How to kill invasive periwinkle
I have discovered quite a large patch of what I have identified as invasive periwinkle (vinca minor), picture found on the internet is attached. The plant has several stems coming from a point of origin at the ground with each stem growing a bit more than one ft long. The leaves are dark green and shiny and the plant produces small somewhat attractive light purple flowers. I have sprayed with both round up and cross bow to no avail, the plant withers but bounces back. I had read that because the leaves have a waxy finish adding mineral oil will help the herbicide penetrate. I also understand that digging them out may be the only option. I am of course looking for the easiest solution preferably with an herbicide.
Yes, periwinkle is a weed commonly seen in patches here and there in the county. It is Mediterranean in origin so very much at home here and not bothered by summer drought. I would expect either Crossbow or similar broadleaved herbicides to do a decent job of controlling this weed, as well as glyphosate (Roundup). I don't know what the product label says about the control of this specific weed, however. And a single application may not be sufficient for control, as appears to be the case here.
Many plants have waxy foliage that does make herbicide use more challenging and Vinca may well be one of them. For these plants an adjuvant of some kind may well increase efficacy of the herbicide. I would follow the directions on the label for selecting one of these, however, as some adjuvants will wok better with specific herbicides.
Other than that, if it is possible to use a string trimmer on the plant to reduce the foliage that will act to reduce vigor and probably make it easier to dig out the roots by making the root clumps easier to find. Another idea might be that although herbicides usually work better with a good complement of foliage to absorb the product, perhaps string trimming followed by a targeted application of an herbicide applied to the cut stems at the roots would help the herbicide penetrate the plant.
Feel free to write directly with questions.