clover is the last thing I would put in on my place. It will fix too much nitro and keep your fruit from ripening. I like grass or just weeds and then if I need nitro I apply a leaf feed. However, I understand that the rocks make it difficult for you to have grass. A low growing native cover crop could be a good idea. A creeping crop such as thyme (about which I know nothing) sounds like it might be good. How low will that grow?
A recent article in the Goodfruit Grower mentioned that sunflowers are great to have around peaches and roses around apples. I always thought roses could exacerbate fireblight.
If you can at all work your soil a program of vetch and grains can be beneficial. Buckwheat, and rye in the winter. You have to be able to turn this in or the grain will make a mouse heaven, something you don't want. But then again, maybe if they have enough grain to eat they will leave your apples trees alone. Some of you need to try this. Tell me how it works.

As a cover crop I use whatever comes up, and it does come up where I am at. On the young trees I turn in the green stuff and on the older trees I mow. I like to see a variety of "weeds" in order to provide food for the beneficials. I do not like broadleaf in the peaches. It is hard for me to keep the alfalfa out.

My orchard already had an old pine tree there when I moved in. I sprinkled hollyhock seeds in the bare spaces, and they look beautiful in the summer. I have to pull the ones that are too close to the fruit trees, though. I also cver the entire area with straw as mulch in the winter, and it looks pretty.
I grow melons and pumpkins, too, which adds some color and greenery.
Stephenie

I use ground cover Comfrey, Symphytum ibericum, in my orchard, it is very tough (and invasive) and is one of the few ground covers that is tougher than weeds. For that reason it could be very hard to get rid of. I like it because I don't have to do anything under the trees where it is planted, no weeding, mulching, mowing, or weed-trimming. Other people might hate it. It doesn't get very tall so I can still find fallen fruit pretty well in it.

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Thanks everyone. Someone mentioned Irish moss to me -- any familiarity? Also doesn't the straw attract mice?

Local experiment station has orchard fruit trees in strips of dirt kept bare with herbicides, with strips of mown grass between.

Doubt clover will keep apples on nearby trees green. In fact, one instance in New York of apples not setting was corrected by applying nitrogen.

The reason for getting rid of weeds is to get rid of competition for nutrients and water. Wouldn't planting a ground cover just replace one competitor with another more attractive one?

The reason for getting rid of weeds, or maintaining a herbicide strip, in immature trees is to enable the young trees to grow better as they have less competition.
The reason for the herbicide strip in mature trees is to create a barrier to mice. They don't want to cross it for fear of raptors. If there are winter snows and weeds near the trees, the mice will make under the snow tunnels to the trunks of the trees and feast. Hawks can't see them.

I let the weeds and grass grow right up to the trees in the summer and mow periodically. I want the grass and weeds to pull nitro out of the soil so that the trees won't use it. I have 2.0% to 2.2% fruitlett analysis anyway. More nitro will (from legumes growing in the cover crop) delay my fruit from ripening. I know this. It might be a week or it might be two weeks. Since I am a commercial grower I don't want late fruit. If you have very bad soil then you might want a legume cover.
Many years ago I was told by an old WSU scientist that the Washington State record for production of apples per acre was about 5500 bushels. For those of you keeping score that is almost 100 tons. He grew them, almost literally, in a field of alfalfa. He cut and let lay the alfalfa. Year after year. "Wow", said the young fruit grower. "Maybe I should grow my apples with an alfalfa cover crop". Well, you should know, said the scientist, that the man was out there picking with snow on the ground. And often his apples suffered from the cold. And his apples didn't color up well. And we all know that red apples bring more money. If you want more nitrogen just throw a sack of urea in the sprayer. It is really cheap and easy.