Winter weeds are a cheap way of improving and adding organic matter to soil. It is costly and difficult to sow a cover crop on the tree row, so let winter weeds do the job for you.
Most weeds support beneficial arbuscular mycorrhizal (AM) fungi, as do most other flowering plants. The hyphae (filaments) of these AM fungi grow in the soil and in the root. In return, the plant supplies the fungi with organic carbon from photosynthesis, all continually mixing organic matter with soil without cultivation.
The hyphae and roots hold particles of sand, silt, and clay together into stable soil aggregates. To see this, pull a clump of weeds out of moist soil, where you will see aggregates of soil clinging to the hyphae and roots (Figure 1).
The hyphae also help the root to take up nutrients and water.