This is Polygonum aviculare, quite coomon indeed.

This is an annual that usually grows in low nutrient, compacted soils. The first step in control is to get a good soil test and look at the amount of organic matter in your soil and then improve the soil so it will grow a good healthy stand of turf that will choke out "weed" growth.

Here is a link that might be useful: Knotweed

Yup, that's the stuff. And yup, my soil stinks.

Funny that gardeners call the stuff "Knotweed", but the people that use it for herbal medicine call it "Knotgrass". It'll be "weed" in my book.

I waged war on crabgrass this year. I saw this stuff coming up, and said "whatever it is will be better than crabgrass". I think I was wrong. This darn knotweed is choking my grass just as bad as the crabgrass.

Oh well, I will have a second enemy next year.

Thanks

Start controlling this with a good, reliable soil test, you can find your local office of your states USDA Cooperative Extension Service frmo the link below. You can alos use these simple soil tests, along with that nutrient and soil pH test, to determine what you do have and what you need to do to make your soil into a good, healthy soil that will grow strong and healthy plants.
1) Structure. From that soil sample put enough of the rest to make a 4 inch level in a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and replace the lid, tightly. Shake the jar vigorously and then let it stand for 24 hours. Your soil will settle out according to soil particle size and weight. A good loam will have about 1-3/4 inch (about 45%) of sand on the bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above that, and about 1/4 inch (about 5%) of organic matter on the top.

2) Drainage. Dig a hole 1 foot square and 1 foot deep and fill that with water. After that water drains away refill the hole with more water and time how long it takes that to drain away. Anything less than 2 hours and your soil drains too quickly and needs more organic matter to slow that drainage down. Anything over 6 hours and the soil drains too slowly and needs lots of organic matter to speed it up.

3) Tilth. Take a handful of your slightly damp soil and squeeze it tightly. When the pressure is released the soil should hold together in that clump, but when poked with a finger that clump should fall apart.

4) Smell. What does your soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant odor? The more organic matter in your soil the more active the soil bacteria will be and the nicer you soil will smell.

5) Life. How many earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5, according to the Natural Resources Conservation Service, indicates a soil that is not healthy.

Here is a link that might be useful: USDA CES offices