I just got a new home with a front yard that may at one point in the past have had a lawn--right now it's mostly dandelions and other weeds. I like the idea of lasagna gardening producing a nice garden plot over sod, but as I have neither an entire winter worth of time nor any compostables, I thought I'd modify that idea a bit.

Would I get good results if I just put down a layer of cardboard over the weeds and covered it with finished compost (the kind I'd get in bags at home depot)? Is 2 weeks long enough of a wait before planting in a bed I start this way?

Also, if I were to plant roses or other larger plants in a bed like this, can I just dig out a hole big enough for the roots? I'm hoping I can avoid having to dig out a large area since that would defeat the labor saving and weed suppressing aspects of this method.

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.

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.

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.

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

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 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.

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

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 =:)