Grass seed is sold in two types of assortments: mixtures and straight species. Choose the type depending on what you want.
Mixtures — contain seed from two grass species or more, designed to balance the relative strengths and weaknesses of each type in order to better solve problems or address different light conditions in your lawn. For example, Sun and Shade is an excellent all purpose mix that performs well in multiple light conditions, and High Traffic is a more wear tolerant blend. Mixtures are the best choice for an average lawn or for a lawn that needs help in a certain problem area, typically delivering better tolerance to varying light conditions, more insect and disease resistance, and more versatility.
Mixtures usually combine grasses with similar colors, growth rates and textures, as well as behavior. Seed formulations may vary by manufacturer and sometimes they add small amounts of other grasses, for properties like drought resistance, quick starting or beauty.
Straight Species — Some consumers know that their lawn is a single grass type such as Kentucky bluegrass or tall fescue and want to maintain that single seed type and match the existing lawn when reseeding an area. Many times the products that are single seed types contain a blend of different varities of that seed type to produce a lawn with better disease resistance, better color and a more uniform texture. Always look for the most weed free seed and be sure to buy a product that contains improved, named varieties to ensure the best seeding results.