Possible Clues:

• 'The Joshua Tree' -- carbon copy by half of U2 in a year looking back
• Plant of the agave family with swordlike leaves
• Spiky plant also known as Adam's needle
• Looking back, current policeman has unknown quantity of Joshua tree
• Flower of New Mexico
• Grower heads for York urgently -- Christmas cactus aboard
• The flower of this kind of plant is New Mexico’s state flower
• Tropical lily genus
• Yale first college getting initial anthem from The Joshua Tree
• Spiky-leaved plant