Horticulturally speaking, crown vetch is not a true vetch. It a perennial legume with rhizomes as its underground root structure. It's considered by the Center for Invasive Species and Ecosystem Health to be an invasive plant and is therefore not recommended by many professional gardeners, landscapers and horticulturalists as a good plant to grow. It is however, planted by farmers as a grazing crop for livestock and often by homeowners for its good ground cover ability. Crown vetch will need to be controlled by both pulling and the use of herbicides such as glyphosate, triclopyr and clopyralid, if desired.
Select which kind of variety you would like to buy. Emerald, Penngift and Chemung are the three main varieties of crown vetch. Emerald and Chemung are taller, have stiffer stems and wider leaves than Penngift. Penngift is less aggressive than the other two varieties and has pinkish or purplish white blooms.
Decide how many seeds you need. Farmers need between 5 and 15 pounds per acre. In the garden, this equals about 1/5 ounce to 3/5 ounce of seeds per 100 square feet. Buy on the higher end of this average if this is your first time planting crown vetch and you are not using any professional planting methods, such as chemical weed control and a seed inoculant, which is sprayed on the seeds at planting.
Purchase seeds, which are available through online and catalog nurseries, in early spring for planting. Purchase seeds from a local nursery as well, but call prior to going to check whether they have them in stock. Few nurseries sell crown vetch due to its invasive nature.
Purchase bare rooted or potted plants in the spring after the last frost as an alternative to seeds. Some online, catalog and (possibly) local nurseries carry crown vetch plants, especially the more desirable Penngift variety. You will need one plant for every 3 to 5 square feet of space.