Arlene, I'm not sure. There are so many varieties of mint and so many uses of each variety I tend to think that it is one of those plants like the dandelion, it's simply a survivor. People used to use it to reclaim land because of its ability to survive and the ability to spread far and wide. I know most of those used for culinary and medicinal purposes are true survivors and that's what I have.
On the other hand, sage, hyssop and lavender are in the same family as mint, but aren't considered invasive. They are reliable and dependable survivors, but they just don't spread.
I had lemon mint one time, it was just like the lavender, stayed in one spot. Maybe the orange and pineapple would do the same.
Did your son give a reason for using it as an underplanting? Maybe it has something to do with deterring pests in the orchard. They are good for that, most of them, I think.
My family used bee balm, same family as the mint again, along with mint on the bank of the creek that ran alongside our house. Spring rains came and the creek would overflow and cause erosion. Mint and bee balm helped solve that problem.
I'm not giving you a good answer because I don't know, but I'll see if I can find anything more about it. If I do, I'll let you know for sure!
