The following is my opinion on your question so don't take it personally. The narrow trench method is a cheap trick that's ugly and serves only one purpose. It makes life easier for the landscaper. I literally hate that look. I prefer colored cement edging with straight and curved sections, old bricks, or the green steel edging. The cement edging is easy to lay and maintain and will look the same forever. My wife and I built three flower beds last week an average of about 35 feet long with the cement edging and it looks great. It was easy and averaged about $1.20 per foot with edging from Wal-Mart. As far as maintenance....That's the key word...maintain...Either you maintain it or ignore it for a few months and come back to repair the grass root invasion. The trench method gives landscapers a wider window of time to avoid work. Why don't they spray Roundup about 2 feet wide and never have to edge?

I do trenching of my flower beds but only appx. 4" (never really measured)so I don't turn an ankle accidently and find it attractive, neat looking and easier to maintain that nice edge. However, you'll still have grass from the lawn grow OVER the trench that has to be trimmed. Which brings me to this question which I had posted in the "Tips and Techniques" forum but got no feedback yet:

Any comments on these grass shears, to be used as edge shears for flower beds in lawn areas? My beds are trenched and I've always looked for a tool to clip the grass that grows across the trench. Check this tool out at the link below, scroll down to "Vertical Blade Lawn Shears 39" (P75)" and tell me if it's worth $66. Currently, I'm using some vertical hand held grass shears I bought at Lowes the other day for $9.99. Just thought the long handles would be a plus. Bacho is suppose to be a great brand,too. No, I'm not spamming, honest!

http://www.bahcostore.com/grass-shears.jsp

hpny2 - you will also have to still dig the trench periodically as the dirt tries to reclaim it's space so to speak. As rdaystrom says, "maintain" is the key word.

If you want the little trench, the technique is simple. Take a straight shovel or half moon edger. Cut straight down into the soil at the outside part of the trench. Then, from inside the bed area, start several inches on the inside of your first cut and make an angle cut going outward towards the original cut. You should get a triangular clump to pop out. That should leave you with a sharp edge that you can maintain with a string trimmer or similar and then a gradual slope up to the bed to minimize the erosion that will eventually fill your trench.

I've used trench edging for years. It's easy to do...just some masons string for the straight lines and a spade. For curves an old hose or some spray paint to mark the edge.

It gives the wood chip mulch I use a space to fall into and I think it is a more natural and "squared away" look.

It's also the frugal gardener's method...

While concrete or brick edging is preferred by those with deeper pockets than mine...it is a nice look. However, the concrete/brick creates a "hot-dry spot" near the edge of the bed just like a sidewalk would do.

Here in "flyover country" of central Iowa the concrete/brick edges can frost heave during the winter and then you have a problem with unevenness.

Plastic and metal edging is just butt ugly... period.

Like Iowa50126 said:

Clean and natural looking.

Cheap.

You don't get the heat buildup - very important here in the south.

Plastic and metal are hideous. Plastic fades and/or cracks. The paint on the good steel edging will eventually flake off, and the steel will rust.

Bricks alone look okay, and they're great to trim against, but I don't like the line they create. I like the smoother transition from grass to flowerbed.

Plastic and metal are hideous.

Ugh, true. However, I cheated a bit with plastic edging by burying it right at the soil surface in one place where I had trouble holding an edge. You don't see the plastic as it's about 1/4" underground. Although the grass does creep over it, it's exceedingly slow.

re:concrete edging. I have a small rectangular bed in one of my sunken gardens, set in a beaten earth floor, and have some concrete edging around it. I've dug a trench, put gravel in it and laid the edging on top of that - it still heaves up with the frost. I re-set that thing every Spring! Still looking for just the right material to replace it with.

I mentioned above I bought hand shears to trim the grass that grows over a trenched bed - the other day I tried them out, they work great, but I felt so "anal" about basically using scissors to make it look perfect, that when a car passed by I stood up and hid the shears behind my back! ha.

I feel, as the old saying goes "use the right tool for the job"

Certain grass cultivars that grow in bunches, ie Rye and Fescues, are perfect for a trenched bed. Others that send out shoots via stolons and rhizomes, ie, KBG, Bermuda etc, will naturally invade a trenched bed by genetics.

Use the right tool (masonry or trench) for the job (cultivar)