Here is a 2020 LSU field day research video on planting orientation of slips. Flat planting allows slips to root at three or more nodes and gives the roots room to develop evenly and increases production. A couple of screenshots showing both ways and link below. I am going to try it on 400 slips I am planting in June.
I contacted Dr. Villordon at LSU this morning to answer some questions I had. Horizontal slips need to be over 12 inches in length so you can get 3 to 4 nodes in the ground. Plant 2 to 3 inches deep and be sure to have irrigation ready as you may be marginal on moisture with shallow planting.
He suggested I try both ways and see what works for me. I doubt the slips I order will be long enough. Probably end up trying it on the variety that I will be producing my own slips with.
