There are many factors which yield a high crabgrass population in any given lawn area. Last season, we had one of the wettest seasons in nearly a decade while this year we are in one of the hottest in nearly a decade. The two extremes are just that- extreme and there are ramifications to a lawn. To understand what happens in a lawn setting we must look at the weather, turf density, mowing height, and treatments. Hot and especially dry weather will cause dormant crabgrass seed to germinate- typically in bare areas first (along roads, walkways, driveways) followed by thin sections in the lawn. Crabgrass loves high heat and low moisture. Seeds can remain dormant for years until the right conditions arrive, and then they germinate. Normal rain, proper fertilization, and cutting height can usually minimize crabgrass in primary lawn areas. Grass that has been treated with high soluble fertilizers and is not as healthy will be more susceptible to crabgrass infiltration. The best defense is still a thick lawn, a high cut, irrigation if possible, and slow release fertilizer among other applications. Some years are above or below average in terms of rainfall and heat- key factors in crabgrass germination.
One year of crabgrass does not undo a lawn. A pre-emergent barrier can be applied in the spring, but even that will degrade by late July or early August. Luckily, crabgrass knows when the remaining growing season is insufficient to complete its life cycle. Said another way, crabgrass seeds will usually not germinate past mid July. So what you see now is going to be it- the plants will just become larger. Again, a pre-emergent barrier can be used to help suppress, not eliminate crabgrass in thin or weak areas. Most of these products are simply dyes and are not harmful in terms of the environment. A pre-emergent product can be applied this fall (often overlooked) or next spring if you belive crabgrass has gotten a firm foothold in your lawn. There is a trade off between putting down a barrier and seeding- so give this treatment careful consideration. When desirable turf becomes stressed by high heat and drought, it provides an ideal growing environment for crabgrass by heating up the soil. Lack of moisture further stresses desirable turf and enhances the ability of crabgrass to grow at exponential rates- real fast.
Mowing at 3”, mulching clippings, watering (1” per week), and a solid lawn health care program are all great defensive measures. Adding a pre-emergent in the spring is another tool to help inhibit crabgrass but not eliminate it. In a normal year, things would be in check and balance like in nature. However, in extreme heat and drought- nature will win the battle and aesthetics will suffer- regardless of the plans in place.