Returning roots in flower bed
I have a flower bed near my house that keeps getting over run with roots. To build the flower bed I dug out the existing soil about 8 inches down as the area seemed to have a lot of weeds and rocks. I then filled with garden soil, peat moss, & some compost. The first year was fine. The next year when I went to re-plant some annuals I found a bunch of roots growing about an inch under the surface. I pulled out as much as I could and re-added soil. Each year they get thicker. I have tried adding mulch on top but they just grow right thru it. They never break surface and develop into any type of plant. The roots seem to be choking out anything I try to plant. The area is edged with landscaping timber to make it easier to mow. The closest tree is 8 - 9 ft from the bed. Not exactly sure what it is. I have never added chemicals to try to eradicate the roots because I did not want to hurt my plants. Any ideas of what these are and how to get rid of?