Typically planted in the fall, catch crops provide ground cover until the land is ploughed the following spring. While they are growing in the fall, they take up rainwater, and thereby reduce the amount of water that can cause leaching. To be a useful catch crop, plants must be fast growing, highly competitive against weeds, and be able to quickly take up substantial amounts of nitrogen. The time of planting defines the choice of cover crops. Buckwheat is killed by light frosts and can only be planted in the summer.
Phacelia is killed by moderate frosts.
Brassicas and spring grains are all killed during the winter. The brassicas, grains, and grasses can all be planted in late summer to early fall. This will give them time to grow enough to take up nutrients, but not enough time to set seed. Fall rye can be planted late into the autumn. In 1996, the Nova Scotia Organic Growers Association began a two-year project, funded by Environment Canada's Action 21, to encourage farmers to try using catch crops.