Question
I live outside Seattle. At the end of summer my mother insists I should mow my yard so the roots will make more grass for next year but I don't think that advice has worked very well - I think it would be better to let it be a little longer for the winter, to protect itself.  My dad comes over and mows and scalps it, in my eyes.  It is a little mossy and not very thick.  Is there an optimum guideline for lenght of grass to prepare for winter?

Answer
Blondie:

I would tend to agree with you.  The grass continues to grow during winter months when temperatures are within proper range.  The more leaf area left, the more photosynthesis takes place, the more roots can grow.  Shock therapy is useful, but tends to deplete resources.

The main enemy in winter, or anytime, is moisture leaving the ground and plant.  If lack of the leaf cover exposes the ground, moisture can evaporate more easily.  There is significant loss during winter months though we are not as keenly aware of it as in summer.

I mow my bluegrass and fescue at the same height all year round - 3 to 4 inches.  I assume that you do not have a summer grass like Bermuda, St. Augustine or centipede.

If you wish to thicken the stand, now is the time to overseed and fertilize.

Best wishes.