Get some real bids from landscaping professionals, not an off the cuff remark from a plumbing contractor.

A simple edging would prevent the grass from spilling over, although I doubt anyone is going to be picky enough to complain about yours. You could smother the grass and replace it with a groundcover if you don't like to mow.

As Sigrid noted, you don't need a retaining wall. At most some solid edging will do the job and is both low cost and a DIY project. Save that 20 grand for something really important!! 😊

Edging cobbles.....sold at home improvement stores

Wood post edging.......also sold at home improvement stores.

Plant a groundcover so you don't have to mow that steep slope. Do it in sections if you feel doing it all at once would be overwhelming. Or at least plant the lowest and steepest part of it.

Here are some ideas that require only one more row of stone to be added to the wall :

As above, the first thing to do is all labor.

Thanks for the suggestions! The person who did our sump pump is a landscaper who we spoke to about the wall. He specializes in drainage and dry basements, but does landscaping projects like retaining walls, in business and well respected for 30 years. So not a random comment from a plumber. Will keep researching. The hill is very steep where it meets the sidewalk (and will likely be about 6” above the sidewalk when the grass is cut back. There are a lot of huge roots from the tall trees above. I’m thinking that a DIY edging/stone wall fix up will probably we the way we go and if it doesn’t hold up either just maintain it or consult someone. We live in a rocky area and have a TON of extra rocks. Also I’m obsessed with those videos of the guy who cleans up overgrown areas for free for people overwhelmed with their yards. ❤️
I’ve also been obsessing over what ground cover to plant. One that is durable, evergreen and not ugly. We have some poison ivy on that side so also wanting something that doesn’t look like poison ivy so it can’t hide as well. Thank you again!

I think this is an interesting situation because of slope + straight ( and public area)
sidewalk. The sidewalk edge is one reason to consider man- made edger stones, though they don’t look as , well, natural, as various rocks & boulders. Also, to set them not encroaching on sidewalk ( much), you will have to cut into the hill, so the angle of the slope affects what size “ stone” needs to be there, whether man- made or natural. For example, gardengal showed a nice edging inspo pic, but that slope in pic looks shallower& hence could use a smaller / shorter edging to contain it.

If you set rocks, you’ll also need fairly large ones & have to cut into hill. That is “ DIY” for some people, but if hill is very compacted, tree roots, imbedded rocks ( as with some development fill dirts, or natural land) , it can be a chore. Then you will have irregular spaces as well, leaving small gaps by sidewalk & needing some backfill behind. A first decision is whether you want to change to groundcover, as you’d likely want to think of it as one big project, with site prep & edging. Or is the slope mow-able for you or hired crew. And if grass, is the grass healthy, or struggling/ weedy. You did not mention your zone/ close city, but if do, you could get Groundcover input to help think through your project. Regardless of edging rock type, you will likely have some eventual weed invasion between rocks/ pavers/ sidewalk that I typically would manage with careful spritzes of herbicide.

It doesn't need to be a vertical retaining wall. Stack some boulders up the hill.

I think someone did a great job of securing your hill. Your stone looks like it hasn't budged in decades and looks very stable except for the one big rock it looks like was purposely tipped over...continue the same stone treatment the rest of the way.

Once locked in together, rocks can be very stable even on a slope.

That you! The one big rock that moved is on the corner at the driveway, I think it gets bumped by cars periodically. 🤦🏼‍♀️

@marmiegard_z7b we live in zone 7b, and I have been indecisive about ground cover, which I do want. Mowing this angle is ridiculous. Our lawn in general is small and I actually use a push mower (no motor) for the backyard with a battery power weedwacker for edges and tough weeds. This hill greatly increases how much effort lawn maintenance takes from very little to too much energy for a slope no one ever walks on. I want something evergreen so the whole hill doesn’t look dead over the winter, won’t take over the rest of the yard and doesn’t look too shabby. Still undecided about that.

Also you are right about soil being very compacted with huge rocks and roots. Not an easy project to “just cut into” for sure.

A shovel, string trimmer, lawn mower, rake and some hard work will do wonders.

So how is your groundcover research going? What plants have you seen during the winter that you like? We (collectively!) are quite good at ID's if you have pictures of things you aren't sure what they are.

I'd recommend buying a mattock.

The ground looks fairly stable where the grass is growing over the sidewalk. Have you tried clearing that portion to expose the concrete? And does the dirt just pile up again? How much ground movement is actually happening? Hard to tell.

I agree, elbow grease and see if you can clear not just the concrete but also an extra foot for the first row of boulders at ground level.

Down where the tree is, the trunk seems stable looking enough not to worry about that area much other than clearing weeds. You want to save big bucks? Dig. Or hire a handyman and tell him what you want done.

I agree with mad_gallica, that driving around to look at groundcovers used locally is a good idea. In addition to neighborhoods, cruise some places like any local university or established office parks wher groundcover may have been installed some years ago. Also see what local nurseries, especially not big- box, have in stock in their groundcover sections ( big box stores are very seasonal whereas larger but more independent will carry the “ old standbys “ year-round in flats of small transplants).

You might be looking at Asian jasmine, or maybe spreading liriope, not aggressive if too near a desirable garden bed but may be good for sidewalk- bordered slope. I hate to mention dreaded English ivy , as it can also escape and also wi shoot runners over sidewalk pretty rapidly ( as would jasmine). Those have various drawbacks as I’m sure other folks will point out. I don’t have experience with the pachysandras. Other less common options but are the bigroot geraniums or epimediums. Depending on how much sun. I think these are lovely but are not as available, are way more expensive to do a large installation, harder to find someone who knows how to obtain and use them. There is the option of a “ mixed” installation of different plants, looks great in inspo pics but harder to execute and nurse along that a monoculture of an aggressive groundcover. These are just a few ideas to start the thinking.

How much sun does hill get ? For part sun & shady , I believe for your zone

In you zone

Cut the dirt back, lay some geotextile fabric, start placing lots of your free rocks, and reset the ones that are there please. It will look and function great and won't cost 20K.