Protecting Mature Fruit Trees

I've been reading all about fruit tree guilds, and planning what I can put with my apple trees (they currently just have sparse grass underneath). Just want to check though: I keep seeing pictures of young trees with companion plants all around them, or mature trees with mature guilds around them (that look like they were planted at the same time as the tree). Will I be damaging my mature trees' roots if I just start planting things around the tree willy-nilly? They are very healthy and productive right now, and I am afraid to damage them by digging or building up the soil under them too quickly so the roots don't have enough oxygen. Anyone have any suggestions of how to start implementing guilds without potentially hurting the trees?

How big are the plants in your guild. Planting smaller plants or seed is much less damaging to the trees root system than planting 15 gallon pots. From seed is the best way IMO but small rooted cuttings or seedlings will be no problem for the mature tree.

Thanks so much, Jordan. So far, I had in mind daffodils and garlic chives to suppress grass, dill to attract beneficial insects, and maybe Jerusalem artichokes for a deep taproot. I've heard so much about comfrey with apples, but it sounds like it's mostly good for accumulating biomass, not sure if that's a good idea at this point. I read about mature trees dying when people started planting beneath them, because of the soil building too quickly and loss of oxygen to the tree roots. Also, we are on rented property, so comfrey may not be a very nice gift to leave for the next tenant.

Grass is the bigger pest plant around my fruit trees. They really don't grow well with it - in fact, they do much better with bare soil in a ring around them.

Other plants I've added don't seem to make much difference.

Grasses can, indeed be a problem under trees.
They are horrible water hogs.
Many grasses form a mat-like root structure which allows very little water to penetrate below it.
This not only robs the tree's roots, but also the deeper soil microbes, which the tree needs for optimal health.

Individual plants under the tree's canopy shouldn't create this problem. As their roots expand outward and down, they will learn to co-mingle with the tree's roots, each of them assisting each other by exchanging nutrients they have in abundance for those they lack.

Just be cautious if digging holes for transplanting larger plants. Don't go in there and begin digging 1' X 1' holes everywhere. I wouldn't do a lot of planting shortly after pruning - if you prune 10% of a tree's branches, it will self-prune 10% of its roots - not a good time to go in there and damage more.

Tree roots are dynamic - except while dormant, every day some will die, and new ones will form. So even if you damage/kill a small proportion of them, the tree should continue as if nothing had happened.

With a wide variety of plants growing around your trees you will be attracting a wider variety of beneficial insects (including pollinators) and also a wider variety of soil microbes which bring life to your soil - a Win/Win situation.

Wow, so it sounds like grass is about as bad as it gets for fruit trees. I did just do a little pruning, so I'll wait a while before digging any major holes! I think many of the things I am hoping to grow will do well from seed anyway. Thanks for your help, all!

I also have an old cherry tree in terrible condition... I am excited to see if replacing some of the grass beneath it with beneficial companions might make a positive impact on it.