I think this information would be better on another forum, but since it is here, I'd just like to suggest that northern climate folks should keep their lawns mowed high. At first when you experiment with this, it may look a little straggly as it is growing in through multiple mowings, but eventually the grass blades will uniformly grow into a deep lush lawn that will greatly shade weed seeds. Other benefits are that your lawn will require less mowing (mowing stresses lawns), water, and fertilizer. Rain water runoff will be slowed and percolate into the soil better than a shorter turf. Never let your lawn be mowed shorter than 3.5 inches except when putting your lawn to bed for the winter. Winter dormancy height should be 2 inches to lessen turf disease and to prevent the grass from suffocating under compaction of snowfall.