What lies beneath?
What has been happening beneath the lawn through a summer of use, dry periods, heavy rains, whatever we and the weather have thrown at it? Soil compaction is likely and the roots need air. If it’s not too damp now would be a good time to hook the scarifier onto the garden tractor or buy or hire a walk-behind to deal with the accumulation of thatch which is potentially harbouring disease while preventing fresh growth.
Conditions can’t always be perfect for scarifying but you don’t want the grass either too wet or too dry. If in doubt wait until later in the month or early October but before it really does get too damp. Cut the grass first. Don’t be deterred by the effect of scarifying, it’s simply lifting out all that dead material.
Run the mower over the area with the grass box on and you can collect up the debris. For smaller areas you can rake out the thatch. Following this you can spike and aerate using powered equipment such as a hollow-tine spiker or aerator or again, for a smaller area simply use a garden fork. All of this will help drainage and act as part of you maintenance regime for a healthy lawn.
If you end up with any bald patches reseed by adding grass seed to a mix of garden compost, soil and sharp sand and spread it over the area. Creating a new lawn using turf gives you an immediate result, although it’s a touch more expensive than seeding. Finish preparing the area ready for laying the turf in October to November when the conditions are more favourable for the grass to root.