As the growing season winds down, it’s natural to start thinking about the health of your garden soil. After months of planting and harvesting, how well has your soil held up? Has it retained enough nutrients, or is it exhausted from the demands of the crops you’ve grown? Fortunately, autumn offers a simple way to revitalize your soil and prepare it for the next season: growing green manure crops.
What Are Green Manure Crops?
Green manure crops are cover plants specifically grown to improve soil health. These plants help restore nutrients, prevent winter weeds, and enrich the soil with organic matter. Depending on your climate and crop choices, green manure crops can either thrive through the winter or die off with the cold. In this article, we’ll focus on those that are killed by winter’s chill, which are often the easiest option for vegetable gardeners.
Winter-Killed Green Manure Crops
Plants that die off in winter offer a simple solution for replenishing the soil. Oats are a perfect example: they grow quickly in the fall but are killed by a few frosty nights. Once dead, the plants act as a natural mulch, and by spring, their roots have decomposed, leaving the tops to be composted or used as mulch elsewhere in the garden.
Other winter-killed options include mustards and daikon radishes. Daikon radishes, known as “biodrills,” have deep roots that break up compacted soil. While I often harvest the radishes for use in fermented dishes, the radish tops can still serve as an excellent base layer for winter mulch.
Several half-hardy flowers can also act as green manure crops, including calendula, phacelia, and poached egg plant (Limnanthes douglasii). These can be seeded in late summer or fall, either by planting directly or by scattering seeds from leftover flowers over the garden.
Hardy Green Manure Crops
Some green manure crops are more resilient and survive winter to continue growing in spring. Crimson clover is a popular choice, as it survives the cold as a small rosette and then grows rapidly once spring arrives. It is hardy down to 0°F (-18°C), but you can improve its cold tolerance by covering it with row covers or garden fleece.
In addition to crimson clover, you can plant hardy annual flowers like bachelor’s buttons or poppies. These not only improve the soil but also provide valuable nectar and pollen for pollinators early in the season, benefiting your entire garden.
Green Manure Crops That Survive Winter
Some plants are even more robust and can survive even harsher winter temperatures, down to -20°F (-29°C). Winter wheat, cereal rye, field peas, beans, and vetches (also called winter tares) all fit into this category. These plants grow slowly in the fall, survive the winter, and burst into vigorous growth in spring.
When growing these crops, it’s essential to know when to cut them down. I’ve learned from experience that they should be harvested when they’re just beginning to bloom. At this stage, the plant tops make excellent green manure for composting, while the roots decompose in the soil, enriching it for the next season.
The Benefits of Green Manure Crops
Regardless of the type of green manure you choose, the results in spring are undeniable. The soil will be full of organic matter, with fine rootlets from wheat or rye making it light and airy. Field peas and vetches will leave behind nitrogen-rich nodules on their roots, which act as a natural fertilizer. These signs are clear evidence that the soil has benefited from the fall-sown green manure crops, and it’s ready to begin a new season of growth.
By using green manure crops in your garden, you’re not only improving soil health but also helping prevent winter weeds and creating a more sustainable garden for the long term. Whether you opt for winter-killed or hardy crops, your soil will thank you when spring arrives.