gecko said:

I have been fighting a weed for a few years now, all to no avail. It
is sometimes called 'wiregrass'. sometimes 'witchgrass', I think.

Unfortunately, common names for plants make them very hard to identify.
Without proper identification, it's even more difficult to find a remedy or
control. Photos of the plant would be most helpful. Posting them online
somewhere, and linking here, would be best. Your geographical location is
helpful information, also.

Looking for "witchgrass" turns up Panicum capillare. Do a search on Google
for the Latin name, and see if that's the plant.

"Wiregrass" turns up another plant altogether (Cynodon dactylon (syn.
Panicum dactylon, Capriola dactylon)), which is actually Bermuda grass.

It grows long 'stringers' below and above ground, and when I pull one
up, sometimes the 'stringer' is a foot or two long! Terrible stuff.

I have tried a few weed killers, such as Weed-B-Gone, and weed killers
in Turf Builder. I think the danged weed feeds on that.

The local nursery tells me that you can't kill it except by using a
vegetation killer, which of course would kill grass and everything.
I hate to think of doing that.

I guess I have a few questions. First, is a vegetation killer my only
choice?

It depends on the plant. If it's C. dactylon, timing is everything with a
non-selective herbicide. It spreads by stolons (the runners across the
surface), rhyzomes (underground), as well as seeds. You need to apply
glyphosate (ex. Roundup) at the END of the plant's growing season, when the
plant is sending food back down to the rhyzomes for storage during it's
dormant season. If you apply it during it's growing season, you'll kill the
top-growth, but the rhyzomes will send up new stolons in no time. Applying
at the end of the season, it will take the herbicide down to the rhyzome,
and effectively kill it.

If it's P. capillare, and it's in your lawn, you should apply a
pre-emergent. It's an annual. If the seeds can't sprout, the plant can't
grow.

If not, what else can I try?

Mow higher, seed in the fall to get the turf thicker so the "weed" can't
compete, and try not to overwater.

If so, when do I apply it? In the Fall?

Apply glyphosate in the fall. Water, heck, even fertilize the invasive
pest. Make it feel welcome, get it growing and producing sugars, then ZAP
it with the herbicide. It'll never see it coming. =)

Pre-emerge in early spring.

Second, how soon after something as drastic as that can I
then plant grass seed?

Glyphosate won't affect the seeds. It's a contact killer. It's absorbed
through the leaves of the plant. If you're really worried, wait 7 days
before seeding.

Seems to me that the killer would exist in the
soil for a while. Maybe years?

No.

HTH,
--

Eggs

-A tree never hits an automobile except in self-defence.