This website is “live”, which means that all results that I get in the garden will be reported as I discover them. So this update is presented as a kind of warning about using cardboard if you have soil that is clayey.
Cardboard would possibly be ok to use if you have a soil that is sandy and well-drained. But the soil I’m gardening on is more clay based. What this means in practice is that any flat material (like cardboard) put on the soil surface will attract slugs - lots of slugs! (In case you just beginning gardening, slugs will eat your tender plants - in fact they can wipe out all your seedlings)
So the way to convert a weedy lawn to a veg bed is to just put more compost on it. This is much, much simpler than having to find a big piece of cardboard to lay on the ground first. The original thought about supressing weeds or underlying grass is achieved by using a thicker layer of compost. This will supress all but the most pernicious weeds and any that still poke through can be nipped of just under the compost surface. This could mean having a deeper wooden border by using thicker pieces of wood round your bed.
The thicker layer of compost will of course be even better for the plants you are putting down, especially the ones that have a deeper root system. The more abundant nutrients in the compost will make the plant stronger and more resistant to pest attacks too. And also the larger volume of growing medium will last out the growing plant till it start to sink its root into the original soil.
Here is a YouTube video of Charles Dowding doing what I have just suggested.
I am trying out a ways of enriching the soil still further and reducing the impact of slugs on vegetable plants. This is in the early stages yet and the results will be reported on this site soon. This does involve you finding a source of clean hay and horticultural grit. More on this later!