Saplings don't exactly have stumps but you do need something intended for use on woody plants rather than herbaceous weeds or grasses. Get something with triclopyr as the active ingredient. Different vendors will have different formulations but I'd look for one in a concentrate to be applied directly to cut stems so as to avert damage to any surrounding plants or lawn.

I had some success against bind weed by putting some RoundUp into a bud vase and sticking a strand of the bind weed into the vase. I left it like that, and about a month later the entire plant was dead.

You'll want an ester based Triclopyr like this:

https://alligare.com/wp-content/uploads/2023/11/Alligare-Tech-Bulletin-Triclopyr-4.pdf

Regards,

Will

Read this. It works. I use Round Up Poison Ivy and Brush Killer.

https://www.gardenweb.com/discussions/2039081/100-000-buckthorns-to-kill