I used to think cover crops were for commercial farmers with tractor attachments to use for tilling. Now, I know they are a basic gardening tool that any gardener can use to improve and protect soil while reducing weeds.
What are Cover Crops?
Cover crops are kind of like an exciting adventure vacation for your soil life. Because they germinate and grow quickly, a round of cover crops is the soil life equivalent of us humans spending “time off” scuba diving or rock climbing rather than gorging at the buffet on a cruise ship. In other words, cover crops are a reinvigorating, life-affirming thrill for your soil!
Plus, while giving all the life in your soil a great adventure — they prevent erosion, preserve nutrients for later use, and occupy soil so weeds can’t get a roothold.
How to Use Cover Crops
Using cover crops isn’t complicated. In fact, it involves less work than not using them because they reduce weeds and improve soil to make gardening easier the rest of the year.
The basic steps for cover crops are to scatter seeds thickly over your bed area. (You need to ensure a high density of plants germinate and grow to carpet the bed.) Water or rake them in. Then, encourage the plants grow quickly to increase their carbon content and root mass in the soil.
Most cover crops don’t need a lot of care. However, regular watering, occasional liquid fertilizer like compost tea, and early frost protection can help plants thrive and produce the most benefit for your soil.
Before or shortly after flowering (or when you need your bed back) but always before the plants set seeds, kill the cover crops and leave their plant parts to decompose in the bed. The easiest ways to do this are by mowing or light deprivation.
Option 1: Use your Mow(jo)
Repeat mow until the plants stop regenerating. Some cover crops die after 2 or 3 mowings, others may need a few extra passes to give up the ghost. Tools for mowing include a string trimmer, scythe, sickle, or even scissors in a small bed. Mowed plant matter should stay on the bed to decompose and nourish the soil.
Option 2: Let there be no light!
Mow plants down to create a flat area. Then, cover the bed with a few layers of uncoated paper and an inch of aged compost. This keeps plants from getting light so they can’t photosynthesize and eventually die.
Avoid glossy paper unless the manufacturer declares it safe to compost. In some cases that shean comes from biodegradable materials like kaolinite, calcium carbonate, bentonite, talc, and cornstarch. However, much of the time synthetic polymers or adhesives made from plastic byproducts are utilized. Those can have detrimental impacts on soil life.
Kill Methods to Avoid
Some people also kill cover crops by tilling them under several inches of soil or spraying them with herbicides. These are methods I would never consider for use in my garden.
Weed killer is detrimental to soil and wildlife and may complicate growing future crops. Tilling exposes soil to air causing a loss of nutrients and carbon while disrupting soil life networks by breaking up soil layers.
If you don’t want to use the mowjo or no light methods, then, skip the cover crops and just cover your soil with cardboard topped with mulch when not planted. This is more like a cruise ship buffet type vacation instead of an invigorating thrill for soil life. But it’s better than just leaving your soil bare or than disrupting soil life with herbicides or tilling.
When Should You Use Cover Crops?
Cover crops can be used anytime you would otherwise have bare soil such as in between planting, over winter, during extended vacations when you won’t have time to garden, etc. You just have to make sure plants get going strong before it’s too hot or cold for them to germinate and grow.
Repeat cover cropping is also a fantastic way to prepare soil for a new garden. To use for new beds, apply your kickstart amendments like compost, aged manure, grass cuttings, etc. Then seed, grow, mow, and repeat for several rounds of cover crops to work those amendments in and increase carbon, air spaces, and moisture retention into the soil before you plant more demanding crops.
What kind of plants work as cover crops?
There are a lot of tried and true cover crops used in commercial farming.
- Cool season favorites include crimson clover, hybrid mustard, tillage radish, oats, barley, hard winter wheat, Austrian peas, and annual rye.
- Warm season favorites include buckwheat, cowpeas, sesame, sorghum sudangrass, and millet.
These are great options because the seeds come in bulk packaging. They’ve also been field tested so you can find lots of information from agricultural extension offices on how to use them and what kind of impact they’ll have on the soil. However, for home gardeners, we aren’t limited to the standard list of cover crops.
We can use any fast-growing, easy to kill plant that makes sense in our crop rotations. I particularly like to use plants that are extremely easy to save seeds from with almost no work. For example, I spread half-rotted tomatoes (that I didn’t preserve in time) over beds in late summer. Within days, hundreds of tomato plants erupt and grow quickly to blanket the soil. When the seedlings are a couple inches tall, I broadcast out arugula or corn salad (mache) over the beds. After first frost, the tomatoes vines die back to reveal and act as mulch for the leafy greens of those cool weather crops that germinated and grew happily in the shade of those tomato cover crops.
I frequently use all sorts of beautiful mustards, kales, storage type radish, collards, and ornamental cabbages instead of the commercial farming hybrid cover crop type mustard or rapeseed plants.
