Last year I dug up an Asian Honeysuckle bush (invasive species) and my husband helped me put a raised garden bed in it's place. Here's what I did, and it worked great - I tilled the soil before I put in the frame of the garden bed. I had a lot of weeds in the area so I wanted to make sure I got rid of as many as I could before I put in the garden. Then my husband helped me put in the frame of the garden bed (4x8). Finally, we filled the garden bed with soil my neighbor had delivered from the city. I don't know where the city gets the soil, but I know it probably comes from construction sites. All I can tell you is it takes a lot of five gallon pales full of dirt to fill a 32 square foot garden bed.

Once I had the bed filled with soil, I planted all seedlings in the bed. This year, my mint, lemon balm, lavender, and loveage all popped out of the ground by the end of March. Once I fill my garden bed with fresh soil (some of the soil washed away over the winter), I will planted chamomile, broccoli, and cauliflower.

As a tip, I would consider putting up a little fence around the raised bed. We tried to do without a fence last year, and the rabbits kept chewing my plants. Then we out up a little green fence, and the rabbits were no longer a problem.

Here are some photos of my raised garden bed:

http://www.flickr.com/photos/veggiefaery/show/with/3746387009/

http://www.flickr.com/photos/veggiefaery/show/with/3746387009/

http://www.flickr.com/photos/veggiefaery/3747176442/

(the above photo shows the green fence we put in)

You CAN do it that way, especially if you like time-consuming manual labor, but...

The easier way is to lay down a few layers of overlapping cardboard where you're putting your beds on the lawn. I would let the cardboard extend 10-12" beyond all edges of the bed. Then set the bed on top of it, and fill it with your soil and any nutrients. Plant.

The cardboard will shut off light to the lawn and kill it. The lawn has nutrients in it, so removing it would be removing useful nutrients. Put some kind of mulch, like chipped bark, on the exposed cardboard around the edges. This 'apron' helps prevent grasses from growing from outside the bed into the box, where they are a nuisance to get out.

By the time any of your crop roots hit the cardboard, it will be soft and starting to decompose -- the roots won't have any trouble working their way through it. You've seen cardboard when it's been wet for a week or two, it falls apart. Soil creatures will break down the cardboard and incorporate it into the soil, and it will disappear.

Lots quicker and easier than digging out the lawn, right?

Sue

I'll just add that I had no lawn to dig up - the grass had all been killed off by the Asian honeysuckle bush. What I did have was a ton of creeping Charlie. I dug all the creeping Charlie up in hopes of doing a little weed control throughout my yard.

Yes, you will be able to plant in the spring.

You are probably going to receive a lot of different answers to your question as there are many options that will work for you. If you are ok with the work, removing the sod and tilling the soil under your boxes will allow your plants roots to penetrate deeper into the native soil.

However, another option would be to layer cardboard as Sue (belgianpup) suggested and just place your wooden box on top and add your soil. The cardboard will block the grass and weeds from growing and break down over time allowing your plants to access the nutrients of the soil beneath the box in time. This is what I have done with my Square Foot Gardens with great results.

I garden both with traditional in-ground gardens and Square Foot Gardening (SFG) using MelÂs Mix which is a mixture of peat moss, compost from 5 different sources, and vermiculite. Others also have great results with just compost in their boxes.

There is a Square Foot Gardening forum right here on GardenWeb that can provide you with more info.

Rachel
GrafixMuse's Garden Spot

Here is a link that might be useful: Official Square Foot Gardening website

I used straw and compost last year, right on top the grass, to make my beds. Then immediately planted. Stuff grew without any problem. When I first started gardening, I tried removing the grass, and double digging. Way too much work for me.