Just pull them out at dirt level, and pull them again if they come back

With a young tree, the bark can absorb enough glyphosate to slow down the tree.

Roundup (glyphosate) can be absorbed by young/thin bark or exposed roots, and damage the tree. But if you are very careful, preferably "dabbing" the glyphosate on the weeds, you shouldn't cause any harm to the tree.

you could use something like at the link.. to DRIP non-diluted RU on green tissue ... 2 or 3 drops per weed ... if killing saplings.. snip at ground and one drop on cut ...

it will become inert on touching soil ...

it since it must be applied to GREEN TISSUE ... it will not affect bark ...

though i dont know how using the precision tool at the link you could spread it all over a tree ... short of a stroke or seizure ...

return unused portion to properly labeled container ...

ken

Here is a link that might be useful: link

".....a few small weeds....."

Can't you just pull them out? It's only one tree and that method will cost you absolutely nothing - monetarily or environmentally.