Prior to spring planting, many folks like to warm their soil to avoid seeds rotting or faltering, attacked by fungus active in chilly soil. Expert after expert tells gardeners to warm soil by placing black plastic over it, but they are leaving something out.
Black plastic absorbs heat for itself but wont transmit that heat to the soil unless its firmly fastened to the earth over its whole expanse. The main advantage of using black plastic is that it bars weed germination. Clear plastic is much more effective in warming soil even without touching it, but it does allow weed germination.
If you have raised beds, your soil will warm sooner than soil at ground level, not because its a little closer to the sun, but because it doesnt have the mass of cool earth surrounding it.
Expanding or creating a new garden bed can be done either by lasagna gardening or by stripping the sod, then adding amendments to the soil left where the sod had been rooted.
Lasagna gardening is easier than stripping sod, and its really not too late.
As soon as possible, though, put down corrugated cardboard or 10 plies of newspaper, overlapping to eliminate gaps, over the area where you want your new garden bed. Top that with a layer of fallen leaves, then a layer of compost and top that with a layer of soil.
You could hold the leaves down with soil, and top that with compost. Whatever herbicide-free organic matter you have will work for a layer.
If you remove the sod, dont discard it because its good earth as soon as the grass and roots decay.
To prevent regrowth, lay sod pieces upside down. You could use them as a layer in your lasagna garden, but until grass and roots decay they will use nitrogen; after they decay theyll provide nitrogen. If you intend to plant that area early in spring, add some nitrogen to the layer of upturned sod.
Some folks cut sod into 3-by-3-inch squares and plant their garden seed in the decaying roots. Theyll hold the square together through several waterings, until its time to transplant outdoors.
Some folks advocate double digging a new bed or one that needs rejuvenation.
What this means is digging out a shovel-deep trench, setting that topsoil aside in a wheelbarrow or on a tarp, then digging down another shovel depth, adding compost and churning that soil with the shovel, then start another trench, putting the topsoil removed on the now-improved and aerated subsoil in the first trench.
Its hard work, but it improves drainage, facilitates penetration by deep-rooted vegetables and increases oxygen availability to roots.
On the minus side, it breaks up mycorrhizae and disrupts or kills microbes, earthworms and beetles in the soil.