Never heard of the term berm bed before, but it sounds great. I have built and planted berms with perennials and groundcovers, but I never used a trench at its base. I would be concerned with it filling in with erosion. Typically I am not worried about grass creeping up my berm but rather my berm plantings creeping out into the yard. A healthy thick lawn can hold it's own against most on-comers though, but soon enough I will be putting in some black plastic edging.

Rose: How big of a berm are you thinking about? Below is a link with some good information on basic berms.

- Brent

Here is a link that might be useful: Building Soil Berms - SULIS

I prefer a clean line with no edging material around any garden bed, bermed or otherwise, so I'd do the trench to outline the area in question. Erosion is typically not a problem with a well-designed berm - the slope is not severe enough to encourage it and without some sort of demarkation, turf grasses will encroach if not kept back.

The SULIS website is an excellent overview on berming. There is no need to remove existing sod if you layer the area first with newspaper - a modification of a lasagne garden - as long as the berm is deep enough to allow planting without disturbing this layer until the sod and paper has decomposed.

Also, bermed or mounded beds just seem to call out for a couple of well-placed landscape boulders to anchor them and provide a sense of permanence. And they are a wonderful addition to allow suitable plants to drape or cascade over or creep around. Just be sure to "plant" them well, with a third to half in the ground so they look natural and not just flung into the planting area.

Would a berm be a method to block water that is flowing from yard to yard? I am thinking about putting in something that woould look natural to block/soak water.

Here is a link that might be useful: How to create a berm