Also, thick beds of lettuce, zinnia, balsam flowers, sunflowers, basil, or marigolds are some of the less conventional cover crops I grow. When planted densely, the plants stay small and grow low to the ground. Most don’t flower due to crowding or only put on a few flowers per plant which makes it easy to prevent seeding. Their foliage is still lovely and they all have different benefits when used as cover crops.
- Lettuce retains moisture and grows quickly in cool weather which makes it a good cover crop to use after my cabbages come out but before my peppers and tomatoes go in.
- Zinnia germinates easily even in hot dry weather and out competes many heat-loving weeds.
- Sunflower roots and leaves left to decompose reduce seed germination for a few months. They make a good natural weed reduction method in beds that I plan to transplant into, rather than direct sow.
- Balsam stems take up a lot of water in our wet periods. When you mow them, all that moisture and stem matter give the soil a cool drink making it easier to start my next crop when our weather turns hot and dry weather.
- Basil has minor antifungal benefits when grown in dry weather (in humid or wet weather it’s fungal prone). Planted in late summer, at high density, during our dry period, it works well as a soil protecting biofumigant and heat protector for carrots or beets started in the shade of its leaves.
- French marigolds left to decompose in soil over winter and early spring reduce the hatch rates of tomato killing root knot nematodes. Growing them en masse in late summer is great bed preparation for where tomatoes will be planted the next year.
You’ll need to do some experimenting to figure out which plants work well as cover crops in your climate and for your soil condition. But here are some tips to keep in mind to help you figure it out.
Tip 1: Make it a vacation for your soil, not just more work!
- Change up your plant families so you interrupt the life cycles of host specific pest and pathogens.
- Alter the nutrient demands on the soil such as by planting nitrogen fixing legumes after heavy nitrogen users.
- Grow fast flowering plants after slow-flowering plants or taproots after plants with matted root systems.
Tip 2: Focus on fast-growing, easy to kill plants.
Cover crops needs to be easier to control than weeds. Plants that can be killed quickly by mowing low a few times or covering with newspaper and compost are ideal. Crops that have a tendency to keep coming back should be avoided.
Typically, annuals are better for cover crops than perennials or biennials since they have shorter lifespans. The one downside to annuals, though, is that they flower and seed quickly. So, you need to kill them or repeat mow right before and after flowering to make sure they don’t produce seeds.
Also, you can opt for plants that will be naturally killed by frost in fall or winter. Just plant them late enough that they don’t have time to flower before the cold comes.
Now for the hard part… especially when you grow edible cover crops.
Tip 3: No harvesting the cover crops!
Cover crops should not be harvested. Roots should be left in the ground to maintain soil structure. And leaf matter should be dropped on top of soil to nourish soil life. Otherwise, it’s not a cover crop, it’s an actual crop!
Okay, okay… yes, you can harvest a few leaves or few flowers for salad garnish. But the goal is to use the cover crops to tie up nutrients so they don’t wash away or become aerosolized through evaporation if soil dries out. Then, as those leaves and roots decompose, the nutrients become available again for your actual crops.
You are also trying to continually improve soil by using a constant stream of fast-growing plants to draw carbon from the air and put it into the soil using photosynthesis.
Increasing carbon in the soil is what improves soil tilth and allows soil to support more plant beneficial organisms like worms, beetles, fungi, bacteria, and mycorrhiza, that extra carbon improves soil tilth. Better soil tilth also allows for more air and water spaces in soil so your crops thrive.
Tip 4: Plan for nitrogen binding.
The other thing to know is that as soil life decompose cover crop residues, they have to borrow nitrogen from the soil as fuel to process all that new carbon-rich matter.
They use much of that nitrogen to create a population explosion of new micro-workers to handle the job quickly. Then, once the job is done, the naturally short-lived microorganisms die blissfully fulfilled. As their bodies decompose, they will release back what was borrowed plus the extra nutrients that were stored in the cover crops.
Ultimately nitrogen, carbon, and other minerals increase as a result of growing cover crops. However, in the short term, the soil will be nitrogen deficient until the decomposition is complete. This period when soil life is tied up is referred to as “nitrogen binding”.
The time it takes for completion of the decomposition process and release of nitrogen back into the soil depends on several factors.
– Carbon in Crop Residues
Younger plants with tender leafy greens have less carbon mass than older plants with thick, sturdy stems. So, younger plants generally take less time to break down than older plants. However, very high carbon matter such as woody stems on the surface of soil may not trigger bacteria into action the same way greener matter does. In that case, plant residues may break down so slowly that it really doesn’t cause nitrogen binding.
For short turnover times in the warm months, mow down plants when very young. To use plants as a long-standing mulch mow down older, woodier plants as temperatures cool so they can break-down slowly over the fall, winter, and early spring.
– Soil Temperatures
Soil life decompose slowly in both cold and hot temperatures. So, it also takes longer to break down matter in winter and the hottest parts of summer than in spring or fall when soil temperatures are between 60-75F.
– Soil Condition
Soil with more carbon and better tilth supports more soil life than heavy clay or sandy soils with low organic matter content. As such, the more ideal your soil is already, the quicker it will process all that organic matter.
