I like to think I've come a long way since back in 2011 when I last wrote about my mulching experiments. No more plastic for me; now I'm mulching with combinations of chicken bedding, cardboard, living mulches, and green manures. Instead of being concerned only with weed control (although that is certainly still my main priority), I now think more in terms of soil building and conditioning.
While green manures aren't primarily meant to be a mulch, they will cover your soil somewhat and minimize new weed seeds emerging. Anna Hess over at The Walden Effect was kind enough to send me some oilseed radish and oat seeds to try out for her new book, The Ultimate Guide to Soil (due to be published in Spring or Summer 2016 - if you can't wait that long, I highly recommend her e-book Homegrown Hummus: Cover Crops in a No-Till Garden). Since she lives in zone 6, she was interested in how these crops would work in my zone 3-maybe-4 garden.
I'm sorry to say that I probably wasn't very much help, since I sowed the seeds quite late in the season, and didn't keep the best records (last fall was extremely busy, for non-garden-related reasons); but I can share the few things I did observe. Both crops did grow (although the oats seemed to have trouble - Anna reported that her oats had a lower germination rate than she'd been hoping for, too). The bed I sowed the radishes in had been double-dug and filled in with old logs and compost the previous summer (2013), then topped off with soil (kind of a hybrid of John Jeavons double-digging method and Sepp Holzer's Hugulkulture).
I'd planted the bed with rye that spring (2014), then after the rye was harvested I covered the bed with a layer
of leaves. When I sowed the radish seeds from Anna in early September, I
simply sprinkled them over the leaves and watered them in well.
Here's a shot of the oilseed radishes, just after a hard frost last November:
For me, the real test was how the gardens look this spring. I went out this morning and took the shot below, showing the radish seeds that apparently didn't germinate last fall but sprouted this spring (I pulled up a few, and they definitely don't have roots big enough to have carried them through the winter). For my purposes, I wanted these plants to winterkill and keep the ground covered over the winter, but leave the ground open for planting this spring. So seeing seedlings coming up this spring was disappointing (but since they pulled up quite readily it wasn't a deal-breaker for me). Obviously the grass coming up behind is going to be a much bigger problem. That does show, however, that the radishes didn't work extremely well as a weed barrier - but I haven't yet found a cover crop that will block out grass!
So in terms of soil conditioning, the radishes didn't seem to have much effect (although that was probably due to how late I planted them - they didn't have time to make much of a root, which in turn would break down and add organic matter to the soil).
As for the oats, it was really hard to tell how they did, since there is so much grass invading the area. I could tell last fall that some of it had come up (there is a slight color difference between the oats and the grass), but again, it wasn't thick enough to keep the grass out entirely, and since I had the leaf mulch already on the bed, the oats didn't do much in terms of covering the soil.
So, Anna, I'm sorry I wasn't much help, but hopefully you can use at least some of the information I learned. It was very interesting for me to experiment with some crops I wouldn't otherwise have tried!