Hi MaryAnn, I had the same thought, remembering we learned they found a couple of pavers about 10' from the remains - actually I thought it said 15', but that is pretty close.
MADJGNLAW's post got me thinking {thanks for that wonderful post MAD ;}. and I could easily envision plant seeds being attached or clinging to concrete. Caught in a spider web as you would find on the underside of the paver. Here in the NW if one lifted up a paver that had been there for awhile, you would see lots of bug activity and stuff attached to the concrete which is very porous.
I think there was a wealth of evidence taken from the crime disposal site that matched up to the A's place, and their taking flash bulb photos in the yard immediately after her remains were found says they knew right away something matched from that yard and they knew what it was.The possible movement of seeds with the accoutrements to the remains findings was just a big bonus imo.
ETA:
A bit OT for this thread, but I think the spot where they found the pavers is probably closer to the original spot the bag was put, and the bag traveled with the movement of water to a spot between 10 to 15 feet away from there.
But any seeds that were on the pavers could have easily dropped off into the bag and sprouted, to grow thru the bones, like the rib cage. Plants will grow with very little light. They just aren't green, but pale yellow.