Additionally, soil life require sufficient air and water to decompose matter quickly. Decomposition will happen fast if airflow is good and the soil is moist. It will take longer if the soil is dry or has too little air because it’s been over-watered or in rainy periods.
– Nitrogen Navigation
Fortunately, you can also navigate nitrogen binding with these strategies.
- Time seed germination or transplanting for when most of the cover crop residues are noticeably decomposed.
- Plant before decomposition is complete, but supply water soluble nitrogen such as by using compost tea or fish emulsions weekly to fill the nitrogen gap.
- Expect your crops to stunt temporarily and take a bit longer to grow to maturity if you don’t use option 1 or 2. But once the nitrogen is freed back up, plants will usually make up for lost time.
Tip 5: Choose cover crops based on soil life needs
(This last tip will make more sense after you’ve grown some cover crops or if you already have a good understanding of how plants and soil life interact.)
Most gardeners think about nutrient use when planning which cover crops to use. But I suggest that for long-term use, go even further and think about which members of the soil life community need encouragement or discouragement.
Let’s look at a few examples.
Brassicas and Amaranth Family Plants
Brassicas like cabbage, mustard, kale, and collards as well as amaranth family plants like spinach, chard, beets, and grain amaranth don’t form relationships with beneficial mycorrhiza in the soil. These plants instead work with some specialized cool or hot season bacteria to garner the nutrients they need for ideal growth.
So, after you harvest plants from these families, your mycorrhiza are going to need a workout. Opt for cover crops from myco-dependent families like grasses (corn, sorghum), nightshades (tomatoes, peppers, eggplants), mallow (okra), zanzibar (ginger, turmeric), and pretty much any other vegetable families that aren’t brassica or amaranth relatives and that also require soil pH of 5.5 or more for good growth. (Low pH plants like blueberries, cranberries, azalea also don’t rely on mycorrhiza.)
There are a number of plants that have known allelopathic abilities to suppress the germination or growth rates of other plants. Some brassicas and amaranths have this ability. But other plants like those in the aster family including lettuce and sunflower have it too. Buckwheat and it’s weedy relatives like smart weed and rye family plants also have some allelopathy side effects.
Allelopathic plants all have different mechanisms for suppressing other plants. Some increase toxic chemicals, others encourage pathogens, and others regulate soil signalling by other methods. Even if you don’t know what form of allelopathy a plant uses, most that we grow in our gardens only have short-term allelopathy that will time out one their plant matter decomposes… unless you grow them consecutively.
For example, if you planted sunflowers followed by mustard, followed by rye, then buckwheat as cover crops in the same bed, you could literally stop weeds from germinating for a year. (I did this as an experiment for fun). Of course, you also stop your crop seeds from germinating and stunt your transplants.
By avoiding planting back to back allelopathic plants you can skip those long-term risks to your overall plant productivity.
There’s also a bit of a misconception that legumes are perfect cover crops in all situations because they have the ability to fix their own nitrogen. Unless you use rhizobia inoculant when planting legumes and have good air spaces in your soil, there’s no guarantee that legumes will fix even the amount of nitrogen they need to survive.
See, the thing is… legumes don’t fix nitrogen at all. They trade carbon exudates for nitrogen that rhizobia bacteria place in nodules on their root system. So, the legumes must grow well to produce high quality carbon exudates and then they also have to access the rhizobia to make the trade. If you don’t regularly grow legumes, your rhizobia count might be low. Also, if your legumes aren’t happy and healthy, their exudates might taste icky and no rhizobia will want them. Often legumes still end up taking nitrogen from the soil.
Next, legumes use lots more than nitrogen to grow. They need a fair amount of phosphorous, calcium, and sulfur to thrive. Plus, because of that sulfur usage and the methods rhizobia us to produce plant usable nitrogen in the soil, high densities of legumes that are fixing nitrogen can temporarily lower soil pH.
As a cover crop, this pH alteration is short lived. Once the plant matter is decomposed and worked back into the soil the pH will slowly work its way back up too. But, that temporary alteration can take a few months to clear up. If your soil pH is already on the low end of what’s good for most garden plants, that could cause whatever is planted next to stunt for months.
Finally, if you have excess nitrogen in your soil and need to keep it from washing into waterways or the fresh water table, then legumes aren’t your best option. They are more ideal after your soil has been depleted from nitrogen by other hungry plants.
In a nutshell, legumes are awesome to use in rotation with inoculant if your soil pH is in the 6.5 range when rotated with other plant families, assuming you have low nitrogen levels in your soil.
Tip 6: In all things moderation…
Which brings me to my final bit of advice. Cover crops are a great tool. But like most things in life, the key to success is moderated usage.
Give your garden a balanced diet of cover crops, actual crops, and additions like compost and green manure for best results. And selectively choose your cover crops based on the current condition of your soil, time of year, past actions, and future goals.
Wishing you tilth, health, and happiness in your garden!