How to Re Seed your Perennial Ryegrass Lawn
First you should prepare your soil as you would for any other planting. Pull out any existing weeds, aerate if the soil is compacted, de-thatch if the thatch layer is greater than ½” thick, add soil amendments if possible, and apply a pre-plant fertilizer to the soil (all of this should be done when your perennial ryegrass lawn is actively growing). Your goal is to give the new seeds the best possible chance of germinating.
And speaking of germination, pick a grass seed product that has a high pure live seed percentage: that indicates the amount of perennial ryegrass seeds in your product that are expected to germinate. Wait to sow the seeds until the soil temperatures are between 55ºF and 65ºF. The optimal time for perennial ryegrass seeds to germinate is either in the early spring or early fall (early fall tends to be better because the grass has more time to prepare itself for the stress of summer).
When it is time to sow the seeds, mow the lawn slightly shorter than usual so that the new seeds do not have to compete for sunlight with tall grass. Use as drop spreader or a rotary spreader to help with evenness. Look on the bag of your seed product for reseeding rates, not seeding rates. Any reseeding rates for perennial ryegrass should be less than 9 lbs. per 1,000 sq. ft. Take half of the total seed you will be using, and spread it in a horizontal direction across the lawn. Then, take the other half of the seed and spread it in a vertical direction across the lawn. You can apply a very light topdressing (no more than 1/8”) to the lawn to keep the seeds in place, but it’s not necessary.
After you have sown the seeds, you should water as briefly and frequently as possible to keep the seeds lightly moist until they germinate. Perennial ryegrass seeds have a very quick germination rate, so if the temperatures are right and the water and seed-soil contact are there, the seeds should germinate within 5 to 10 days. After the seeds have germinated, you can begin watering as deeply and infrequently as possible. Begin mowing the lawn when it is 1/3 higher than the optimal height, and apply a light fertilizer to the soil a few weeks after planting.
Reseeding your perennial ryegrass should not be a fix-all for every problem that arises, but it is a good general maintenance technique for keeping your lawn thick and healthy.