Buckwheat

Buckwheat is a fast growing weed suppressing cover crop for fall. This crop should be planted late, or mowed before it can go to seed, because it will actively outcompete most other crops in spring through fall. It will die with the first hard freeze, so you will need a stronger overwintering cover crop like oats or mowing the buckwheat in early winter (before the first hard frost) to provide a mulch layer to prevent early spring weeds or soil damage. Buckwheat will go to seed in 45-55 days after planting, so planting should be done late fall.

Cornell Buckwheat Handbook

Cover Crops are sold in pound bags.