Dealing with a crabgrass problem requires a very specific strategy. The heat we’ve experienced the last few seasons plus the exposed soil resulting from heavy grub damage has caused many Ottawa-area lawns to develop large patches of crabgrass.
This problem grass is an annual and dies out in September leaving large bare patches full of crabgrass seeds. The crabgrass returns with a vengeance the following spring when all of its seeds begin germinating. A popular approach was to use corn gluten in the spring which inhibits seeds from properly germinating and producing roots. That strategy continually fell short of expectations. The very use of a seed inhibiting product preventing us from applying our golf course quality grass seed in these thin areas to establish desirable grass types was counter productive.
We now recommend our topdressing and aeration overseeding service for these areas in the spring and then again in the fall once any existing crabgrass has died out. The need to keep adding more high-quality seed to the area until the desired grass outcompetes the crab grass depends on the extent of damage and the proper care of the seeded areas.
A soil test might also be required to help identify any underlying issues preventing the desired grass to establish and grow. Often we find crabgrass growing along driveways and sidewalks where salt damage occurs from winter road salt. This salt can be reduced with a Gypsum treatment. Crabgrass loves poor soil conditions - Kentucky Blue grass cannot tolerate it.