Possible Clues:

• Plant Bill in a secret agency
• Plant a group of US agents to protect California
• Plant succulents, passing time in borders of arboreta
• Spiny tree or shrub with small yellow or white flowers
• Spies appear after hiding odd bits of larch and oak tree
• Tree that giraffes love
• “____ Avenue” is a clichéd suburban street name
• Ark of the Covenant wood
• Plant’s account supported by a covert US group
• Mimosa tree, e.g