dean wrote:
Is it possible to just throw a bunch of grass seed onto an existing
lawn, one that is not worth starting again from scratch, but that
could use thickening up?

Would I have to throw some soil down on top? I need to be able to
continue cutting it every week. I have around 1/2 acre to do.

Waste of time and money. You need to address why it is thin. If the
conditions are right for a good lawn it will not be thin. If the conditions
are wrong, putting more seed there will not change things.

Get a soil sample tested, and take a look around. Heavy shade or
shallow tree roots or poor compacted soil etc. can cause the problems.

Work intelligently. Don't just through fertilizer or chemicals or seed
at the problem. Define the problem first.

--
Joseph Meehan

Dia 's Muire duit