organic gardeners often plant hairy vetch (a nitrogen-fixing legume) as a companion plant to tomatoes, as an alternative to rotating crops in small growing areas. when it is time to plant tomatoes in the spring, the hairy vetch is cut to the ground and the tomato seedlings are planted in holes dug through the matted residue and stubble. the vetch vegetation provides both nitrogen and an instant mulch that preserves moisture and keeps weeds from sprouting.