Kudos for thinking of the tree! Smothering the vinca can also smother the tree roots, so I don't recommend putting down cardboard or anything impermeable over the root system. The tree needs to be able to pull moisture and oxygen from the soil. The root system on a 100 year old tree can be something like 3 times the diameter of the branching, so it's hard to know where the feeder roots of the tree are at this point. They may be well beyond the spot where you planted the vinca. I'm not a fan of chemicals, but this may be an instance where careful application of glyphosate (active ingredient in Round up) only to the vinca as it pops up may be the way to go.

Since I'm not well versed in herbicide chemical use, I hope others will chime in.

Barb

Thanks Barb. I have used Roundup on some of the pieces that I couldn't dig up. I am sure this is going to take a long time to eradicate.

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I have mulched heavily near trees to smother grass and other plants. I was a little nervous thinking about the oxygen deprivation thing. I believe the smother technique worked excellent. The areas that were heavily mulched didn't seem to be lacking in oxygen. If anything with the amount of worms it was more oxygenated. I have been careful not to mulch against the trunk.