Do plain popcorn kernels need a hechsher?

Answer: No.

Reason: Plain popcorn kernels are fine without a hechsher.

Source:

• cRc: popcorn kernels are acceptable without hashgachah if there are no questionable additives; already-popped popcorn requires kosher certification.

Related questions:

• Does basmati rice need a hechsher? No. Rice from Pakistan should be checked for bugs.
• Do loose roasted peanuts need a hechsher? Yes.

More from Rabbi Tahan: Q&A: Hechsher on Popcorn Kernel

References: cRc (askcrc.org), “Popcorn kernels” · cRc (askcrc.org), “Popcorn (already popped)”