how do I rehabilitate a lawn w/mixed grasses and oodles of crabgrass -- temperate climate but hot, dry summer.

Answer 1

crabgrass (poa annulis) is an annual and will die with the first hard frost. In the spring, apply pre-emergent crabgrass killer and fertilize and water throughout the summer. The grass will grow and fill in. Crabgrass seeds can remain dormant for several years, so you will probably have to follow this regimen for 8 years or so. Quick drastic fixes will be more expensive and wont solve the problem long term.

Answer 2

The best time of year is in the fall.
The first method is to use Roundup and kill everthing off, wait about two weeks, fertilize, and then wait another two weeks, or after a couple of good rains and reseed. Water Well. By spring you will have a beautiful lawn.

The second method is to rent a machine to de-thatch the lawn in the fall and then come spring use a herbicide, fertilize and reseed.

Good Luck!