Question
QUESTION: I have some weed plants I'm trying to kill..Roundup browns them but doesn't kill them.  What's the most effective herbicide for killing hard to kill plants?

ANSWER: Are you trying to kill EVERYTHING in the yard, or just a few things?  And where (state or zip)are you using these WMD's?  Are they growing in your Lawn -- and if so, what kind of Grass are you growing?

I can give you an answer, but I will need more information.  Right now, the most effective weapon for those Weeds would be to solarize your soil.  Foolproof.  And cheaper and healthier than RoundUp.

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QUESTION: The weeds/brush are in a stand of trees which is my front lawn.  No grass.  Some rhododendron and mountain laurel bushes.  Where there are no bushes there's just old mulch and decomposed leaves on the ground.  In one small area there are some ferns I don't want to kill.  I live in Hendersonville, NC-just south of Asheville.  What is solarizing?  I've never heard that term.
ANSWER: Solarizing is a foolproof way to wipe out ALL vegetation without killing most of the microbes and the earthworms in the soil.  It involves spreading clear plastic over a SUN-drenched plot and sealing the edges until the Weeds bake.  It's an easy and cheap way to prepare a plot for new seed or sod.  Clearly this is not going to work for you in this particular case, but keep it in mind in the future and I'll give you the details then.

Your Round-Up by the way OFTEN drifts around to neighboring plants and inflicts damage that may not show up for months.  Keep that in mind next spring if anything looks out of whack.

Your best solution here is a roll of landscape fabric, which you can pick up at Lowe's or Home Depot or even your local garden center.  This is the best thing since sliced bread.

First you want to kill the vegetation, though.  You don't want one of those little green monster-type Weeds - Wiregrass or similar noxious alien vegetation - to rip through the fabric (those monsters are known to break through concrete).  So you have to get rid of those first.  A sheet of black plastic (like a box of unused black Hefty garbage bags) placed over a soaked plot and left there for 30 days will block out all light and wipe out anything you did not personally put there; any that break through the plastic can be dug up by hand.  Just an insurance measure.  Then you can remove the plastic and in its place roll out your landscape fabric, cut as needed, and cover with mulch.  

Then relax.

Cheap, low sweat, safe and foolproof.

Any thoughts about that?

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QUESTION: The only reservation I have is that we're talking about a pretty big area on which to put down black plastic.  And, since the plants I'm trying to kill are brush, I'll have to cut them down to the ground before covering them.  I was hoping there was a super herbicide that would kill anything it touched to make the job easier.  I could cover the plants I don't want harmed when I spray the nuisance plants.

Answer
In that case, the most effective way to accomplish this mass destruction would be the time tested slash-and-burn treatment of your plot.  Even unwanted brush would burn to the ground under those flames.  Wikipedia describes it well:

http://en.wikipedia.org/wiki/Slash_and_burn

Why do I have this funny feeling you are just going to reach for the nearest herbicide?

I have to tell you, my friend, any chemical with a skull and bones on it has no business being poured by a human being on a plot of ground.  RoundUp works on most vegetation, but it is not safe and is banned in many areas.  F.Y.I., People die from RoundUp exposure -- it's not non-toxic.  A professional landscaper would be licensed to purchase more dangerous chemicals that are not available to the consumer; you should consult one of them for a chemical that is legal in your state and county.

Also, I would suggest that you ask the other Lawns AllExpert for a second opinion; he is not averse to that kind of solution and may have something still on store shelves for consumers that you would be satisfied with.  Make sure you tell him where you are writing from.