Time for a backhoe.

Does a lot of water run in the ditch in the winter? If so, the brush is probably holding the soil in place. This is a big problem.

If erosion isn't a problem, I would water the brush to get it actively growing, then spray with Roundup, wait a couple of weeks, then cut the brush down. If it resprouts, spray again. That will kill most of it. You will have to keep watch for the occasional sprout over the next couple of years. And the weeds will try to take over as soon as the ground is bare. You can use a heavy mulch of wood chips, assuming it won't get washed away. But weed control in a ditch, without chemicals, doesn't sound like fun. Maybe a weed control fabric would work, anchored with something heavy, like stones. That might also help control erosion.

Do not use glyphosate (RoundUp etc.) where the spray will fall on water.

A backhoe, even a small one, may not be a great idea up next to the road. The soil is sodden in the winter, but there's rarely running water. We have basically used lilydude's method on blackberries, and I had thought of something similar for this area. We used the cutoff blade on the string trimmer to cut the brush down in one area. The soil is pretty dry right now, so I did think about spraying the new growth. Otherwise, I could dig out what I can, plant the hedge a bit farther back from the road and then just commit to cutting the salmonberry and hardhack down repeatedly. I don't know how DH would feel about that. Maybe he'd make me learn to run the trimmer.

You might consider creating a mixed hedge of hardhack, salmon berry, and cornus alba. Rather than having to clear the entire stretch you could hand-dig the space needed for each c.alba. I think mixed hedges are more visually interesting.

Of course the situation would have to lend itself to a backhoe. It also remains true that if you cannot dig them out by hand or kill them with herbicide then what you are left with is digging them out with a machine or killing them with tethered or fenced goats. This last method would also not be too good right next to a road.

What's wrong with Salmonberry? It has pretty flowers in spring that attract hummingbirds, and berries that attract birds. Besides, dogwoods often get anthracnose.

While not impervious redtwig dogwoods do not have the disease problems of flowering and Pacific dogwoods, and do have a suckerous, thicket-forming habit--as do salmonberries, which also have numerous easily detached prickles which are ideal for penetrating and remaining stuck in certain kinds of rubber gloves.

Salmonberry is a native, which makes it inherently better than blackberries, but it still behaves like a thug on my property. The existing hedge of salmonberry is old and has continually been hacked down to 3ft by the PUD, so it's less than attractive. It's not much of a bird habitat, since it's right next to the road, and I have lots of it in the woods. I don't feel guilty about removing a native and replacing with a friendly non-native in this case.

My plan is to cut down the grass and fresh growth with the weedwhacker, then cut the salmonberry "trunks" as low as possible with the loppers. Then I'll use the weedwhacker to keep it all low. I may spray it, but I'll decide that later. The new hedge will probably be planted inside the old hedge line. It will give a buffer from the traffic (which is not heavy in our rural area) and be safe from the PUD that way.

There's also one in the White House. Deer can be reluctant to stick their heads into large evergreen shrubs, hiding favored browse among big dark bushes can make it possible to grow them.

Oh, I get it, a talking Bush! Very funny. Anything to thwart deer is good in my book. I can't understand why they prefer the things I plant to native plants they are supposed to eat. I'm resigned to their eating the lower apples on my apple trees, I wish they were content with that.

Plants that have evolved with them have defensive chemicals that others may not.