How did you apply? For something fast growing / invasive like a Bradford pear, or a honeysuckle, you really need to get Glyphosate into the roots for it to die. I was taught this trick by an Arborist:
Cut the tree down and leave 4-6” above the ground. Take a small drill and put a 3/8” drill bit in it. Try to find the small hole in the very middle of the trunk and drill down into it. This is how the sap flows through the tree. Carefully spray 3-4 good sprays of Glyphosate into the hole with gloves and eye protection. The tree and gravity will take this down into the roots where they will die. The small amount of Glyophosate will stay in the roots for approximately eight years, and it won’t leak out into the soil as it’s held by them.
That makes me think of some bamboo eradication advice: You allow new growth to progress so that it consumes energy from the tricky root system, then cut it before the fresh sections can provide much in photosynthetic return-on-investment. After a few years of losing calories with each attempt, the plant runs out.
That might only work for plants with "bursty" regrowth though.
I spent 3 years combating bamboo in a similar fashion and it certainly didn't work for me. I've never experienced a more frustratingly invasive plant in my life. Even wild blackberries are easier to deal with.
You may appreciate the fact that Bamboo sprouting is edible. I add rings of fresh bamboo to my pasta dishes and is a nice touch for a summer salad.
Just use only the new stems, the parts that can be sliced with a sharp kitchen knife using your hand and reasonable force, and discard everything else.
Best advice is not planting it unless you have a lot of space and something to do with it or build --strong-- concrete root barriers. Alcatraz jail level.
Supposedly painting it on the cut end of the trunk on a periodic basis (and on the cut ends of any sprouts that do come up) is more effective than a one-time drench. Glyphosate has a pretty short half-life.
Also, a spot treatment of the ends means considerably less herbicide entering the soil.
I believe the best herbicide for many situations may very well be patience and elbow grease.
Disclaimer: While I am an avid gardener and I enjoy learning about the agricultural sciences, I am a computerologist, not a botanist, biologist, or chemist.
I don't know all there is to know about herbicides. I know there are broad-spectrum herbicides and selective herbicides, but I am only aware of two of them by name.
There's the well-known broad-spectrum herbicide glyphosate, of course, but it is absorbed by foilage and has low persistence in soil. This is why the application instructions typically say to apply in sunny weather when the winds are light and rain is not expected for several days.
Then there's tebuthiuron, another broad-spectrum herbicide. It's been almost fifteen years, now, but there was a famous case of landmark oak trees in Auburn, Alabama, being poisoned with Spike 80DF -- a specific formulation of tebuthiuron made by Dow -- by a rabid sports fan upset by the recent success of a rival. Because tebuthiuron is absorbed through the roots, has high persistence in soil, and inhibits photosynthesis, the trees were assured to die. My understanding is that this is an industrial herbicide, though, and may be difficult for consumers to obtain. It's banned in the EU.
Don't do what I did, which is to throw caution to the wind in frustration.
I was successful in killing a tree under my roofline. It was planted less than a foot away from the foundation and was difficult to prune due to its location amidst yews and aborvitaes. The trunk grew to about 3" in diameter before I decided to cut it down. I cut it down to about an inch or two above the ground. It was nothing if not insistent, though. It continued to put out new shoots and I would cull them and/or spray it with glyphosate when I noticed it. After a couple of years of this dance, I escalated things. I cut the trunk down beneath the surface so that no part of it was above the ground. I doused it with a number of chemicals based on advice I received. Eventually, it stopped putting out new shoots. Over the next few years, though, the pair of healthy, well-established arborvitaes about ten feet away on either side began to brown and die. I think it's quite likely that whatever I did to rid myself of the tree also contributed to the demise of my arborvitaes.
If I had to do it over again -- having the benefit of hindsight as well as the patience that comes with age -- I would have tried to suffocate and starve it instead of saturating the soil with something that may hang around to impose unintended consequences. Perhaps I would have had success cutting it beneath the ground and capping it with a sturdy container impermeable to light, water, and air. I would have periodically removed the cap long enough to eliminate any new branches or shoots that should threaten to breach the barrier. I would have done this until it exhausted all of its energy reserves trying to reach the light.
Cellulose is constructed from chains of glucose -- probably the most common carbohydrate produced via photosynthesis. Plants synthesize carbohydrates, store them in tissue, and later metabolize them via respiration. If one is both diligent and patient in limiting the plant's capacity to photosynthesize and respirate, I am sure it will eventually die.
Cut the tree down and leave 4-6” above the ground. Take a small drill and put a 3/8” drill bit in it. Try to find the small hole in the very middle of the trunk and drill down into it. This is how the sap flows through the tree. Carefully spray 3-4 good sprays of Glyphosate into the hole with gloves and eye protection. The tree and gravity will take this down into the roots where they will die. The small amount of Glyophosate will stay in the roots for approximately eight years, and it won’t leak out into the soil as it’s held by them.