Lawn needs major help
When we moved into our new home 12 years ago, the builder had not planted any grass in the back yard - there was muddy red clay between scattered ugly weeds. (Had been farmland prior.) We spread mushroom soil (~4" thick) and planted turf-type fescue seed in that. The grass that has grown there is so fantastic that weeds can't even get a foothold among the densely packed blades of grass! However, we initially didn't do anything with the back half of the back yard, since there was enough vegetation there to cover up the mud. We have since tried several times to improve that area, which is 90-95% weeds (all kinds). We even went as far as killing it with Roundup, bringing in top soil, and re-seeding. Still >90% weeds! Wondering if we can just spread mushroom soil back there and plant seed in that, like we did up next to the house. Questions: how deep would the mushroom soil have to be to smother/kill the weeds? Or would this not even work, since the weed seeds would still be down there? If it could possibly work, should we lime/fertilize the existing soil first, according to soil test recommendations? Or would that be a waste, since we'd be putting down a new top layer?
It depends whether the weeds are perennial (dandelions, clover, etc.) or annual (crabgrass, chickweed). Annuals could be covered with 2 inches of mushroom compost and smothered. Perennials would still come back through the compost. I suggest, killing the existing vegetation (Round Up is probably best option) and then adding the mushroom compost (2-4" layer). Correct the pH of the existing soil first because you would like your grass roots to grow down 6-8 inches, so they will be in contact with that soil. You might also have a compaction problem, so it would make sense to core aerate once the mushroom compost has been added. That will help mix the soils a bit. The soil quality really matters! Good luck.