A: In your situation, crown vetch (Coronilla varia) probably is not the best choice for an orchard ground cover. Potential problems include its competition with the fruit trees for water and nutrients, its possible invasion of your garden space and the unpalatability of the legume to your geese.
When choosing a suitable cover crop, you should first look to species that would provide nutritious forage for your geese. The grazing habits of these web-footed birds make them excellent orchard managers, as they'll not only reduce your mowing chores, but clean up diseased fruit as well. While the literature on this subject varies, it's generally agreed that the large birds prefer grasses over broad-leaved plants (the latter group includes legumes such as crown vetch). The geese at New Alchemy, for example, graze alfalfa reluctantly, and only if there's no grass available.
Furthermore, though grasses don't have the nitrogen-fixing capabilities of legumes, this disadvantage will be somewhat compensated for by the soluble nitrogen contained in goose manure. Moreover, it's been found that, in certain situations, legume understories in orchards can actually provide too much nitrogen for optimal growth.
Also, it's true that certain ground covers might invade your garden, but in this situation, again, your geese would be an asset. Although geese will devour some vegetables, the birds will carefully weed around other undesirable (to them!) crops, such as potatoes and tomatoes. Therefore, rather than concentrating on keeping a ground cover out of the garden, you might actually want to locate some geese-suitable crops on the birds' side of your fence and let your feathered livestock do the weeding!
I've stressed the importance of geese rather than the values of specific ground covers because the large birds are extremely useful in orchard management, and without information about your climate and soil, it's difficult to recommend a suitable crop. You should check with local growers and the agricultural extension service in your area for advice, and establish several small-scale plots to test the growth and palatability of your chosen ground cover. If you then decide that legumes are necessary, select a legume/grass mixture. (In such a case you might consider introducing ducks to your orchard, as they will graze some legumes and are effective insect eaters, too.) You'll also find that it's a good idea to divide your orchard into two sections, and then rotate the birds to allow better pasture utilization and to avoid overgrazing.
Finally, because many ground covers compete with fruit trees for water and nutrients, mulching around the trees out to the drip line is a valuable technique. This method reduces water evaporation and provides organic matter and nutrients. Furthermore, the labor involved in mulching around the trees is minimal. We use partially decomposed leaves and add lime to the mulch when necessary. We've observed yet another advantage of using mulches: The geese keep the ground covering weed-free even when there's plenty of pasture available elsewhere.