop.

Red clover can be another gentler option, and it can be planted even in early spring, if you didn’t get around to planting a cover crop at the end of the summer season. Red clover is so resilient, you can broadcast the seed onto frozen ground. The seed will survive ongoing cold temperatures and will sprout as temperatures warm and the soil softens. This is known as frost seeding. As long as you broadcast the seeds before the first sign of weeds sprouting, the red clover will have time to germinate, grow and be ready to work into the soil before you plant your edibles after the last risk of frost has passed.

Here I’m in the process of turning the cut cover crop into the soil of my GardenFarm raised bed. It definitely took a little muscle, as you’ll see when you watch the episode of Growing a Greener World® where I demonstrated this.

So, when is a cover crop ready to work into the soil? Jack explains that the plant is storing up nutrients right through the point at which it sets flower. Once the crop has set flower, it’s at the apex of benefit.

The sugars produced by the plant are focused in the flower. While in that form, everything you want transferred from the flower into the soil food web is at the perfect stage. It’s also easier to kill the cover crop while it’s in the flowering stage, and ultimately, cover crops are grown to be killed and put back to use in the soil.

When a flower transforms to seed, the sugars in the flower turn to starch. As a starch, the energy and nutrient benefit are no longer available to the soil. So, the ideal time to cut down a cover crop is after flowering and before the seeds set.

It’s these same principles that apply when cultivating the weeds in your garden as a cover crop. There are nutrients stored in the foliage, flowers and roots of the weed. As long as you turn it back into the soil before the weed goes to seed, you’re turning lemons into lemonade by preventing seeds from spreading – and using the weed to feed your soil.

Cut the growth of any cover crop close to the soil surface, and then, you have two options:

Turn the plant material and the roots into the soil to take advantage of the nutrients stored in all the plant materials.
Add the cut foliage to your compost pile and turn just the roots into the soil.

Option one feeds the most nutrients and organic matter directly into your soil, but it can also take more effort to mix that much material into your garden bed. Option two can provide plenty of benefit with less physical effort. The nutrients within the foliage will still find their way into your soil once the composted material is added to your bed later.

Once you turn the plant materials into the soil, there are certain microorganisms which are triggered to go into digestion mode. They go to work to break down all that fresh organic material. If you sow seeds during this period, the seeds may actually be digested right along with the cover crop.

By the time these edamame have set seed, they are no longer as valuable as a nutrient-rich cover crop. You can still enjoy the delicious seeds, and the remaining foliage and roots would provide some benefits to your soil if you cut them back and turn them in. However for optimal benefit as a cover crop, it would be best to cut the edamame plants to the surface before they’ve set their tasty seed pods.

Jack recommends that you allow 2-4 weeks for the cover crop to be broken down by the soil food web before you sow any edible crops. Roots break down more quickly than foliage, so if you are short on time, turning in just the roots will reduce that digestion period.

Once the “digesters” in the soil have done their work, other microorganisms will go into action to do different things, and it’s safe to plant. You will likely still see some dry plant material in the soil at that point. That’s not an indication that the digestion period isn’t over, it just takes longer for some of those materials to be broken down.

So how does all of this impact soil structure? After all, most gardeners now know that it can be detrimental to break up existing soil structure, which is why a no-till approach is becoming commonly-accepted as most beneficial for a healthy soil food web and, therefore healthy soil.

In Jack’s experiences at Stone Barns, he’s found that cover crops and the no-till approach can be complimentary. Vegetable crops prefer soil which has been disturbed and aerated, but tilling tends to go deeper than is beneficial. When cover crops are turned in, it’s just the first several inches of the soil which are impacted. The complex soil structure further down is left intact.

Tillers tend to reach further into the soil surface, breaking up more of the structure without the benefit of all the organic material and nutrient benefit provided by the cover crop.

Jack and his Stone Barns team have developed a complex cover crop system to work in tandem with crop rotation for optimal soil health.

