Buckwheat can be used both as a substitute for baby spinach and as a fast-growing short-term summer green manure/weed suppressant that germinates in about 3-5 days. It produces a dense canopy of foliage that smothers & out-competes weeds.
 
If using as a green manure, cut down before flowering when the stems are nice and soft as they decompose quicker, retain more beneficial nutrients and are easier to incorporate into the soil. They can be dug into the soil by turning over into the top 15cms or left on the top as a mulch and once dug into the top layer of soil (15-18cm) rapidly decomposes helping to improve the soil structure & organic matter content so it can retain more moisture. 
Prepare the soil by roughly digging it over and removing any weeds. Lightly tread the soil, and then sow the seeds by broadcast sowing (i.e. scattering evenly) for good coverage. Rows can be sown but are not as good for weed suppression. Rake the soil and water well. Water in dry periods.
To use as a green veg, sow small patches throughout the season, pinching out the tops for more bushy growth. It has an inoffensive bland flavour that is useful to bulk out stronger flavours or foraged leaves. Use immediately after picking as it wilts quickly. 
Can also be used as an ornamental which grows to 80cm, and the pretty little pink flowers are loved by bees & beneficial insects.