Perennial rye grass has no rhizomes; it does not creep. But each plant will form a bigger clump through tillering. potentially, each grass blade could form a clump of about 4 to 6 inches in diameter. If conditions are excellent in regard to water, nutrients and plenty of sunlight, along with warm temps--this should work out fine in a few weeks. Weed control may be needed in a few weeks. Tall weeds will be killed by mowing. I suggest mowing when it gets about 2 inches tall. Mowing helps the grass to spread sideways and reduces excess vertical growth. You may need to withhold water for 24 hours when first mowed to make sure the soil is firm enough to support the mower.