In reply to a customer question about controlling gorse in amongst Flax.
 
I don't have a clear answer, at least not from practical experience.
 
I once tried several weed sprays on some unwanted flax (a small stand of flax that was going to have to be removed) to see what was safe and what wasn't.  It was not a very illuminating experiment!  
 
Glyphosate definitely damaged the flax.
 
Less certain was GrassMate and Triclo, which are both grass-friendly herbicides.  Flax is a grass-like species, and I'd expected that these two herbicides would be safe.  But there was still some damage … and it wasn't consistent; on a couple of flax 'bushes' the effect was zero-to-minimal damage from these sprays, but on one or two others there was moderate damage.
 
One herbicide I did not try, but should have, was MSF600 gorse & brush weed spray.  It does not damage, or at least does not seriously damage, many native grasses in both NZ and Australia, so perhaps it may not damage or at least seriously damage flax.
 
I abandoned my flax experiment without ever having reached a clear conclusion, I'm afraid.  And nobody has ever asked me the question since then anyway … about 10 years ago!  Until yourself, I mean :)
 
Approaching the problem from the 'opposite' side (i.e. what does control flax?), there's no references that I can find anywhere about herbicides to use to deliberately kill flax.  Not surprising; it's a native and is therefore not normally a target for herbicide control.  Nobody in NZ registers any herbicide for that purpose of course.
 
So here's what I can say:
• Glyphosate kills the gorse, and will severely damage the flax.
• MSF600 kills the gorse, and MAY be acceptable around the flax.
• GrassMate and Triclo will kill the gorse, but they have a variable and unpredictable effect on the flax.
 
I'm afraid I can't suggest anything for certain.  If you are willing to try a small scale trial to see for yourself what will do the job best for you, I think I'd try MSF600 first, and if that's causing flax damage then perhaps next try a small scale spray with Triclo, or with GrassMate.