I would be in favor of removing any really invasive roots like quack grass or Canada Thistle. Even then you can put all those roots in a pile to rot and later add back to the soil.

Other sod I think I would want to invert and cover about 7 inches deep.

Many grasses will regenerate from a segment of root left in the soil, so his may be the more conservative approach. If you are being careful about burying the sod as deep as possible, it may not be able to reach sunlight before its resources are exhausted.

Due to a problem with his septic tank a neighbor had to dig it up to work on it and when he covered it up again he put Quack Grass sod at the tanks level (due to several factors that tank was 10 feet down). Some years later his repair failed and he had to dig down again and he found that the Quack Grass roots were growing back toward the surface.
1. Determine whether the roots will grow new plants.
2. If they will do not bury them.
3. If they will do not save them for later use as organic matter.
Someone, many years ago, told me that Quack Grass roots put into a sealed black plastic bag and left in the sun would be cooked and then they could be composted. Before the end of that, very hot, summer those roots had punched open the black plastic bag and started growing.

Well, it should be no surprise to me that DH takes the more conservative and cautious approach. I may be following him in this instance as I am usually the one to do the weeding. I am not certain if I have seen quack grass around here but in this case, it will be easier to be safe. Thus far, I have only seen one thistle plant out here and it was on a neighboring acreage, not mine. I do have far too much sagebrush to compensate.

One thistle can make for a HUGE weed problem if the seeds start blowing around. Sneak over there and kill that thing.

Good for you to be double digging, by the way. Very few people seem willing to put in the time and effort - hence the popularity of "lasagna" gardening - but I think the results are worth it, in terms of deep fertility and creating good tilth. You will find it easier to weed, too.

Lasagna gardening does look attractive, but, I live in a high wind area. I could just see me piling up layers and all of my hard work blowing to the neighbors. I am also impatient. I understand that lasagna gardening works but it takes some time for it to work it's way down. The problem besides the time too, is that I want water (a very precious thing in the SW)to penetrate, which it certainly does quicker with double dug beds. Also, at around 18 inches, I tend to find some good sized (cobbles or larger) rocks, I like being able to get them out.

I will put the thistle on the hit list, if it sprouts this year. Fortunately, that lot is not built on so I will go over with a big shovel (in case of rattlers) and some herbicide!

Knowing how far to dig or not to dig is a question I have questioned myself on over and over. I have some beds where I just put new soil on top as the top soil was only maybe an inch deep and other I went to the extent of taking a backhoe to just breaking up the clay hard pan below. Several years latter I see no difference as far as plant growth is concerned.
Now when putting in a new bed I will take a fork and turn over the top 4 to 6 inches and remove all weed roots. I amended the soil with compost. From then on everything goes on top leaving mother nature with her microbes and worms to do the job.
The decission to dig deep or shallow will often be determined by how deep the top soil is. I have found that turning over the hard pan is better left to nature and worms. Planting deep rooted plants and cover crops helps.
This year with all the rain after drought seem to inspire the thistle seed to germinate. I believe all the seed that hit the ground for the past hundred years has sprouted, along with wild oats and rye.

I cannot afford to buy soil or trucks full of manure. I must sift my own dirt. In many cases I, literally, sift the topsoil and even subsoil through a galvanized steal screen. I live in a rural area with many native invasive species of weed and grass. I cannot always go to this extreme but every time I do it is worth it. It is always best to err on the cautious side concerning weeds. As mentioned above many are so invasive it is near impossible to kill it. Likely, the darned things would survive a nuclear holocaust. Creeping charlie comes to mind...

I have to admit, I may be taking a slightly easier approach to the double digging. In one book I was reading this weekend, it said for double digging to dig down one shovel's depth, then loosen the bottom layer with a garden fork and mix compost with that. Well, for my lower layer I have to use the pick but that is probably what I will do which will hopefully leave me with enough energy to go ahead and sieve out all the roots. Lord willing, I will get all of the vegetable beds down this year so it will just be topping off with compost after this.

When we started our "share crop" gardening we double dug down about four feet, layered in horse manure, then filled the bed with the dirt from the top layer. The next year we built wood framed raised beds, breaking up the ground layer with a garden fork, layering in sheep manure, coffee grounds and straw. Last year, we were tired of all the work of those other methods, grabbed a couple of waddles from the floods we had around here. We tacked them into the ground, layered cardboard, some of the remaining sheep manure, semi-finished compost and a layer of straw, to sit over the winter. About a week ago I put the shovel to it and the soil was loose to well below the original layer of cardboard and full of worms.

Double digging works, but it is work. If you want to do it I say go for it, but you could just as easily go the lazy way - lasagna is lazy - and let the beds get worked over by the worms over the winter.

To a degree how much good double digging does depends on how deep ones good soil is.
I did a whole garden once, and small areas since then but my good soil goes down at least eighteen inches.

I see my post from a few years ago. Let me tell ya.. With bermuda roots, johnson grass roots and field bind weed I'm sick of double digging. I still double dig, but selectively. Having the soil constantly covered with mulch .. a deep mulch ... helps by keeping the soil moist on a regular basis and when I need to weed, they pull much easier. But their growth never stops. Only until I plant something which is allelopathic to the weed roots - like potatoes - will their growth ever be sufficiently stymied to stop excessive growth. Only pulling and weeding are effective, in general. I'll be glad when this area of my garden is somewhat weed-free. It's a lot of work !

...something which is allelopathic to the weed roots - like potatoes ...

That's an interesting idea. I've not come across references to potatoes being allelopathic. Sweet potatoes, yes, but not potatoes. Can you elucidate?

Since unwanted plants, what many call "weeds", throw seeds out, attach them to animals and us, and allow the wind to blow the seeds around no garden will ever be "weed" free. That is a delusion if one thinks it can be. I have had potatoes, both types, have roots from Quack Grass and Johnson grass grow through them so I would doubt they have any allelopathic properties. I have seen some tracts that appear to indicate the sweet potatoes may have some allelopathic properties toward a few other crops.

kimmq is kimmsr