I may not be understanding what you're trying to do, and live in the south, but I think putting soil on top of the grass would just encourage the grass to grow. You really can't have a flowerbed without getting rid of that grass, it would be a nightmare to try to maintain.

I just put a 70' flowerbed along the east side of my house, and here's how I did it. First, I used Roundup to get rid of the grass, used the lasagne method, put cardboard down, landscape soil on top, and then mulch. This has worked out really well in controlling weeds, do get some, but easy to control. Every year I put more mulch in, and it has really made a difference in our rock-hard clay, have lots of worms, and can conserve water (sort of!)

Like I say, really think I'm not understanding what you're trying to do!

I laid heavy cardboard over the grass where I wanted a new bed and covered it with bark mulch. I left it for several weeks and when I was ready to plant, I just pushed the mulch aside & used a spading fork to dig through the cardboard. I then used my garden spade to dig the planting holes. The grass wasn't completely dead under the mulch/cardboard but it was pretty easy to dig out. I planted an area roughly 18' x 24' with shrubs, peonies & assorted perennials over the next couple of years. Everything is thriving there and the peonies bloomed a year after being planted in that bed.

If you add topsoil thickly enough it will smother the grass but I can't say how much is needed. I enlarged a bed this year and didn't use cardboard and the grass is coming up at the edges where it is not as deep. In the center where the soil is over a foot deep there is little grass coming up. I think 12" of soil would smother the grass and even it if doesn't you can lay down cardboard between the plants with mulch over top as I am. Make sure you overlap the 'seams' of the cardboard very well, that's where weeds and grass will come thru if there are any spaces. It's necessary to remove grass and weeds from around planting holes or they will get into the plant roots and next year you'll be digging up every plant to remove them. I lined the planting holes with layers of newspaper when I planted in this new bed.

The leaves, especially if shredded make excellent mulch. Left whole they also work well altho take longer to decompose. I just rake all my leaves into my shade garden bed under the trees - I never have enough as most of my trees are coniferous.

I used cardboard covered with a few inches of compost and it worked great. If you don't let the cardboard dry out too much it will break down nicely by the time the grass underneath is dead.

Will the cardboard be sufficiently broken down to plant say a month after it is put down? Will planting things on top of it be ok, the cardboard might break down by the time the roots want to go through it?

I know how eager you must be to plant those pots out but I think you may save yourself lots of future headaches with weeding out grass by not rushing it. Maybe you could consider potting the plants up into bigger pots and plant in early Spring?

I've done both - dug out all the grass for new beds, mulched, then planted. And in another bed I used the lasagne method. Even where the grass was completely dug out, I find some comes back. The only time I haven't had this happen is when I did the lasagne bed, and gave it six months to break down. I now love that bed best of all as not only do I not have to weed it but the plants grow much better there than anywhere else in the garden, due to the soil being extra good for all that mulching and building up.

cardwellave - the cardboard won't deteriorate in a month's time which is why when I was ready to plant, I cut through it to get to the soil underneath. The cardboard is important because worms love it and when you have worms, you generally have healthy soil. Wherever I dig in my garden, I find big, fat, healthy worms. I don't see how you could plant things on top of the cardboard either--the roots wouldn't be able to reach the soil.

You could lay down newspaper first - it won't break down in a month, but it will be easier to cut through to plant. I love the lasagna method but it requires more patience than I have sometimes.

We successfully smothered grass, creeping Charlie, and a host of other noxious weeds 3 y ago by laying down about 12 layers of wet newsprint, 6" of shredded leaves, and then about 8" of compost on top of that. I planted seeds directly into that after making it, and made a point not to turn it over (so as not to expose weed seeds underneath) for two years. I think if you have potted material, your best shot is to make it deep enough so that you don't break the paper or cardboard barrier during planting. A month is not enough to kill off the roots, and that is what your grass will spread from if you don't roundup it. I would definitely not do what you proposed in your original post - that will just result in a ton of needless work and frustration for you.

It all depends on the quality of soil you have beneath that grass. If hardpack, the best way is the double digging method. If not, the lasagna method is fine. But you will have straggler grass to deal with. It would take all season to effectively kill off grass.

I have had great success with lasagne gardening.

The first 2 pictures are what I did. Notice the dates.

Kathy

Here is a link that might be useful: lasagne

I've done lasagna gardening before and planted right away, but it only works if you add enough layers, and plant relatively small plants. I've done it with cardboard weighed down with pieces of sod (dug up at the edges of my bed). Then, I've used dry leaves and fresh grass clippings, and a couple inches of compost on top. (I had to buy a truckload when I did a bunch of beds - I don't normally have enough finished compost at one time). I've found that having more material has made more of a difference than the amount of time I let it sit, fwiw.

If you just dump dirt on top of green grass, however, I can pretty much guarantee that it will come back with a vengeance.

You can plant and soon. What I have done with great success is to dig your hole where you want to plant (big enough around so you can amend the soil), spray around the hole with roundup, let dry well. Plant your plants, then take wet newspaper - at least 6-8 pieces thick and place up against your plants, and around the area you want to mulch etc... Wet newspaper is important, so you can mold it etc... Finish your bed with your additional layers of soil/mulch etc. This way you can plant and make a new bed. I've done just this way many times and it works well. Just let the roundup dry completely or whatever it says on the container and you will be set.

If you lay the cardboard to smother the grass (mow it as close as you can 1st) & top with enough organic matter you can plant in the layers without digging through the cardboard. If you're going to dig through cardboard you're going to have the grass grow through, so will have to use round up like others suggested.

I've made several beds this way never digging down to the hard clay & rocks just building up. Improves drainage & the plants really take off when you give them good soil.

Here's one a few months after planting last year (wide leaves of Crocosmia leaning to left)

close up 1 year later in July, 2011 - Crocosmia in bloom & some other plants removed as the growth was amazing & the bed overfull by the end of summer last year.

About the leaves in the fall - Are you saying that they will fall upon your new gardens? If you keep up with removal every few days of the larger leaves they won't smother the new plants. It is a maintenance job to keep large leaves off of small plants, so if there's a way to prevent it from happening you'll save work.