I live in North Carolina so you may need to check with your local extension office for specifics for your area. Here the best way to control crab grass are the following steps.
- Fertilize at the correct times. For us Labor Day, Thanksgiving and Valentines day for fescue. If you fertilize into the summer you are just aiding the weeds.
- In early spring put down a pre-emergent.
- Mow to a height of 3.5-4 inches. There is an NC State study on mowing height and crab grass validating the importance of not mowing short.
- There are a number of post emergent controls. If our state has an agricultural manual check it otherwise you can Google and look at the one for North Carolina.
Watering may be a factor but I suspect its not the prime consideration.
Good luck.
Joe Spears
NC County Extension Master Gardener