Phacelia tanacetifolia is definitely my favourite. It seems to grow well down here in the South no matter when I sow it. I strewed some seed over what will be a new veggie patch a week ago and it is already romping away merrily. When chopped into the soil the tilth it produces is beautiful.

The reason I choose Phacelia instead of the others is that it does not affect the crop rotation. Fava Beans are Legumes and count as such in the rotation so you need to be careful where you sow them. Mustard is a brassica and presumably has all the same pests and diseaes. Alfalfa, Clover, Lupin, Tares, etc are all Legumes which will fix Nitrogen via their root nodules.

I think the only other ones that don't affect the rotation are Buckwheat and Hungarian Grazing Rye. I've sown some Buckwheat for the first time this year but the Rye I sowed last Autumn did not grow at all.... maybe the birds ate the seed!

The HDRA do a brilliant booklet about Green Manures, what to grow where, when, how and what the benefits etc of each one are.

Regards,
Mo