Banana stem (plantain stem / vazhaithandu) is a bit different from banana fruit in Jain view:

• It is a stem, not a root, so it is not in the same category as potato, onion, garlic etc.
• But practically, to use the stem you usually cut down the whole banana plant, which is then destroyed after one fruiting.

Because of this, some stricter Jains prefer to avoid banana stem, seeing it as causing more himsa than necessary.

• Many householders do eat it, treating it like any other above‑ground vegetable, especially in South India.

So:

• For normal Jain shravak life, it is not clearly forbidden like root vegetables.
• If you are trying to follow more ahimsak / stricter diet, it is better to avoid banana stem and choose fruits, grains and simple above‑ground vegetables where the plant is not completely destroyed.
• Finally, follow your own guru / family maryada. If your sangh avoids such items as “more himsak”, you should follow that.

For a general Jain food overview, you can see this helpful explanation on Jain foods list