This is going to sound (unintentially) kind of negative, but it seems like you are trying to treat perennial woody trees and shrubs the same way as minor tender annuals. They live different lives, the most important difference being what layer of soil they abide in.

The soil is sandy, and I'm surprised that some of the larger trees are doing as well as they are with the 3" or so of mediocre topsoil

They donâÂÂt live in that three inches. Their roots are beyond such little things! Mighty Mighty!

Do I dig the "used" mulch in with the compost? Or should I rake it up, dig in the compost, then use the old mulch to protect this compost layer ⦠Again, I'm not certain how to dig in the compost without damaging the roots

Compost and mulch are not really interchangeable concepts. The compost is in the soil and decomposed and it does nothing for trees and nothing for looks. The mulch is on the soil and not rotten yet. It protects the soil somewhat, and can help rain to soak in a little better, but is really used for looks, especially with trees. I would think composting efforts wouldnâÂÂt take into account that one would want to encourage downward root growth in woody perennials.

The term "topsoil" is pretty meaningless and can be anything the seller of "topsoil" wants it to be. The only definition of "topsoil" I can find is that it is the top 4 to 6 inches of soil from someplace and may contain organic matter.
The only way you will ever really know what is in the compost you put down is if you make it your self. The compost sold in garden centers ranges from very good to very not worthwhile. Compost is digested organic matter, that may also contain animal manure, and should smell like good rich earth, even if packed in an air tight plastic bag. The compost should be light and fluffy and just moist. I have seen some packaged compost that was quite wet apparently to increase the weight so if 10 pounds of a 40 pound bag is water you are not getting what you actually paid for.
Grasses, often, can be killed off just by covering them with an impervious material, like newspaper or cardboard, and covering that with a mulch material to hold them in place. That way the grasses which have been growing there and taking nutrients out of the soil will be putting them back into the soil.
Just put the compost or mulch on the soil and the Soil Food Web will work that into the soil for you.

I do realize that the roots of trees and shrubs go further into the soil-- I was just commenting on the fact that mine are planted in sandy, poor soil and are doing well in it. And yes, I do understand the difference between compost and mulch. The fine stuff that the builder used to pretty-up our beds and around the trees has decomposed to the point where it looks like soil and since anything organic can be called compost once it has broken down enough, I just thought that it could be added to the soil to improve the structure.

Thank you, kimmsr , for your info on compost. I've only made it before, not bought it, but I need a lot and my current compost maker is at our old house where the compost is being used on my planting beds there. Once we sell that house and move, I will bring along my bin and once again be able to make my own compost at the new house and use it. Buying compost is a one-time thing at this point.

So, since I've only ever "lightly" dug compost into my annual and perennial beds (we have lived on old farmland in PA where the soil was quite nice and I didn't feel the need to amend the soil around our many trees and shrubs), please tell me-- if I layer 2-3â of purchased compost on the empty bed, and around the perennials and trees that have already been established at the new house, do I not need to dig it into the soil to incorporate it? I can keep it on top, and it will percolate through the crappy topsoil and still amend it? Seems almost too good to believe... ;)

I do not have enough cardboard (and I don't like to buy plastic) to make 12 large circles needed for killing the existing grass under our trees and larger shrubs. I was going to use glyphosphate to do that, and then break the clumps up a bit, and add compost then mulch on top. Does that sound as though it would be okay, since gylphosphate doesn't remain in the soil, to kill the grass but leave no residual effect? Or am I missing something?

The glyphosate products are not as innocuous as the hype promoting them makes it appear. Studies by the Environmental Protection Agency have found glyphosates in soils and in water long after they were supposed to be gone. Where glyphosates are used heavily (farms) there have been found genetic modification of lower life forms such as toads and frogs, three and five legged development, indications of harm. Thus far, to my knowledge, no one is proposing studying what happens to humans exposed to glyphosates.

For the most part I agree with previous comments. I would only say trees do have roots in the top few inches of soil,lots of them. ALL the plants we grow require air in root zone to survive and there isn't much deeper down. Not only will plastic and cardboard cut off light to soil,it prevents moisture getting to soil. Sufficating grass will sufficate tree roots as well. Depending on how long the cover is in place it might not to kill trees but will undeniably harm their overall health starting day one.
Short of digging an olympic pool size hole and backfillihg with improved soil,composting has little effect on a tree over it's life,EXCEPT the first years. Because of shock from transplanting,it is wise to afford every advantage possible to new plantings.
For future planting sites I suggest tilling an area aproximating drip line of mature tree in question. If such large area seems to conterdict what I said above regarding lack of benifit to mature trees,I'm allways concurned about compactation during construction. It also alows puting turf in so roots arn't desturbed latter. If one has the equipment,chisel plowing intire yard should be considered in cases where mixer trucks and/or other heavy equipment has packed soil, You should already have soil test results in hand. Incorperate admendments as deep as tiller will reach. Set up bed retainer for at least 25% size of tiiled area. Plant annuals in bed and desired lawn and/or ground cover in remainder. Bed should be mulched minium of 3 years following placment of tree. Any time after 3+ years,retainer can be pulled allowing turf or ground cover reaching trunk if desired. If one uses a lawn service,leaving retainers will minimize weedeater damage to trunk. See if your city offers compost. That allows getting it fresh and from a likly professionally supervised operation.
Since your trees are in place,do the above EXCEPT the following difference. Till only outside area where roots have grown to (3' dia circle).

I generally add compost to my beds in the fall by spreading a couple inches of compost then spreading a couple inches of mulch on top of the compost. No digging.

When I need to regain control of my yard I've used Roundup around the trees and then put mulch down around the base of the tree once everything has died--just as the OP proposed with no issues. The occasional home usage of glyphosate has no comparison in quantity as to how it is used in commercial agriculture. Statistically, I'm probably more likely to do permeant damage to my body with an accident involving a shovel than with the tiny amounts of chemicals I use. To each his own.

For a host of reasons, I was unable to empty my compost bins this spring, and just now finished spreading everything in them all around the garden, and started a new batch of compost. I put the compost down as a top dressing, over deteriorating pine bark mulch on the perennial beds. I'll put down more pine bark when I get my firewood delivered and clean up after that. I also spread the rest of the compost out on the now-empty veggie garden, and will mulch on top of that with leaves and grass clippings when I clean up the yard for the winter after the leaves fall.

IMHO, its all good. The worms will incorporate both the deteriorating mulch as well as surface compost for you.

As for using Roundup to kill a bit of grass around trees before laying down mulch. I garden on 3 acres, composed of 1/3 cheat grass, 1/3 bind weed, and 1/3 brome grass. I've used it now and again.

For some reason, I haven't been informed about the most recent posts. THANK YOU all for your advice. It's really difficult to have 2 houses and trying to make both look their best while also dealing with everything else and also my aching back... it's a bit crazy, really, wanting the landscaping around my new home to look great (since most of the neighbors use landscape services) while also needing the landscaping around our "old" house to look nice enough for potential buyers. Not to mention the fall maintenance issues on both homes... oy... ;) I do appreciate your input, very much.

Hello Food Fiend.

It will get better :)

Chuck

Thanks, Chuck-- I would certainly like to believe that!