A Cover Crop Alternative

Perhaps you have limitations that prevent you from working with cover crops at all? Mulch is a great low-maintenance alternative. Natural mulch materials protect the soil from harsh elements and erosion. The mulch will break down over time to provide organic material and nutrients to the soil, and there’s no need to expend effort to turn anything in to the surface.

I teach on the benefits of natural mulch all the time. It’s one of the single best things you can do for the health of your garden. I plant cover crops at the GardenFarm occasionally, but not every year. More often than not, I use mulch instead.

I have to say that the seasons after I planted a cover crop, my soil is even richer and healthier than the years I don’t. Frankly, it’s amazing. But some years, working with a cover crop just isn’t feasible for me for one reason or another. You know what? My soil is still amazing thanks to my organic amendments and the natural mulch that I never fail to use. It’s not quite as amazing as post-cover crop, but it’s pretty darn close.

So don’t feel the pressure to cover crop every season – or any season – but never skip the mulch.

Jack and the team at Stone Barns rely on mulch too. He loves using straw and, in spring, he and the team simply pull the straw to the side and plant in the soil beneath it, leaving the straw on the exposed soil areas.

There are living mulches too – which might be the perfect compromise between cover crops and mulch in your garden. Alyssum and white clover will protect the soil and feed nutrients back to it. They can be planted around your edibles without out-competing them, and they have the added benefit of drawing in pollinators.

The team at Stone Barns Center often plants alyssum and clover under the tomato crops. Did I mention they look great in the garden too?

The Stone Barns Center complex includes this large state-of-the-art greenhouse, which continues to produce food through the cold winter months.

So Many Cover Crop Options

If you plant late fall season crops, like kale, try adding crimson clover or oats around your kale plants. If you sow the seeds in early fall, the roots of the cover crop won’t be mature enough to compete with your producing kale. Once the kale is done for the year, allow the plants to die in place, the cover crop will grow up around the dead kale plants to protect the soil through the winter and be ready to turn into the soil in spring.

Did you know some cover crops, like mustard, release chemicals into the soil which deter some harmful nematodes and grubs? There are so many ways the natural world can work for us – without ever requiring that we reach for a treatment product.

At Stone Barns Center, the team often uses cover crop mixes. The diversity of the mix adds to the diversity of the benefit to the whole system. They also use Dutch white clover as a living mulch on their pathways. The clover isn’t aggressive enough to outgrow the space, but it’s tough enough to take foot traffic.

There is so much more to the world of cover crops than you have probably ever imagined. In fact, it can be a bit overwhelming. Don’t get bogged down in paralysis by analysis. There are great resources at your disposal to help.

Many seed companies sell cover crops, and their staff is often happy to guide you in your selection. Their advice will be based on your unique needs – the area in which you garden, the season in which you want to plant a cover, the other plants you are wanting to grow, and your cover crop goals (like building fertility or breaking up soil compaction).

Contact your local seed company. They can be an invaluable resource to get you started, but then, don’t be afraid to just keep experimenting. Seeds are fairly inexpensive. Try something new and observe the results. Everyone’s soil and conditions are different and will change from year to year. A gardener’s most powerful tool is the willingness to experiment.

Just look at the space between these mature kale plants. This is a great opportunity to scatter seeds of a cover crop to germinate and grow to take over the space by the time the kale is spent for the year.

As we try new things and observe results, we learn what works and what doesn’t. We also learn how to be more precise in our choices and our timing. Don’t fear failure – those are learning opportunities. Embrace the journey.

Speaking of journeys, I can’t wrap this up without suggesting that you make a journey to Stone Barns Center sometime. The Center is open to visitation and holds frequent classes to teach more on cover crops, crop rotation, and all the unique agriculture management techniques they employ. It’s also just a beautiful place to visit. I loved the time I spent there with the Growing a Greener World crew in 2017 and am looking forward to getting back soon.

Be sure to listen in to my conversation with Jack. He offers so much information and other specific examples for you to consider in your gardening. Scroll to the top of the page and click the Play icon in the green bar under the page title. I hope you enjoy. I sure did.

Which cover crops have you used in your garden? I would love to hear about your experiences in the Comments section below, and our shared stories help other gardeners in their journey too.