Buckwheat (Fagopyrum esculentum)
Buckwheat is an annual that grows very quickly. It should be sown between April and June. It is not frost hardy but is good at scavenging phosphate from the soil. It is a weed supressor, but not a nitrgoen fixer.
This is a short term, aggressive weed suppressor. It is good at scavenging for phosphate in the soil, breaking it down and then making it available to subsequent crops after incorporation. It's especially useful in soils that suffer from phosphate lockup. With its quick growth, the plant can establish from seed in 8 - 10 weeks.