I have a small border between a shingled parking area and the top of a steep bank of soil (covered with old grasses now). I planted it all at the same time, with the same plants from right to left: Aptenia cordifolia cuttings and Nerium oleander as small potted plants. At one end, both types of plant are doing very well (dark green, spreading and growing well), in the middle they are not so good (leaves a little yellow, less growth), and at the other end, they leaves are very yellow on the Aptenias and growth stunted.
Irrigation is similar throughout.
All the soil was put in place at the same time, raising the level somewhat (new soil deeper at the good end).
The good end also gets a little shade from a small wall.
The poor end is closer to a Myoporum laetum shrub and cypress trees (Cupressus sempervirens var. sempervirens; not fastigiate) (allelopathy issue?)
Any thoughts?