As much as I appreciate the beauty of beautiful dark soil full of organic matter, I always feel a deep sense of discomfort when I see it. If this is the case, it’s because I’m worried about the health of this soil that isn’t covered. As far as I’m concerned, mulch of any kind (preferably organic) is, in most cases, absolutely necessary in a garden or vegetable patch. It preserves moisture, reduces weeds, regulates temperature, prevents erosion, reduces compaction and a decomposing mulch adds organic matter to the soil.
But there is an alternative to mulch, a technique that has been used for millennia in agriculture: cover crops.
What Is a Cover Crop?
A cover crop is a plant used to protect and amend the soil. Traditionally, these plants are sown in a vegetable garden or farmland when it is not being used by another crop, either before, after or even in between. As part of a crop rotation plan, cover crops can even be grown for an entire season to allow the soil to regenerate.
The benefits of using cover crops are very similar to those of mulch.
- Soil improvement: the above-ground and below-ground parts of a cover crop will decompose, adding organic matter to the soil, improving its structure and ability to retain water. Some plants, notably legumes, absorb nitrogen from the air and release it into the soil as they decompose.
- Weed reduction: a green manure competes with undesirable plants for light, water and nutrients.
- Erosion control: roots hold the soil in place, reducing erosion caused by water and wind.
- Nutrient conservation: a cover crop absorbs nutrients from the soil, which might otherwise be washed away, and conserves them for the duration of its life. These will return to the soil when it decomposes.
- Biodiversity and pest management: some cover crops harbor beneficial living organisms that help reduce insect pests and diseases. What’s more, when a green manure dies, it feeds the microscopic flora and fauna present in our gardens.
How to Use Cover Crops
Start by deciding what type of cover crop you’re going to use. Legumes fix nitrogen; the deep roots of grasses prevent erosion, and so on. The time of planting could also determine which green manure you use, as some grow faster than others or grow better at different times of the year.
Next, prepare the soil to receive the seeds: remove weeds or the previous crop (or not!) and scarify the soil to allow the seeds to penetrate.
There are various types of seed drill or spreader that can be used to plant the seeds, but you can also do it by hand if you don’t have large areas to sow. Depending on the type of plant chosen, the seeds must be incorporated into the soil at different depths or left on the surface. You can use a rake to bury them.
Now all you have to do is watch your green manure grow! Be sure to keep the soil slightly moist, especially in the first few weeks. Keep an eye out for undesirable plants and remove them if necessary.
To Bury or Not to Bury
Opinions are divided on whether it’s better to bury a cover crop by ploughing the soil, or to mow it and leave it on the surface.
Ploughing accelerates decomposition, since the organic matter is in direct contact with the soil and decomposing microorganisms and insects. But ploughing destroys certain weeds, allthough it can bring buried seeds to the surface increasing the germination of undesirable plants… Ploughing can increase erosion and partially destroy the flora and fauna living in the soil.
When a cover crop is mown and left on the soil surface, it acts like a mulch with all the same advantages. Decomposition will be slower than with burial, however, and there may be more nutrient loss through leaching.
Tip: Cover crops can be left in place over winter, as long as they don’t produce seeds that could harm the following crop. By doing so, your soil is protected from spring erosion and temperature variations.
Types of Cover Crops
There are three categories of commonly used cover crops: legumes, grasses and brassicas.
Legumes such as peas, vetch and clovers can fix nitrogen from the air to help them grow, with the help of rhizobium bacteria present in the soil. It’s best to inoculate seeds with this bacterium to ensure its presence in the soil. This type of product is sold in garden centers or online.
Grasses have deep roots that seek out nutrients and bring them to the surface. These roots also improve deep soil structure. Barley, rye and oats are popular choices.
Mustard and radishes, including daikon, are brassicas used as green manures for their roots, which help loosen compacted soils.
You can also mix several types of green manure.
Popular Cover Crops
Buckwheat (Fagopyrum esculentum)
Buckwheat is often used because of its rapid growth. It attracts pollinators with its flowers and competes with weeds. It can be sown year-round, but prefers warmth and is intolerant of frost. Sowing rate: 6-10 lbs/1000 ft2 . Sowing depth: 1/2″ to 1″.
Crimson clover (Trifolium incarnatum)
Like other legumes, red clover can fix nitrogen. Its red flowers attract pollinators. It is often sown in early autumn. Sowing rate: 2-4 lbs/1000 ft2. Sowing depth: 1/4″.
Oat (Avena sativa)
Oats grow quickly, which helps them compete with weeds. Sow in early spring or late summer, as they are cold-tolerant. Sowing rate: 4 lbs/1000 ft2. Sowing depth: 1/2″ to 1″.
Lacy phacelia (Phacelia tanacetifolia)
Phacelia can be sown in early spring or late summer. It produces beautiful purple flowers and loosens compact soils. Sowing rate: 2-4 lbs/1000 ft2. Sowing depth: 1/4″ to 1″.
Daikon radish (Raphanus sativus)
The broad root of the daikon radish breaks up compacted soils and brings nutrients to the surface. Sow in late summer or early autumn. Sowing rate: 2 lbs/1000 ft2. Sowing depth: 1/2″ to 3/4″.
Ray-grass (Lolium multiflorum)
Ryegrass is a fast-growing grass, good for preventing erosion and controlling weeds. Sow in late summer or early autumn. Sowing rate: 4-8 lbs/1000 ft2. Sowing depth: 0 to 1/2″.
White clover (Trifolium repens)
White clover forms a dense carpet fairly quickly, smothering unwanted plants. Like other legumes, it fixes nitrogen. It can be sown at any time. Sowing rate: 2 lbs/1000 ft2. Sowing depth: 1/2″.
Hairy vetch (Vicia villosa)
Another legume, it’s good for weed control. Sow in late summer or early autumn. Sowing rate: 4 lbs/1000 ft2. Sowing depth: 1/2″ to 1-1/2″.
Rye (Secale cereale)
Rye is an excellent choice for weed control, as it grows quickly and forms a dense mat. Cold-tolerant, it is sown in late summer or early autumn. It survives winter and can be mowed the following spring. Sowing rate: 4 lbs/1000 ft2. Sowing depth: 3/4″ to 1-1/2″
Of course, this is not an exhaustive list of all the green manures available. You could try other species, as long as you make sure they don’t establish themselves permanently.
Or, why not use a mixture of old seeds rather than throwing them away? It might make a nice salad at the same time!