Well, nobody else has chimed in, so let me take a stab at it. Getting rid of anything that spreads from underground roots is a royal pain, as you have guessed, and sometimes must be done with a ton of digging and going through your soil with a fine-toothed comb. Before you get out your shovel, maybe try this: take full strength Round Up and put it in an empty squeeze bottle (dish soap bottle, condiment bottle); mix in a little something to make it thicker - I use Dawn, Ken from Adrian who gave us all this idea uses motor oil. Cut one stem of dewberry down close to the ground, then squeeze out a tiny bit of RU, enough to cover the cut stem, then move on to the next one, and the next one....it will work better if you get the RU on a freshly cut stem.

Obviously, be careful with the RU and WEAR GLOVES. The RU will not spread through the soil, as far as I have found. Depending on how much root strength the plant has, you may have some random growth later, so treat that the same.

Hope that helped. Good luck!

Just make sure to get the greatest strength you can, Mike.

Any luck, Mike? I am completely overrun by this crap in my garden, and I need a definitively effective measure of getting rid of it. Preferably without things that will cross contaminate to my fruits and veggies.

Sorry, SLSP... no luck with the dewberry invasion. It's worse than ever. I have tried spraying with both triclopyr and glyphosate. The above ground vines do die back when I spray, but new shoots and vines continue to emerge relentlessly. I honestly don't think there is anything resembling an "easy" way to get rid of dewberry once it's well established. The only method I think that could completely eliminate it is a very labor-intensive dig-out of all the underground runners. To treat/spray every single shoot and sprouting vine individually in my garden with herbicide (lisanti's method) would take weeks and would require essentially spot treating every square foot of the garden with toxic chemicals. I would rather dig out all the soil in my garden and replace it than do that.

First, there are multiple species of dewberry native to North America so while they may be aggressive and pernicious spreaders, they are NOT considered invasive.

Many types of Rubus have a similar behavior and pattern of growth. The PNW deals with Himalayan blackberries (a true invasive species), which are literally everywhere and can develop into thickets 10-15 feet tall and many, many feet thick. The only recommended method of elimination is to cut back any visible canes to a few inches off the ground and then paint the cut stems with undiluted herbicide. Because this is a direct contact application rather than a broadcast spray, there is no risk of cross contamination.

But just treating the cut stems will not resolve the issue fully. These vines develop a sizeable seed bank in time as well as develop a vast, interconnected root system. You will need to stay on top of the situation digging out every speck of new growth that emerges for at least a couple of seasons and maybe for as much as 3-5 years.

I'm always battling this too. Painting cut stems does work, and seedlings are not hard to pull but you have to spot them right away or they start to form a tuberous root that becomes too difficult to pull. In the meantime, make an effort to at least eat the berries because they're delicious and if they hit the ground, new sprouts will appear. But I'm well aware of how tedious and painful it can be to find them, and that birds will keep delivering new seeds to the area. String-trimmering them to the ground often can help keep them from becoming more vigorous until you have time to cut and paint individual stems.

If there are entire areas with a hard border/barrier, smothering the whole thing could be an option if you also make the effort to go all around the edges with your shovel to sever all connections between that within and that which is outside of the border. If you covered it well now, I would wait until at least next spring to test a small area. I would use 2 layers of corrugated cardboard, overlapping edges by at least 6", covered with at least 9" of some type of heavy organic matter, like shredded wood, or a much thicker layer of leaves or something more lightweight. You could augment with sawdust, shredded paper, kitchen scraps, shrub trimmings, whatever organic material that is available to block the light and physically prevent growth from occurring.

Hack em down to ground level, 6 mil black plactic. no water, no light. no growth.

If one is going to go to the trouble to kill dewberry vines, they probably don't want to ruin their soil in the process.