The grass grows around and into the edge of the pile of wood chips. If I try and mow this grass, the lawn mower throws wood chips everywhere and it dulls the blade. If I don’t, I end up with a ring of really tall grass around each tree … which is the current situation and it looks like a half-assed mowing job.
Does anyone know a trick to getting rid of this ring of grass?
You will need to install lawn edging around the tree.
Use a string trimmer to cut the tall grass missed by the mower.
I use a string trimmer in the vertical position to edge wood chip covered beds. This keeps the beds shaped nicely too. It does throw a small amount of wood chips and it doesn’t completely keep the grass out, but it does slow it down. The grass that gets in is pulled by hand before it takes over. Looks good to me.
There is always Roundup, but it’s not for me.
try vinegar and epsom salt. my brother has had good luck with it. i use just a tiny bit of roundup and i only do it in spring and mid summer and the weeds stay out. i have too many plants mulched to have time to weed wack them all.
In this case you may not be wanting to completely kill the grass, so a vinegar knock-down is probably fine. The Epsom Salt bit though, hmm…
Every time I see a friend on “social media” share the meme about “weed-be-gone”, I reply with the article below. The meme that says to use Vinegar, Epsom Salt, and Dawn brand Dish Soup[SIC]. 
I let the grass/weeds whatever grow right up to my trees now. I tried woodchips for probably a decade and lost that battle to creeping charlie. For new trees i’d still do it…for at least the first couple of years.
Could you raise your mower height to stay just above the woodchips. I’m thinking i did that in the past.
idk he swears by it. maybe theres something else he adds to it that i don’t know about. never tried it myself. i know that lightly sprayed roundup keeps the grass and weeds out and so far no ill effects on my plants of trees going on 6 yrs now and i use it very sparingly.
That article suggests that somewhere along the line someone heard “salt” and inadvertently changed it to “epsom salt”. Many people have dutifully followed those instructions and seen “success”. From the acid in the vinegar sucking moisture out of the plants greenery and quickly turning them brown. Unless the grass or weed is already weak, there will likely be new greenery in a matter of days though. Little if anything is done to affect the root. Unlike Glyphosate (Roundup, etc) and other herbicides that enters the cellular structure of the plant and kill root and all.
Table salt (Sodium Chloride) will “salt the earth” and render it inhospitable for most plants. Epsom salt (Magnesium Sulfate) will provide minerals many plants need to grow. We spray it on Tomatoes and many other plants every summer.
I can’t help it, seeing epsom salt in weed killer instructions is just a trigger for me 
Thanks everyone for your comments.
I have too many trees to buy edging to go around each tree, the cost would be prohibitive.
I do like the idea of digging and maintaining a trench around each tree. I’m leaning towards buying a machine designed for this purpose rather than using a string trimmer. I don’t trust my manual dexterity. I have started researching what to buy. Hopefully this will work but a little voice is telling me, I will eventually give up and follow Warmwxrules’ lead.
I’m not opposed to spraying RoundUp, however I live in Ontario and it’s banned. Because of the ban, I was forced to try the salt & vinegar trick. Didn’t work for very long, the weeds came back from the root within a week.