Possible Clues:

• 'Every two ears, by the sound of it (8)'
• In even-numbered years, maybe
• Lasting two years
• Plant that lives two years
• Plant with a two-year life cycle
• Plant book has unusual alien in
• Poison hemlock or Queen Anne's lace
• Abroad, well rested, rising every two years
• Broke bail coming back over most of 14 down every other year
• Happening every other year