The fastest growing grass seed in colder climates is rye seed. In warmer climates, Bermuda grass seed offers the fastest growth and coverage. Depending on the size, a lawn seeded with Bermuda grass achieves full coverage in 60 to 90 days.
Ryegrass seed generally falls into two types, annual and perennial. Lawns sown with annual ryegrass seed live for one season and then die off. The lawn must be replanted the following season. Because of its shorter life span, ryegrass is generally used as a fill-in grass that is mixed with other slower-growing, more resilient grass seed in order to establish the overall look of a lawn quickly. In comparison, perennial ryegrass grows throughout the year in colder climates and does not need to be re-seeded every season.