There are a number of old sayings when it comes to gardening, such as “the corn should be knee high by the 4th of July” or “you have to add lime to sweeten the soil”. However there are not too many old sayings when it comes to lawn care, but one that often gets thrown around at this time of year is “apply your crabgrass preventer when the forsythia bloom.” As we often find out, these old sayings are not usually based on scientific evidence and, for the most part, cause more confusion than necessary.
For the most part, the corn in my area is always taller than my knees by the 4th of July, with the exception of a severe drought. Lime should only be added to soil if the pH is well below 6.5, which is determined by a soil test. As much as lawn care companies try, there is no way that all of the lawns we service can have their crabgrass preventer applied by the time the forsythia bloom.
The crabgrass statement is not really based on any hard and fast rule. I am sure some turf professor was asked as to when the best time to apply a pre-emergent weed control product and he picked a plant that was popular and one that everyone knew the name and that bloomed early. It can be used as a reference point, but it is more important to understand that crabgrass will germinate when soil temperatures are greater than 55 to 60F degrees for 7-10 consecutive days. Depending on the year, this may not occur until mid-May in the Midwest.
For those in the south, your soil may never drop below 55 degrees, so crabgrass can be a problem for you from the fall until soil temperatures climb above 95 degrees, which is when crabgrass stops germinating.
It is similar to the mowing rule of not removing more than one-third of a grass blade when mowing. This is not based on any studies, but it was an observation made by a Department of Agriculture employee who thought the grass looked best after mowing if not more than one-third was removed.
We have all been faced with mowing grass that is very long because of rain or other outside commitments that prevented us from getting to that task. As long as the lawn is mowed tall, it will be fine.
One final word about crabgrass germination and mowing. Mowing high will help prevent crabgrass and many other weeds from germinating by shading the ground and prevent the sun from heating up the weed seeds that already exist in your lawn. Of all the cultural practices, mowing has the most impact on the health and quality of your lawn.
Do you have question about your lawn and how to best take care of it? Contact your local Spring-Green for more information.