Cut as in the top sliced off or cut as in severed in half? Although not wonderful, the first is much less of a concern than the second. I'd suggest covering with a light mulch of wood chips or bark.

You may want to designate a 'no-mow zone' area surrounding this tree if surface roots seem to be prevalent. Damage to the root zone of a Japanese maple is something to be avoided if at all possible, especially in the PNW where soil borne pathogens abound.

Thank you so much for your response gardengal48! I'm not sure I understand what you mean by your description of the cuts. So I figured a picture would help. As you can see in the first shot, we planted the tree on a mound due to the clay soil that gets pretty soggy in winter. We planted it about 4 years ago and it's done well (so far).

Here's a close-up of the cuts that the mower made. Should I make cleaner cuts before covering with the wood chips?

Toad, that doesn't look so good :-( That's the type of cuts I was hoping NOT to see. I would make clean cuts with sterilized pruners just behind the damaged portions; clear out the weeds, grass and other crap from around the planting and cover all with a wood-based mulch (couple, three inches). Keep moist but not wet and hope for the best.

Make that mulched area as wide as necessary to avoid any further possible damage to the root system from the mower or any string trimmers you might use.

Here's another opinion: Not mower cuts.

Instead, a long-ago wound that occurred from who knows what.
The rolls of tissue at the sides indicate the tree is attempting to close the wound.
If you trim that area, you *will* make things worse.

I suggest you clear the grass in at least a 20-inch radius at the base of the tree. Doing so will avoid any chance of mower blight and more.

Next add a mulch in that cleared area -- coarse wood chips are fine, as are other materials. Just don't place against the trunk.

jean, I am pretty sure toad is referring to the severed roots clearly evident at the base of the second photo, not the damage occurring to the trunk (although I don't like that much, either).

One can attempt to rectify the root damage - there's not much to be done re: the bark/trunk.

Yes, gardengal48 is correct. I don't know how or when the trunk damage occurred, but I'm pretty sure my husband didn't bump into it with the mower. He got too close but not THAT close. (sigh).
Anyway, I've followed gardengal48's directions: made clean cuts, cleared the grass and weeds, and laid down wood chips (and watered). So now I say a prayer to Mother Nature and wait.

You're correct, I didn't notice the severed roots.

And here's my opinion:
With such large roots so close to the surface, I wonder about several things:
-- how the tree was planted
-- the condition and depth and water status of the soil
-- the tree's future health status: perhaps relatively short prior to the injury, now even shorter

Perhaps it's wise to consider removal & replacement. Could wait until next year to see how it does. If not vigorous, then replace.

Bottom line: The longer one delays removal, the more difficult it is to do because the gardeners in charge have invested large amounts of $$$ and emotions into keeping the thing upright and alive.

Oh yes; if you keep it, I still recommend the 20-inch clear zone all around the tree.
(P.S.: That also mean no flowers.)