Heavy plastic and products such as BioBarrier which combines a physical barrier with a slow release chemical that inhibits root growth should work for some time if installed very carefully. That means a lot of digging, removal of any stones that may cause tears, not even a pinhole size puncture, close attention to the surface where the barrier overlaps the existing soil. Even if done perfectly, I would not bet against plants with ultra aggressive root systems such as Norway maple or many species of bamboo. These will attempt to exploit the tiniest opening and try to go over, under and around any barrier. Things may be fine for 5 years or 10 years, but then... Special in-ground pots for bamboo used to be sold. They were made from heavy plastic and had an internal shape that allegedly kept the bamboo confined. How well they worked or if they are still available I don't know, but it might be worth checking.

Thank you Embothrium! This is not a forest region - just a single tree in the middle of my yard. I planted an elderberry on the other side of the stump at about the same distance this spring, and it seems to be doing fine (although I didn't come across any roots of this size when planting that one). The weeds seem to like the area as well. :) Is honey fungus a problem outside of thickly forested areas?

welcome wanda.. perhaps you should have started your own new post ... then at least.. replies could be set to come to your email... anyway .... im not rereading all the above.. but the first thing that pops into my mind.. is that a mature tree has put it roots at the surface.. because that is favorable to the tree .... adding a little soil above those roots.. is only going to encourage those roots to resurface again ... though i tree time.. it will be a very long time before you see big roots there ... but you might start seeing fine feeders roots.. faster than you might imagine.. a year or two in tree time .... and every time you dig a hole below the new soil ... and into those roots.. the tree will respond by putting new feeder roots into the new and fresh soil ... not to mention those roots cut due to the garage construction ... should you try it.. of course ...perhaps it will work ...and even if in a few years.. the plants seem to be struggling.. you can always move them somewhere else ... hosta are water hogs.. and if you ever start noticing.. them wilting... and you just cant seem to water enough ... then you have tree root problems... but there are many other things that will grow there.. [and dont stain that brick works with ugly water ... the plants arent worth it...] ken btw: i would make the bed as wide as the brick work.... i think it would add some symmetry ... it would look pleasing to the eye ... rather than a 2 or 3 foot ribbon of soil ...

I am removing these roots. Almost every resource says to just make sure to plant a few feet away, and many testimonies say their new tree is doing fine years later, but even 8ft away I am hitting like 3x3 fresh lumber I can make furniture with lol. Ultimately I think this is a unique scenario because I checked youtube vids like below and although some of those trunks are much smaller than the Silver Maple, the ratio of roots-to-trunk size seems way different as if this Silver Maple made clusters of much thicker and longer roots for its size - I think it's because the sandy/clay grading soil when they built the development makes things root shallow in this location. This is how a lot of developments are built, they use cheap sandy/clay stuff for the main grading and then only about 18" of good soil on top for the grass and trees, and also I've heard this whole side of town was basically a swap or something once upon a time, google maps even shows a blue line (river) along this fence line but there's never been a river, maybe an underground spring or something hence the map's river line starts and ends with no body of water and I've replaced fence posts here when it hasn't rained and basically hit an underground river (the wood underground was solid because no oxygen is why fence posts usually rot at the base only). I removed the stumps and roots of a row of blue point junipers that were chopped down like 7 years prior and they were still solid. If the roots look like the below vids, then yes go ahead and plant close and don't even worry about that little amount of roots in fact yes it'll just add organic matter I'd prefer to leave some roots if it's sandy/clay which is what I'm doing adding small roots back into the mix and mixing the sandy sub soil with the dark rich compost topsoil. Green giants are well-rooted and unlikley to tip over from windthrow, and they might root deeper into the sandy/clay stuff just below the old maple roots, but bottom line is there's too much big roots in the way that I know it'll be better without them, I can't even dig a 1ft wide hole for a new tree without hitting something decent. Some trees cost too much to risk it IMHO at least get some of the old roots out, setting up irrigation, transporting the trees, fertilizing etc better to not risk it if the conditions and old roots are like this. Another thing is if a previous tree/roots had disease, it could enter the new tree, but as long as there's not a ton of old root mass, then maybe it's not a big deal and the soil microbes will correct it, but just wanted to note that about planting near an old stump. And well, you can't just make quick work of roots with a chainsaw because it'll dull in 5 seconds and you'll spend more time sharpening than cutting (although I might sacrifice a chain for this and test my dremel chainsaw bits for once because usually I just hand file), but it's not too too bad removing roots. It rained hard and turned my project into a pool that I bucket'd mostly out and then filled the finished areas back with soil and it was like working on quicksand but it's getting there and the farther away from the stump the easier it becomes. Sawsall pruning blades dull really fast too and I resharpen them on a bench grinder but it's still takes a while. I've been picking at them with an axe, splitting wedges etc, rain-soaked compost topsoil makes it a lot heavier and everything slippery, been going at it with a jack hammer with a sharpened chisel bit does decently. This Silver Maple stump was like 3ft wide at the base. I just removed a different maple's stump and roots from the sidewalk grass strip area to plant new sweetbay magnolia and that was about a 1ft maple stump but the ratio of those roots to the large silver maple doesn't correlate, I was able to rather easily remove the whole stump and what seems like the only main roots it put out due to being confined between the sidewalk and street compared to the large Silver Maple really put tons of big roots out. sorry so long, Another things though is Green Giants are an extremely QUICK growing tree so I would still prefer not to have even medium sized roots near them. Tree services should offer the option to grind the roots too if they're this big. The Silver maple roots I'm removing are much bigger for the trunk-to-roots ratio than these trees it seems likely because of the site conditions https://youtu.be/N3RUifC7Uo8?t=310 https://youtu.be/Fjg7ieBY7Lk?t=434 https://youtu.be/bRTIwWUg4fc?t=152 https://youtu.be/vM24klVyxFk?t=477