If you remove the sod. Just rough up the soil a bit there.
They sell garden soil at walmart . Fill them up with that. No need to mix the bottom with the top.
You can plant veggies an
flowers seeds soon as frost quits.
April isnt too early for onion sets or radish or peas , carrots an parsnips.
They can go in soon as you want in april.
Also strawberries.
Annuals you can start in pots on your porch 6 to 8 weeks before the last frost.
Ihave lots of annuals started inside, but end of april is a good time to start annual
seeds on your porch.
Hope this helps
dodge