Overseeding your lawn is a really important part of having a thicker, healthier and greener lawn with less weeds. It really isn't difficult to do, but it does require some knowledge to get the most germination for your buck. Read on to learn the why, what, how, when and where of overseeding your lawn. Keep in mind, we're a West Michigan-based company, so our recommendations are targeted for this region.
When should you overseed? The short answer: either the spring or late-summer/early-fall. Late-summer/early-fall is ideal, however.
The longer explanation is that the grass that grows in Michigan is called "cool weather turf." That means it grows best in cool temps-- when daytime air temperatures are between 60 and 75 degrees. (I like to call this "open window weather," when you need neither your furnace nor air conditioner to make your house comfortable.) When temps drop below 60 for an extended period (late fall & winter), the grass goes dormant. When temps rise above 75 for an extended period (summer), the grass also goes dormant. Winter and summer are terrible times to put down grass seed in Michigan; the seeds just will not germinate.
So the ideal time is to put down seed in the late-August to mid-September. This is when the nighttime temperatures start dipping into the 50s again, which will help lower the soil temp to an optimal point for seed germination. Fall is also a great time to overseed because of the life cycle of annual lawn weeds. Annual lawn weeds germinate in the spring, drop their seed in the summer, and die off in the fall. If you can put down some grass seed in late summer or early fall, it'll have time to germinate before winter. This will help fill in gaps in your lawn, so next spring when those weed seeds are looking for a place to sprout, you'll already have grass growing there.
While fall is ideal for overseeding, you can also do it during the spring growing season (typically late April to late May). The competition for nutrients, water and space is just a bit more fierce in the spring.
Fall and spring are also the time to pull up weeds (or, if you absolutely must, put down an herbicide). If you're removing weeds from your lawn, you're typically opening up a space on your lawn as well. Now is the time to do the overseeding, so that grass will fill that gap instead of a new weed. Something is going to grow in that gap-- let's make it grass.