Fastest Cool-Season Grass. Perennial ryegrass seed germinates in seven to 10 days, compared to 10 to 14 days for tall fescue seed and 14 to 21 days for Kentucky bluegrass seed. Perennial ryegrass is one of the cool-season grasses to grow to **obtain a thick lawn turf** **quickly**.*Select a grass seed suitable for your [grass climate zone](http://www.greatdayimprovements.com/when-to-plant-grass-seed-in-your-state.aspx)**, or else your new grass may be fast-growing but fleeting. In the United States, grasses are assigned [U.S.