Yes, it's hard to believe.

What will work when used that way is Blackberry and Brush Killer, an herbicide avaiable in most garden centers.

A full-bodied balsamic vinegar has a much more complex bouquet. And may I suggest extra-virgin olive oil as a sticker-spreader?

Kathy, I think the salesperson was having a little fun with your DH.

Horticultural vinegar! Undiluted is useless. I doubt that the horticultural vinegar would have a big impact on blackberries, and the product is hard to come by. It can be an effective weed killer and it's sold as "environmentally friendly."

Undiluted roundup painted onto the freshly cut stems would do the trick. The new shoots are usually not the plant returning, it's almost always seed that has germinated. The blackberry seed can live for 50 years and still be viable.

It doesnt seem like it would work, but possibly the idea goes back to things that like to be either acidic or not, like putting lime on the garden for some things, and mulching with fir or pine needles for others. I have to admit, I have never considered which black berries prefer. While this seems kind of "too simple to be true", its interesting to wonder where the idea came from.

It likely came from claims for higher strength "vinegars" (10-20% acetic acid) as weed killers. However even these high strength (and quite toxic) acids have little effect on plants such as blackberries. They act essentially as dessicants for foliage and have no effect on established root systems of most perennial plants, including blackberries.

Crossbow on leaves will kill the plant. Be very careful using crossbow. It is very harmful to you and the environment if not used properly.

There is no such thing as a safe herbicide. They are all poisons. And, no matter whether or not you follow directions to apply, they all drift off target as vapor, mist, droplets via air, soil and water and killor damage everything they come in contact with, including you, your children and your pets, bees, birds. Some are ”forever” chemicals, others have a half-life of 265+-days. Some have been detected 500 miles off target. Vinegar does work; if the plant has no leaves, then it will have no fruit. Salt also works. After application, keep out. Vinegar and salt are irritants.

" There is no such thing as a safe herbicide. They are all poisons."

So are about 99.8% of medications doctors prescribe to treat medical conditions.

The dose makes the poison. Acetic acid at 5% is vinegar, a widely enjoyed foodstuff and condiment. At 100% it would be a poison to ingest.

The website really need 'ancient thread bump protection', although I'm kinda enjoying the ludicrous notion of acetic acid being able to control weedy Rubus. Even if you sprayed them with 100% acetic acid - which would be a very, very dangerous undertaking compared to correctly spraying any normal synthetic herbicide - here is what would happen: the plants would defoliate. Three weeks later, they would grow new leaves. Even 25% acetic acid is quite dangerous, and I think people not versed in chemical handling (I once worked in the pharmaceutical industry) should be very, very careful with it. Many of them should probably avoid it. I'm actually surprised to see big box stores selling it now w/o making people sign a waiver to buy it.

Crossbow is a good herbicide to use for them. Like Gardengal says, READ THE LABEL.

I do use 25% vinegar very rarely, about once or twice a year, for extremely niche use cases. For example, if I discover a swarm of Microstegium that has formed seeds, I will kill the top and quickly dehydrate it, because that's what 25% acetic acid does. Then a couple days later, if it is hot and dry, I will use my weed torch to burn out the seeds. (I don't believe the acid can actually deactivate them. It just helps dry everything up.) If you just tried to use the weed torch on green grass, it would take forever. And if you just pulled it up, you'd release the seeds back to the soil. So, like I said, this is a very rare use case.

I would do the above if rain were forecast in, say, 4-5 days. If I just urgently want to top kill a weedy grass and I have a little more time, I would use diquat dibromide. It takes a little longer to kill and dehydrate the top growth. Maybe 3-5 days instead of over 2-3 days.

Of course this is for annual grasses that are seeding; if you have perennial weedy grasses you need to kill off and they aren't setting seed yet, obviously, you'd just use glyphosate.

There’s no such thing as a safe herbicide; they are all poisons. Some are so damaging, ag. labs. test for parts per billion, e.g. Rimsulferin. Fresh/frozen organic blackberries sell for $10.00 per pound. They are an excellent source of nutrition for people and other animals, and can be used as natural fabric dyes, and they provide habitat for animals. Blackberry vines need monitored/controlled, however, that is easily accomplished other ways besides using poisons..