Hello James,

There are herbicides that affect monocots (grasses) while not injuring dicots (ivy, myrtle).  One of these is fluazifop.  When you buy it, look for fluazifop alone.  Some general non-selective herbicides use a mixture of fluazifop in combination with a dicot herbicide.

I would suggest to try an inconspicuous test area first to make sure that there is no effect on the ivy and myrtle.  I consulted Dr. Erin Hill at MSU about your question and she added this additional concern:

"I would say that yes, it would be good to try fluazifop on a small-scale basis first. It should work, but some perennial grasses may take more than one application. I was a little concerned about the irises in the photo, but according to Purdue, we shouldn't see injury on this from fluazifop, even though it is a monocot like the grass."

https://purduelandscapereport.org/article/killing-grasses-in-grasses-how-to-control-grasses-in-non-grassy-ornamental-plants/

Out of an abundance of caution, I would limit overspray on the ivy. myrtle and especially the iris. 

Hope this helps.

Hello James,

You need to follow the label directions on the product you find.  The label is the law.  I found one fluazifop herbicide whose label said to not apply in July or August.  There were also guidelines concerning the height of grass being treated.

I would expect to apply in September or later, but ultimately you need to apply according to the label instructions of the product you buy.

Hope this helps.