The photograph in the banner above doesn't really look like a retaining wall, does it? Well it is, simply because it forms a physical boundry between two levels of earth. In this case the bottom of the wall is at sidewalk level while the top is at the level of the lawn. The difference is only about 3 feet. At some point someone decided to allow plants to flourish amongst the rocks, which is often seen here in Seattle.
If you walk up close to the boulders, you will notice that they are almost always dark gray, and usually jagged. I'm quite sure, but not positive, that the rock is basalt. Unless someone goes to the trouble of fitting the boulders into space filling shapes, like cubes or parallelepipeds, the face of the wall will have gaps. At this point one can either fill in the gaps with mortar or leave them be. If one chooses to leave them be, plants will naturally start growing in the gaps. Unfortunately, aggressive and invasive plant species like Himalayan blackberry often take over unless the plants are attended to.
From a structural perspective, large stone walls are effective at resisting the hydraulic forces that earth, especially wet earth, can apply simply because of their huge mass. Consider that the average residential concrete slab wall might be just eight inches thick. Compare that with the thickness of a boulder, which might be two feet thick! Unless the gaps between the boulders are filled in, water can flow out from behind the wall during a heavy rainfall and relieve some of the pressure applied to the wall. Contrast that to a concrete slab, which retains the rainwater behind it, unless drains have been specially installed at the base of the wall.
The photograph below shows a more robust retaining wall, and one which is by and large free of weeds. I have no idea if the property owner goes to the trouble of periodically weeding the wall, or simply uses herbicides.
The stone wall show below is the nicest one I've yet found in Seattle. In front is obviously a vintage civil cast concrete wall, but behind that is the only stone wall I've yet found where the stones have actually been cut and fit to minimize gaps. This helps resist collapse, as there is more stone-on-stone contact, and it naturally resists weed infestation. Plus it looks very professional. Bravo!
