“Prince Harry is valiant; for the cold blood he did
naturally inherit of his father, he hath, like lean, sterile,
bare land, manured, husbanded, and till’d …”
— Falstaff in William Shakespeare’s “Henry IV, Part II,” Act IV, Scene 3
This is one of several passages in which Shakespeare referred to the virtues of manure for improving soil fertility. Although we typically think of manure as animal waste, “green manures,” or cover crops turned into the soil, also provide several garden benefits. According to the OSU Extension Service, cover crops and green manure:
• Enrich soil by adding organic matter and nutrients;
• Prevent erosion of soil, reduce compaction and improve soil structure;
• Suppress weeds, pests and diseases
• Support biodiversity by providing food and shelter for insects, wildlife and soil microbes.
A variety of cool-season grasses, legumes and other broadleaf plants make effective green manure when they are planted in late summer or early fall, allowed to overwinter in the garden, and are then worked back into the soil before the plants set seed in the spring. Root crops such as carrots, radishes and turnips can also be left in the garden soil to decompose.
Overwintering cover crops can be planted in our area until the end of September to give them time to establish and grow before fall frosts set in. Plant cover crops in between rows of late-season vegetables to give the cover crop a head start for growing. Cover crops planted earlier capture more nutrients from the soil, cover the soil more quickly and produce more organic matter.
Many gardeners recommend combining as many plant groups as possible for cover crops, and annuals are often suggested because of their rapid, vigorous growth.
One example of a cover crop mixture includes rye (grass), crimson clover (legume) and Phacelia (nonlegume broadleaf). The grass establishes and covers the soil quickly, whereas the legume grows more slowly but “fixes,” or converts, atmospheric nitrogen so plants can use it. Phacelia produces attractive blue flowers in the spring, which provide an important food source for native pollinators and other beneficial insects.
Here is a list of common cool-season cover crops for the Rogue Valley and how much in cups should be planted per 100 square feet:
Winter wheat: 1 cup
Winter oats: 1.5 cup
Annual ryegrass: 1/2 cup
Legumes and other broadleafs
Vetches: 1/2 cup
Crimson clover: 1/4 cup
Austrian winter peas: 1 cup
Fava beans: 1 cup
Phacelia: 1/8 cup
Before planting a cover crop, it’s a good idea to test your garden soil. This will allow you to amend the soil’s pH as needed (use lime to increase pH and sulfur to lower pH). Conducting a soil analysis before and after adding green manure will provide important information about the effectiveness of the cover crop.
To prepare the soil for a cover crop, remove garden debris, till the soil to a depth of six inches and rake even, and add one pound of balanced fertilizer per 100 square feet or 20 pounds of manure per 100 square feet. (If your cover crop consists of only legumes, use a low-nitrogen fertilizer.) Legume cover crops benefit from coating the seeds prior to planting with an inoculant called Rhizobium bacteria, which stimulates the legume’s roots to grow nodules that “fix” nitrogen.
Plant the cover crop in moist soil. Plant large legume seeds in shallow, closely spaced furrows; broadcast small grass seeds over the surface of the soil and rake in lightly. Water the cover crop until fall rains begin, and be sure to monitor the growth of your cover crop to see which plants grow best in your garden.
In the spring before the plants produce seeds, turn the cover crop under with a shovel or tiller. Tall cover crops may need to be mowed first. Allow at least three weeks before planting anything else so the organic matter has time to decompose.
In Othello, Shakespeare’s villain, Iago, says gardeners make a choice to keep their gardens “sterile with idleness, or manured with industry.” Iago is certainly one bad dude, but he makes a good point. Growing cover crops and using their “green manure” takes time and effort, but our work will be rewarded with healthy garden soil and plants.
Rhonda Nowak is a Rogue Valley gardener, teacher and writer. Email her at Rnowak39@gmail.com. For more about gardening, visit her blog at http://blogs.esouthernoregon.com/theliterarygardener/.