The linked webpage depicts a large-scale method of bramble control requiring heavy machinery.

The principal is good though. It's given me food for thought, except that most of our bberries are in places that aren't flat enough to run machinery.

The method above works, for the most part, on rather level ground. Operating a brush hog is not for the timid or elderly.

My ten acres is rather hilly. I take a pole pruner and reach in to the crowns and cut them off and then pull the vines out. A gaff hook works for the crowns. What's left is easily pulled.
A common misconception is that Blackberries spread underground and then sprout. They do not. They spread like strawberries. I try not to let any vine tips root during the fall and winter on areas yet to be conquered.

Mike

When I lived in Seattle I had some Himalayan blackberries trained on a fence. They never spread underground in six years. Maybe they do, but it hasn't been my experience.
Mike

The root leads back to her house, or a section of branch does? Rooting of branch tips is common, as is branches growing flat on the ground and getting buried.

I would say that wild blackberries certainly can spread underground, but this is not their natural habit. I have dug up blackberry clumps and invariably many sprouts will pop up from the roots left behind, despite my best efforts to get all the roots.
A 20' long root that was severed from the mother plant would most certainly sprout canes. Even if not severed, I would think under some conditions they would sucker up.

The root leads back to her house. I'm religious about keeping her invasive plants on her side of the fence but can't do much about the underground raspberries until they show up. The 20 footer was an anomaly, most are in the 10 ft range.

Raspberries are a whole 'nother ballgame.

While I did this method for a large area, I also modified the idea for a smaller would-be gardening area about 25 X 50 feet.

1. First I pruned accessible large canes to the ground,
2. Then I weed-whacked the thinner canes.
3. Moved forward and repeated.
4. I removed the debris large enough to impact mowing.
5. And mowed to mulch the loose canes
6. Planted grass, watered, and mowed regularly to control brambles until grass took over.

I've heard that if you mowed them down at 2 weeks interval, they eventually have no energy left to keep growing.
But you must be diligent and keep mowing them for an entire season.
Good Luck

Goats.

I've been mowing my blackberries in a grassy field diligently for almost four years. They have some massive roots. They haven't given up yet, but neither have I.

I'm retitling the topic for easier reading.

I don,t post here very often but this has been such a big problem for me I had to chime in. The bigger leafed ones can be killed by cutting down the canes the smaller leafed ones spread under ground. I have been mowing my field for years and the small ones just won,t give up . The bigger ones were killed the first year here. I read a article somewhere a few years ago about the different types of blackberrys and the roots can spread and come up as far away as 80 feet with the right kind of berry. I just don,t remember thier names. But it does seem to be my exp. with the smaller leafed ones.

I used a hedge trimmer to tackle blackberries that were beyond the scope of a lawn mower. I made a video about it here: https://youtu.be/l3V6JtbMfyY

Loose sweatpants and a shirt with no sleeves doesn't seem like the way to go about it no matter what you use to cut the vines.

Mike

My biggest problem is that the birds seed the brambles within an existing herbaceous border or garden area in unreachable spots where they grow to mammoth size before I can crawl in and try to uproot them.....

Placing a large enough tarp or other opaque sheeting over the brambles/roots would starve them of sunlight. If the spots not too big, may be worth a shot.

I agree with Botann about not letting them touch the ground anywhere. My husband has gained good control over them at the edge of the woodland near our lot by going out each spring with a mattock and getting the rootball. I haven't found them to spread much underground either. My neighbour's salmon berries on the other hand are always popping up many feet away in my yard. Argh!

At least Salmon berry is easy to pull.........unless they're real big. Then I hook on to a root or two with the back of a shovel and pull them out. (the place where you put your foot)

Unfortunately the big mother root is in my neighbour's yard and she likes it! So I get the stray shoots. Amazing how far away from parent plant they spring up. I am sometimes a bad neighbour and lop off what I can get to by sticking my pruners through the fence. LOL. Maybe that's only making more shoots!?

Where the blackberries are thick I reach in to the crowns with my pole pruner and cut them off. The vines pull out pretty easy then. Next I give the cut off stubs a straight dose of brush killer.

I find blackberries easy to pull when compared to salmon berries. They don't seem as deeply rooted and their roots don't seem to travel as far. But thanks for the tips!