Cover Crops Reduce Weed Seeds
Once summer harvesting ends, cool season cover crops can restore soil health in the veggie patch. Deep mulches of compost or aged manure can nourish tired beds, but cover crops can do even more. Where new beds or lawns are planned, cover crops can eliminate the need for destructive tilling, acting as living mulch to restore soil fertility and reduce winter erosion. Recent research shows that cover crops can also reduce weed seed numbers by providing cover for seed-eating beetles and rodents.
Ick? Not really; both critters consume significantly more weed seeds when cover crops provide shelter and protection from predators. Purdue researchers found that when beetles and rodents had cover crop protection, the weed seed burden was reduced by as much as 400% compared to fields without cover crops. Surprisingly, the soil cleaning effect continues even when rodents eat high numbers of the beetles.
More Moon, Fewer Critters
Ian Kaplan, a Purdue University associate professor of entomology, and Carmen Blubaugh, who earned her doctorate at Purdue and is now a postdoctoral research associate at Washington State University, have been using field experiments to discover the effects of fear and habitat change on both the food chain and seed burden reduction through critter predation. Like most small mammals, rodents and beetles forage more extensively when night are dark, since moonlight makes them more visible to their own predators.
To figure out how this might affect soil seed burdens, they altered night lighting to mimic full moon levels. Surprisingly, they learned that seed counts still went down. Brighter nights reduce beetle action and make them more visible to rodents, yet beetles balance their reduced numbers and foraging range by eating up to 50% more seeds when there’s more moonlight.
Cover Crop Blends
Fortunately, beyond planting cover crops on fallow ground, we don’t need to go to any special efforts to bring seed eaters into our gardens. Whether you want a pioneer planting to open tight clay soil, a nitrogen fixer to bolster soil quality, or bird friendly cover that combines shelter and food, there are plenty of good cover crop choices to suit your need. Many farmers create their own cover crop seed blends, picking fast growers that will leave the soil richer than they found it. For instance, winter rye has sturdy roots that keep soil from washing away in winter rains. Its tall stems shade out weeds and provide cover for game birds like quail as well as play space for backyard chickens. Chopped up and left to rot in early spring, rye provides abundant “green manure” to enrich bed soil. Rye is often blended with vetch, an efficient nitrogen fixer that’s also a fine soil conditioner once it’s chopped up in late winter. A combination of winter rye and vetch is a classic pre-turf blend used to prepare heavy clay soils for lawn use.
The maritime Northwest is a marvelous place to garden, but constant harvesting can leave veggie patch soils nutrient depleted without annual amendment. As beds empty out, replace crops with annual crimson clover, Trifolium incarnatum (NOT perennial red clover!). A summer crop in cold winter regions, here in the maritime Northwest, it can be sown from September through October, growing through our mild winters. Clovers are nitrogen fixers, capturing nitrogen from the air and storing it in little white-ish nodules in their roots. When we chop or turn under red clover in spring, the stored nitrogen will nourish the soil. Nitrogen-fixing legumes like fava beans and Austrian field peas are traditional cover crops for vegetable beds. Field peas are also grown as hog feed, but favas are a delicacy in Europe (I’d personally give my share to the pigs, though). Both grow well in clay soils though fava beans tolerate wet feet better than field peas.
Preparing The Ground
If you’re planning new beds or a new lawn, prep the area with mulch and sow cover crop seed at 2 pounds per 1,000 square feet. You can use half that amount in established veggie beds, after turning under summer crop residues and raking out the soil. Sow your cover crop with a whirlybird or by hand, making two passes (north-south, then east-west) over each bed to get even coverage. Rake a little soil (about 1/4 inch) over the seed, water it in and keep it fairly moist.
In warm, dry weather, cover seed with woven row cover cloth. Sprouts should appear in week or so and beds will be fairly well covered in about a month. Instead of tilling in the residues, which can damage soil life, use a weed trimmer to chop plants down come spring, then let them rot in place. Wait a week or so for the cover crop to break down, then rake out your beds and plant right over any remains. If clumps of rye or peas persist, chop them with a hoe or turn them over to expose the roots, then plant as usual. The result will be healthier soil with fewer weed seeds, thanks to those helpful beetles and rodents. Who knew?