As stated, the Muhlenbeckia are both rampant growers and will tend to overwhelm whatever is nearby and slower growing. The Lotus maculatus is not a particularly long lived plant unless it has perfect drainage and is in a coastal influenced area that stays mild rather than extremes of heat and cold, and generally tends to die after the second year, or decline and need to be replaced. Lysimmachia also tends to be winter deciduous, so is not a year round evergreen, but something that only looks its best in the warmer months, even in USDA zone 10 conditions. For cascading plants that would like the wet feet that you imply from the lawn runoff comment, I would be looking at plants that like constant moisture. Rosemary would be a poor choice for a wet soil site. Myoporum parvifolium 'Dwarf Pink Form' would do well here, and Rubus calycinoides would also love the situation. For a flowering cascading shrub of a scale similar to the Lotus maculatus, you might consider using Russelia equisetiformus. Pelargonium peltatum would also do well here, as long as the soil was also sufficiently porous that it didn't stay constantly wet, but also allowed air to penetrate. Lotus maculatus does not like to stay too wet, and will quickly get root rots if the roots don't also have good drainage.

Wow many fantastic and interesting suggestions and great pictures from scottyboipdx and herblady49,..just this year i added two low walls to the garden collected some old socks i had and filled them with soil then added Campanula then stuck them in between the rocks,..some are already blooming but by next year they should have thrived. Philip

Northern Lights Azaleas are hardy to Zone 3. The whole series is amazing, and definitely worth looking into. I have three of them: Mandarin Lights, Northern Hi-Lights and Tri-Lights. They are very floriferous and fragrant too. They are in different, but all difficult points in the garden. I have had them now for five years now, and the Mandarin azalea (the one on the northern slope)is the biggest- about 1.2 meter high and likewise in diameter. Regarding Hydrangea, I will probably go for Endless Summer. It is also blue, it's hardy to zone 4 and blooms for the whole summer supposedly. Yes, irises don't like it in the shade, regardless what the experts say. Thanks for the daylilies idea- I think that they will look good there. I already have the euonymous and vinca there, some hostas, clematis (three types)out of which Guernsey Cream is the most amazing in that shade, because it is big and white. I might try Astilbe. Thanks!

Old standbys for the top of a retaining wall are Creeping Phlox, Basket of Gold (I think it's now called Aurinia saxitalia) which is pictured here with variegated Wintercreeper. I don't recommend the Wintercreeper however, it is invasive and requires a huge amount of pruning to keep it in its bounds. You might also consider one of the low creeping junipers (eg Blue Rug) which we use on the long stretches of retaining walls we have on our property. They are an excellent groundcover and require very little maintenance other than an occasional haircut when they reach out to grab unsuspecting passers-by :-) Other plants that like the excellent drainage of stone walls are Dianthus and Snow in Summer.