Typically the pohaku used to retain heat in your imu are smooth lava rocks taken from fresh water streams. They should not be any larger than an american football. A friend told me the story of a few guys who stole river rocks from Maui park land for imus. That crime was made possible by the ill fated Superferry.
The key to cooking your food is the steam. Banana tree stalks have a tremendous amount of moisture to make steam. Hacking them up a little right before putting them on the fire releases the water. The banana leaf and stem give imu cooked food a classic flavor. Kailua pig would not taste the same without it. Slash up the leaves and stalks into size appropriate chunks.