Buckwheat grows fast (about a month) and you can easily "chop and drop" after it blooms. (Or let it go to seed if you want more crops.) Cowpeas will fix nitrogen in the soil - you can plant them and forget about them and let the cold kill them.

I love buckwheat/oats/vetch, but only because I typically cover crop the last half of August and September (vetch does not die in winter in MI, but is manageable). But starting now sorghum is superior, in biomass but also in dying on its own (you still need to knock it down). Given that you have sand, I would go sorghum chickpeas sunflower. guaranteed to give you max bio-mass, and both sunflower and sorghum need knocking down anyway. Oats and buckwheat will go to seed for sure.

In retrospect, if you live on Lake Michigan, it is possibly too cold for sorghum (the Detroit area where I live for sure is hot enough this year). But if you think you can stay above 78, it will grow more than anything else because it is a C4 plant. Corn is more tolerant of 70s temperatures, and is also C4. In sand, you don't need the big roots of sorghum as much as in clay, and anyway you have the big roots of sunflower. Both chickpeas and sunflower grow well in the 70, and chickpeas is THE legume for sand.

What to grow as a cover crop in most of Michigan depends on what seed is fairly readily available. Tractor Supply and Family Farm and Home, as well as some locally owned garden supply stores, will have Buckwheat and Oat seed but not some of the others mentioned. If you mail order the time for shipping may be problematic.

Buckwheat has some allelopathic properties others do not have as well and if allowed to blossom (about 30 days from planting) is a very good nectar source for the bees, however, peak nutrient levels in Buckwheat would be just before it blossoms. According to the people at Michigan State University Sorghum grows quite well in Michigan, I've just not seen seed very readily available around west Michigan.

good point Kim. However, I have always used bird seed for sunflower cover cropping (and also for edible shoots) because they are cheaper. I checked Urban Farmer seeds in Indiana, where I get most of of my cover crops, and they do not have chickpeas, and of the two sorghum relatives, they have sudangrass but they are out of egyptian wheat (which is a sorghum). They do not have corn but they have various types of millet, another C4 plant. To me using C4 for summer cover cropping is a no-brainer.

they have a great selection of legumes but many cowpeas are in general for warmer climates. so, get everything from the pet store and/or the food coop? Besides sunflowers, I have tried lentils and fenugreek from the store in my garden with good success.