If you want your retaining walls with foliage on them to receive adequate watering, and to keep dry lifeless roots at bay, the best thing to do is to install a drip irrigation system.
This will help with the reduction of erosion, and the liability of runoff in high traffic areas. Sprinkler Warehouse is your one stop shop for all things essential to your drip irrigation.
A1-112 .95 gph 16 mm Pressure Compensating Drip Line (100 ft.)
Rain Bird XCZLF100PRF Xerigation Low Flow Control Zone Kit (1 inch)
Rain Bird ARValveKit 3/4 in. Air Relief Valve Kit
Drip Irrigation Universal Fittings
Rain Bird TDS-050-BEND 1/2 in. Galvanized Tie-Down Stake (20 Pack)
How to Install:
Helpful DIY links:
Step by Step Drip Irrigation System Installation Guide
All You Need to Know About Drip