["Followup-To:" header set to rec.gardens.]
On Wed, 26 Jun 2013 04:42:03 +0000 (UTC), Danny D. wrote:
On Wed, 26 Jun 2013 02:01:03 +0000, Kay Lancaster wrote:

"Natura abhorret a vacuo"

In fact, as you surmised, on my unwatered lawn, are basically these
two plants (wild mustard and some kind of other nasty looking thing):
http://www4.picturepush.com/photo/a/...g/13411202.jpg

Looking closer at the nasty looking thing, it has nasty leaves:
http://www2.picturepush.com/photo/a/...g/13411200.jpg

And, a nasty purplish headress:
http://www5.picturepush.com/photo/a/...g/13411198.jpg

I think it's some kind of horrid thistle all over my fescue lawn:
http://www3.picturepush.com/photo/a/...g/13411201.jpg

Try pulling one up... you may find they're attached to an underground
root and stolon system, in which case you may be dealing with
Canada thistle, Cirsium arvense, a noxious weed* in California, and
you might want to consider some minor chemical warfare, as fragments of
the underground portions of the plants about 3/8" long can start new
ones, as can all the seeds. http://www.cal-ipc.org/ip/management...mber=182.ph p
They can basically take over an area in a few years.

Seeds last up to about 20 years in the soil, and can travel miles on the
wind because they have a little "parachute" of hairs (pappus), and also
many songbirds eat the seeds.

*Noxious weed is a legal definition, meaning the plant is a peril to
agriculture. I think C. arvense is a class B, but it's been 30 years
since I lived in CA, so you might want to check it. In some counties,
everyone may be required to control it, in which case you're legally
obligated to deal with it.

I control it here in my Oregon yard with heading the flowers**
as soon as I see them, and spot applications of glyphosate on
established plants in the fall. Heading has to be done vigilantly--
at least once a week.
**Canada thistle is a member of the Asteraceae (also known as the
Compositae) the dandelion family -- each of those purple "petals" is an
entire flower, and the flowers eventually develop one-seeded fruits
that are dispersed by birds and wind.

FWIW, I had a bunch of downed trees a couple of years ago, and burning them
was the only practical means I had to get rid of them. So I built the
bonfire on top of a big Canada thistle to get an idea of what might happen
in a wildfire. The fire burned for about 6 hours, got very hot, and left
a lot of very alkaline ash. Next year, guess what I had under the bonfire
site? Only the Canada thistle survived, and it was doing well.