The GARDENA gardening expert
Dear Mr. T., this really is a different variety of grass which has taken hold in your garden. However, it need not have been consciously brought in by yourself. It is sufficient for a few grass seeds from a different field, which for example have adhered themselves to a shoe sole or to an animal's fur, to have been distributed there.
However, this must have occurred a good year ago, and have been present before that, as a square metre of lawn cannot settle in overnight! It is therefore of no avail to look for the cause. You cannot use sprays against the problem, as a herbicide would not only kill off the sweet grass, but also the lawn.
The only remedy is to hack out the “damaged” parts with a spade in 30 x 30 cm pieces, and to remove them on a shovel as flat pieces of turf. After that, you should loosen up the underlying soil to a good depth, form a sludge using 20 litres of water per square metre, leave it to dry and then even it off.
It is then best to roughen up the surface using a rake, sow the grass seed and (instead of rolling [is not worth the effort over this small area]) gently tread the seeds into the ground (or pat them down with a shovel so that the seeds and the soil contact each other). Re-sow your lawn as soon as the lawn has reached the optimum germination temperature of 14°C.
Use the same lawn seeds as you initially used. Using this method, you can definitely prevent a patchy-looking lawn.