An inexpensive solution for large areas is to buy treated 2x1" battens (or treat them yourself) fix these vertically to the wall at 600- 1200 centres with masonry nails(depending on plant density etc). Then fix light gauge galvanised tying wire at 300c/cs vertically, fixing with small fencing staples at each batten. Trail plants through the wires, tying with string as required, once plants become established the trellis becomes almost invisible. This is a very cost effective solution for large areas of wall and the trellis looks quite "architectural" and even when bare takes the grey look off a concrete wall.