The weed killer Roundup and its active ingredient, glyphosate, have drawn much unwanted attention of late. But glyphosate still has its advocates.

“As it stands, glyphosate is really the best tool we have for fighting invasive plants,” says Brendan Quirion, an invasive species specialist with The Nature Conservancy, the world’s largest conservation advocacy group.

With those plants becoming a bigger threat, conservation groups and land managers worry that glyphosate might soon be off-limits or severely curtailed. Across...