I suggest lay some evergreen rock,make the wall.don't dig.

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Or,add some shrub.
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Geez, just when you think Ideasshare has gone away, and the forum starts working as it should again, s/he shows up again. To the OP: you can ignore ideasshare with a clean conscience.

When you say "properly dig," how deep and wide are you thinking of? And how close the maple are we talking? I can't figure out what tree you're referring to on your photo.

Finally, what's your climate?

KarinL

Given the size of the slope, it isn't going to take much in the way of a retaining wall to help. I've build several teeny tiny dry laid stone retaining walls. There isn't a lot of digging involved, just deep enough to partially bury the bottom rocks.

The usual advice about building retaining walls mentions 'what happens if the thing collapses'. Since in this case, what happens is that some people get their feet muddy, and it has to be rebuilt, it doesn't have to be a major construction project.

I'm not a pro or an engineer, but if I consulted one, I'd ask about a simple wooden retaining wall -- hoping that only a few uprights, set several feet apart, would need to go below the bottom of the cement and possibly encounter major roots.

As I said, I'm not a pro or an engineer; if I were, I'd know to what extent the winter freezes/thaws would impact a simple wooden wall. You didn't tell us what USDA zone you're in; that would give us some idea of your winter low temperatures. Your can enter your zip code and find out your zone here:
http://www.garden.org/zipzone/index.php

I agree that this seems fairly minor. I built a dry stack retaining wall that's about 12 - 18 inches high and only partially buried the bottom row. It's held up for a few years and the biggest issue is grass growing in the crack. And the time a delivery guy ran over the corner of it.

If some cinderblocks held it back then I doubt it will need major engineering. The trick will be to make it so that the wall leans back into the dirt to offset the dirt wanting to lean into the wall.

@karinl When I say "properly dig" I mean what I did in my back yard with pavers. I used Allan Blocks (12" w x 8" h and like 80 lbs) and dug a trench 12" deep, filled with 4" of packed gravel, and laid one row of pavers below grade. I dug out another foot behind the pavers to backfill with packed gravel. (see link I posted below)

I live in Massachusetts. You can see the sidewalk in the photo, which is about 3' wide. The maple is on the other side of that sidewalk, out of the photo. So if I dug a 1' deep x 2' wide trench along the front yard for a permanent wall I'd be only 3' from this maple and there would be a lot of root damage, and these street trees can be a bit unstable anyway (one down the street came down in a storm last year).

@tanokicki The cinderblocks didn't so much hold it back as much as there was also a chain link fence there. With the fence gone, the cinderblocks are kind of just propped against the slope and fall over a lot.

I think what I might do is partially bury some square stones to form a sort of garden border. Then above that it will slope down some which I can mulch. This should help erosion and since it's more of a border than a wall, it won't be a big deal if one stone falls over from a frost heave. If I actually built a wall where there was more than 1 row of stones, it might start to look ugly and be a hassle if I had to keep repairing the wall.

Here is a link that might be useful: Retaining wall in backyard I built last year