Tuesday, June 10, 2014

Kill crab grass, goat heads, and star thistle

Crab grass didn’t used to grow in Grants Pass when I lived here in the 80’s.  Neither did goat head, known locally as puncture vine for what it does to bike tires.  Star thistle was out in the country on neglected farms.  Now we have all three all over town, the last two mostly on business, government, and vacant lots; people mostly don’t tolerate them around residences.



          All three can be hard to pull, but are easy to kill with a good pair of gardening scissors.  Kengyu pruning scissors are the best.  They are super-hard Japanese steel.  They can cut through dirt and gravel and still cut branches; they slowly wear down, but they don’t knick.  They may lie outside and get surface rust, but it wears right off with use.  They can not only cut weeds out of gravel; they can cut weed crowns out of cracks in pavement.  They can be found online.
          Any annual weed can be killed by cutting it off under the crown when it is blooming.  You don’t have to get the root; it has no food in it at that point.  Once it flowers, it has put all of its energy into its crown and above.

Green crab grass with dry foxtails and cheat

Young crab grass, spreading variety.  Some stand tall.

          Crab grass (Digitaria) likes water, but it can live on dust and dew.  It is related to Bermuda grass (Cynodon dactylon), with the same fingered seed head, but no underground rhizomes.  Where it has not been watered well, it has just enough root to hold it in the ground and pulls easily.  Where it has had good water, it roots deep and hard, and roots along stem nodes as well, the opposite of most annual grasses, but crab grass acts more like a tender perennial, and doesn’t die until frost.  Its roots are thin, tough and wiry, and don’t pull when mature and rooted deep.  Cut them off though, and the plant is gone; they have no food storage.


Goathead leaves and bloom 

Goat heads in bloom

          Goat head, Terrestris tribulus, literally “ground trouble,” crawls along the ground in a spreading mat with inch-long divided leaves and ¼” five-petal yellow flowers that open with full sunlight.  It laughs at mowers, spreading its tack-shaped seeds on their tires.  Also called tack vine, puncture vine, caltrops, and other names not suitable for print, its seeds can puncture bike, wheelbarrow, and some trailer tires.   It grows well in wet or dry ground, in bare soil or thick grass, spreading up to 3 feet across.  But its crown can be cut off its root, and the root will die.


            Yellow star thistle, Centaurea solstitialis, can be seen taking over vacant lots around the newer parts of town, its bluish foliage standing out among the drying grasses.  Its spines are only on its flower buds and fold down when they go to seed, but they are enough to declare it a noxious weed.  It doesn’t spread its seed on the breeze like other thistles, but on animals, lawn mowers and landscape maintenance workers; they cling to the plant all through the winter, long after the spines are gone.  It loves dry ground, but contrary to rumor, it can take heavy watering and come up through considerable mulch.  Mowing just makes it eventually seed out beneath the blades.  But its crown is often several inches above the soil bent sideways, handy for cutting off its root.