by CKR
Pest is defined by the Oxford American Dictionary as "an insect or animal that is destructive to cultivated plants or to stored food etc." I would add weeds to this definition, although, the more I think about it, probably pest is to animal as weed is to plant.
Weeds of the West was useful to me this week. Here's a tumble mustard (Sisymbrium altissimum, not at full powers, will be pulled now that I've taken the photo) and another sort of mustard that I can't find in WoW now, although I was pretty sure I had seen it earlier. Amazing how many weeds are mustards--four-lobed flowers, long skinny seed pods, leaves very variable, related to cabbage and brussels sprouts and broccoli. Even the model for plant genome studies, Arabidopsis thaliana, is a mustard. And it's a weed.
This is a flourishing stand of cheatgrass, the weed I have spent the most time with this week. This bed is somewhat shaded and obviously retains more moisture than some others. The cheatgrass has several different growth habits, and the contrast between the height and greenness of the cheatgrass in this bed and in another 
tells me what kinds of nice plants will do best in these places. I think there is a drip irrigation system, but I don't know where the emitters are. Subject of a more extensive experiment when I get to the place where I can see the soil in the beds...
That second photo shows something else about cheatgrass. It has many different ways it can grow. I've weeded out some so you can see that when the plants are very close together, they raise their seed stalks vertical. But when they are further apart, 
they sprawl.
I can see two things that this adaptability does for cheatgrass: it helps to spread the seeds, launching them on the wind from a thick stand, and dropping them far from an isolated parent; it also allows the cheatgrass to spread its own sort of herbicide to keep other weeds from growing.
Many plants spread chemicals to prevent other plants from growing close to them. Walnut trees are particularly notable, but many trees do it. As a friend said to me, trees hate grass. I suspect that cheatgrass does this for two reasons: it forms almost monocultural stands, and I now see other plants coming up where the cheatgrass was. 
WoW tells me that this is woollyleaf bursage (Ambrosia grayi--Ambrosia, the food of the gods???), a perennial. I was suspecting it to be a perennial because the roots on the plants I've pulled seem to continue down to some infinite bursage source. There wasn't much of it when the cheatgrass was there, but now it's showing up.
Organic gardeners and lovers of furry creatures stop here. You will be offended by what follows.
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