"The soil underneath looks quite healthy and is full of worms."

In most cases the worms are a good sign, indicative of OM in the soil.
Apart from possible pH adjustment(s), sounds like you're good to go.

The mulch strategy is OK, no need to wait two weeks.

Don't be surprised by the persistence of weeds though.

I'm not sure why so many people think you need several months after building up a Lasagna bed to plant because as Pat Lanza states they planted in that bed right after they built it.
If you think your soil looks quite healthy and is full of earthworms it may be. What does the soil smell like? How much organic matter is in that soil? How well does that soil drain? How well does that soil retain moisture? What is the pH of that soil? Are the nutrients in balance?
A good reliable soil test would be a good idea and these simple soil tests can help answer those other questions.

1. Soil test for organic matter. From that soil sample put enough of the rest to make a 4 inch level in a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and replace the lid, tightly. Shake the jar vigorously and then let it stand for 24 hours. Your soil will settle out according to soil particle size and weight. For example, a good loam will have about 1-3/4 inch (about 45%) of sand on the bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above that, and about 1/4 inch (about 5%) of organic matter on the top.

2) Drainage. Dig a hole 1 foot square and 1 foot deep and fill that with water. After that water drains away refill the hole with more water and time how long it takes that to drain away. Anything less than 2 hours and your soil drains� too quickly and needs more organic matter to slow that drainage down. Anything over 6 hours and the soil drains too slowly and needs lots of organic matter to speed it up.

3) Tilth. Take a handful of your slightly damp soil and squeeze it tightly. When the pressure is released the soil should hold together in that clump, but when poked with a finger that clump should fall apart.

4) Smell. What does your soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant odor? The more organic matter in your soil the more active the soil bacteria will be and the nicer your soil will smell.

5) Life. How many earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5, according to the Natural Resources Conservation Service, indicates a soil that is not healthy.

Here is a link that might be useful: Lasagna Gardening 101

I think it's a mistake to think that in all cases you must Do Something, and not only that, but something drastic, to make your soil usable. It depends on where you live, and the specifics of your yard, but some soils are actually pretty decent to start with.

As others have said, it sounds like your baseline soil is pretty good. I'd start from a perspective of maintenance and general improvement, rather than feeling like you need to create something usable before you even start.

I'm less worried about my soil quality than I am about the weeds. I was hoping that by waiting 2 weeks, they would get nicely smothered, but perhaps I am being overly optimistic. My other options, as I see them, are to yank them all out by hand or rent a tiller, but then I'd still want to lay down some mulch to prevent new weeds. I'd prefer not to use roundup or other herbicides in my garden.

Why not just plant in the ground and use a very thick layer of mulch to smother weeds? If weeds appear, cover with more mulch.

An old-fashioned spading of the sod and/or weed cover isn't so difficult for a small area and isn't a bad thing to do once.

Make sure the spade is good and sharp, with a file.

Go forward with your plan. Waiting a couple weeks will probably help kill a few weeds before you start planting, so if you can afford to wait, do it. Otherwise it won't hurt to plant right away.

I did almost the exact same thing for most of my beds. Over solid lawn, I put down either cardboard or paper bags and on top of that whatever mulchy stuff I had around - wood chips, half composted leavaes, or even lawn clippings. I had a few bits of grass coming up here and there, but they are few and so easy to pull. Overall, it's so much easier and cheaper to do it this way than renting a tiller or digging up all your sod.

I have many times made a new garden bed by laying cardboard over sod, framing in the raised bed with whatever edging I happen to be using, and filling with finished compost.

I have planted the same day and had fabulous results. Only problem is it is hard to plant tomato plants deep because once you hit the cardboard you cannot really go any deeper without cutting the cardboard, but I just set the roots on the cardboard and planted as deep as I could.

When I have had to plant something deeper than the cardboard (in this case potted blueberries) I used a box cutter to cut a hole in the cardboard, and then dug down into the sod. You get a few more weeds creeping up through the compost along the edge of the cardboard but not too much.

I don't worry about amending the soil under the cardboard but I have excellent soil everywhere in my yard.

I would spend the extra time and effort to turn over the soil by hand - spade or fork - removing the dandelion roots and other weed roots. This will make garden maintenance a much easier task. After that, dig in the compost and make your beds.

If you are inveterately lazy, them the lasagna method is a better choice.

Merilia

No one has answered your question yet about planting roses. Please research the proper planting method. If you dig a hole just big enough for the roots, you're likely to have the poor roots going around in a circle instead of spreading out, and your rose bush will die.

BTW How big an area are you doing? When I used the lasagna method, I hand dug the weeds first. I just thought it was more likely they would never come back.

Good luck =:)

Merilia, thanks for posting this question. The answers should be good news to all of us who are interested in starting a spring lasagna bed and planting it right away. For me, here in St. Paul, "right away" means a good month or more from now. By that time I hope to have several layers working and will be keen on seeing how the garden (8' x 8') fares this season. Good luck to all trying this. Gary