Have you ever gone away for a weekend and come back to discover that your yard is full of weeds? How could there be no weeds one weekend, and nothing but weeds the next? It's almost as if weeds have little internal punch-clocks telling them when to go to work. That punch-clock is soil temperature. Here are some temperature tipping points for common lawn weeds.
If you have crabgrass in your lawn, you can thank the US Patent Office. In 1849, it imported the species to this country from Europe as a forage grass. One plant can produce 150,000 seeds, and they start to germinate when the soil temperature reaches 55 degrees or more for 4 days. Of course, germination doesn't mean success. You can keep new crabgrass in check by putting your lawnmower on one of the highest settings and watering deeply but infrequently.
Dandelions are tough. Some dandelion seeds can start germinating when soil temperatures are as low as 50 degrees. Most will start germinating when the soil is around 77 degrees and moist.
Goosegrass, yellow foxtail, and nimblewill are nasty-looking weeds. They resemble crabgrass, and can ruin the look of your lawn just as easily. These grassy weeds show up a little later in the growing season because they start to germinate when soil temperatures are in the 68-95-degree range.
Of course, other factors play a role in weed growth. Crabgrass doesn't care for shade. Goosegrass often appears in poorly drained or compacted soil. Nimblewill needs a little more moisture than some of its brethren. By setting your lawnmower high, feeding your lawn regularly, and watering deeply but infrequently, you'll be doing a lot to keep weed seeds from gaining a foothold in your lawn.