Goat heads: Trouble on the Ground

Flowering goat head, courtesy of Forrest and Kim Starr

          Goat heads have started sprouting, flowering, and setting seed over the last few weeks in the hotter places in Grants Pass where it has been allowed to grow.  Locally known as puncture vine, it is also called bull head, bull thorn, tack vine, caltrops and other names too numerous to mention.  Its Latin name is Tribulus terrestris, Trouble of the Ground. 

Ripening goat head seeds, Forrest and Kim Starr

Caltrops were weapons of war made to lame horses, from which we get the old game of jacks, and the seeds of this plant will quickly lame your horse, dog or anyone going barefoot, as well as puncturing bike and wheelbarrow tires.  Mowers spread it around on their tires as they are taken from one neglected lot to the next if the mower doesn’t get cleaned off between lots.

Goat head seeds

          It is such a trouble that it is generally not allowed to grow for long on occupied residential property, but is common around here on neglected lots, gravel parking lots, and all over our downtown core in parking medians and along sidewalks, spreading out very flat except where it piles on top of itself.  It will soon be spilling over curbs, scattering its seeds under feet and tires.  It comes up a bit later in cooler lawns and lightly shaded woodland, and grows well in thick healthy grass.

Goat heads growing in brick sidewalk.

          It grows very fast in the hottest, driest places, setting seed as soon as it starts to flatten out and spread.  Those seeds come in clumps of 5, which quickly break apart into very hard, ¼-3/8 inch three-pointed seeds that remind people of horned heads of goats or cattle.  They come from flowers that are bright yellow, ¼ inch wide, with five rounded petals.  The leaves are small and pea-like, about an inch long, divided bilaterally into about 9 leaflets.  It can start making seed at only an inch wide, but can spread out to 3 feet in every direction, with hundreds of seeds on its underside, waiting to be stepped on and carried away.  Its seeds are also spread by runoff.

It puts down a thick taproot that is hard to pull from dry ground, but you don’t have to pull the root; cutting it under its thick crown, from which its branches spread, will kill it.  Like most annual weeds, it stores no food in its root. 

Gardening scissors are best for this job, but a knife will do.  BiMart has long-bladed titanium gardening snips that don’t chip or dent when cutting weeds in gravel.

I throw these weeds in the trash, not my truck, lest I spread its loose seeds around in my travels from one customer to the next.  This is the only weed that I do not take to the composter, though it would be killed by hot composting like any other weed seed.