Since mint is so invasive, and you can mow it and it just grows more, and it smells wonderful... would it make a good orchard ground cover or would the root system be too deep/invasive to the fruit trees?
Mint requires between 50 and 60 inches of water per growing season for optimal performance, which translates into about 1 to 2 inches of water per week. Of course, temperature, soils, and type of irrigation affect results.
Can the type/kind of fruit you are growing in your orchard match these soil and irrigation requirements? If you can answer yes, then you should probably take a look at nutrient demands to see whether there will be a conflict or match.
Matching multiple plant demands (legumes, maize, melons mentioned earlier, for example) can be tricky.
No matter how badly I needed a cover crop or a ground cover under trees, I would not plant mint. Mint is very, very invasive. I have never seen it recommended as a cover crop in an orchard of any type and I believe that is because it would suck up moisture the trees need and it would compete with tree roots, especially the roots of young trees, much more than the commonly recommended cover crops.
The type of cover crop you would grow under trees would vary, depending on the type of trees in your orchard, the kind of soil you have, your annual rainfall, and whether or not you irrigate during dry spells. Normally, though, your cover crop needs to include at least one legume.
If your trees are very young, it is better to have mulch than a cover crop, at least until the trees have a few years of growth and larger root systems. Studies have shown that young trees with grass underneath them, for example, grow more slowly because the grass is competing with their roots for moisture and nutrients.
If you'll tell us what kind of orchard you have, maybe we could come up with some suggested cover crops to grow underneath them. Also, knowing the soil type helps and, if you know your annual rainfall and if you irrigate (and how much you irrigate) makes a difference too in what might be recommended for your orchard.
My orchard is pretty big, I tried to 'till' and remove weeds before I started but the "tall prairie grass" rizomes and bermuda are relentless. My solution has been to weed and heavily mulch circles (3ft from trunk, 6 feet across)for each tree. Then I mow single paths between rows of trees and up to each tree. So far I have allowed the grasses and weeds to grow in between as beneficial insect habitat. Its not pretty = you can't even tell its an orchard - someone saw the huge field of tall prairie grass and complimented my "garden" thinking it was corn (it gets 6 - 7' tall). The up side is that the orchard is filled with bees, butterflies, ladybugs, frogs and turtles. The other up side is, since the trees are young, the tall grasses and weeds shade their root/mulch zone, but the tree gets full sun. The 6 foot circle allows plenty of air circulation. The reason I asked about the mint is because I love mint, and planted a patch between some trees... of course its naturalizing. So I was wondering if I should try to get it out or let it go and just mow it. I was weeding the tree circles last week and it smelled to wonderful even at a distance. I'm not sure I'll ever get the weeds under control enough to plant a proper groundcover in that area. I put in some low white clover and recently ordered the "Herbal Lawn Mix" from Peaceful Valley for a different/new small garden area I'm putting in this year where (hopefully) I've won the battle.
Tall prarie grass that has rhizomes and looks like corn sounds to me like....Johnson Grass, second only to Burmuda as the bane of gardeners. In our orchard we tried using weedmatting covered with mulch, but the edge of the mower caught it on several trees and on several others the Fescue penetrated the mulch and we removed it. We started using Roundup to keep a circle cleared around our new trees. It's a 2 person job. One operates the sprayer andthe other one holds the shield--a large piece of cardboard--next to the tree to protect it.
But I agree with Dawn. I would not introduce mint to a garden or orchard where it can roam free, although if you keep it mowed regularly, you will keep it restricted somewhat. My mint bed is confined to a bed that is bordered with boards sunk into the ground which we weedeat around several times a season.
We planted our orchard in a field with Fescue, Johnsongrass, Sedge grass, Purple top, and a little bermuda. I keep it mowed just like a lawn all summer.
It does sound like Johnsongrass, which is the bane of my existence here.....second only to bermuda. I try to keep it mowed down short because that seems to more or less control it, but those large, finger-sized rhizomes (yours might be wrist-sized since you get so much more moisture, LOL) creep into the edges of the veggie garden and flower beds and drive me bonkers. To make it worse (just the name association) our land was once part of the Johnson family farm, we live next door to the Johnson son and his family, we bought our land from one of the Johnson daughters, and our road is known locally as Johnson Road. So, when I am digging out Johnson grass, I am always wryly amused and ask myself if the road was named for the Johnson Family or the Johnson Grass! (And I love every member of the human Johnson family I've ever met, including in-laws and nieces and nephews, but the grass is a different story!!!)
I used to have a lovely bed of daylilies that I put in the first year we moved here, but the Johnson grass kept infiltrating that bed over and over during our hot, droughty summers and I finally gave up and abandoned it and the Johnson grass choked out almost all the daylilies. One day, I will have a daylily bed closer to the house, but only after I have removed ALL the grass and taken measures to keep it from ever invading again. That first year, I was naive in thinking that if I removed all the rhizomes and enriched the hard, clay soil, the Johnson Grass would be manageable. It might have worked closer to the house...but this bed was about 200' from the house, down by the driveway/roadway, and it doesn't get any supplemental irrigation, so the Johnson grass, being more drought-tolerant, "won".
I have planted a lot of trees around the house because we built in the only open area (not wanting to bulldoze huge trees in the woods) so we had no shade. I didn't do as good of a job as I should have at keeping the grass away from the roots of the young trees back then, but I'm doing better now and I think it does make a tremendous difference in how quickly they grow and also it makes a difference in how well they tolerate drought.
I planted mint in clay soil in several areas and it lived for several years but it eventually died out during drought in 2003, a year in which we only had 18" of rainfall here (our worst year here by far, and that was the year the grasshoppers ate everything in sight, including fruit on the trees, the cotton rag rug on the porch, and our fiberglass window screens). I would never plant mint in the "good soil" in my flower beds or veggie garden for fear it would outcompete everything else, but I am going to plant it near the chicken coop and guinea coop where the combination of the hens "digging and scratching" and the increasing amount of shade have left a lot of bare soil in an area that used to have bermuda grass. (Yeah! Some of the trees are finally getting large enough to shade out bermuda.)
When our fruit trees were younger I planted nasturtiums and garlic underneath them (nasturtiums to attract beneficial insects and garlic to repel borers), but the chickens liked the nasturtiums a little too much and the garlic merely disappeared.
When I was a young boy (1950's) my grandfather had a farm and apple orchard. He had mint growing all over the place and used to refer to it as "Borrowers Mint". It was a spiecies of Wolly Mint, though I have never seen its like anyplace else. It never seemed to impact his apple crop, he also used to let the cattle graze in the orchard when there were no apples on the trees claming that the cattle eating the mint would counter the milk taste effect of any wild garlic the cattle might eat. The mint made some mighty good tea as I remember it.