I used alfalfa and annual clover (make sure it isn't a perennial clover or
you will be fighting it forever). I had raised veggie beds with 24" sides. I
rotated the beds yearly so one bed, with winter crops that needed a cold
snap to develop flavor (Brussels sprouts and some root crops), never got the
cover crop sown that year, that bed was then ready for cool temp crop
planting, like peas that liked the cold soil to germinate and early starts
of cruciferous veggies that can stand early season cold, . Once every four
years a bed didn't get it's planting of green manure but I had plenty of
compost to make up for that.
I live in the PNW so I would plant a mix of these two (alfalfa and clover)
about the last of September and into October, depending on harvest times
and weather. About mid March I whacked up what was still standing with a
weed eater, covered the beds with clear plastic and what was left alive or
any seeds on the surface were cooked by solar heat. About mid April I just
used a fork to turn the decomposing crop over so it was toes up and any
green was covered with soil and put the plastic back over it to cook some
more. I generally started planting out the end of April and into May.
Depending on your zone, your mileage may vary.
What had originally been really crappy builder fill turned into fertile loam
in about 3 years and just got better every year. I also bought 25lb bags of
alfalfa pellets from the Feed & Grain store, much cheaper than alfalfa meal
at the garden stores and easier to work with than the expensive "bag o'
dust". I spred this every winter around my flowers, fruit trees perrenial
veggies and garden beds in general. The rains would dissolve most of this
and a shallow scratch or tilling worked it into the soil in the spring.
Green manure crops are good stuff, it's living compost!