Possible Clues:

• Subtropical shrub or tree
• Tree from which giraffes browse
• Gum arabic tree
• a wattle.
• A cold continent reportedly providing growth
• It takes a double century to start to get growing
• How a sound of 21 down gets to an intelligence with thorns
• Bills for one with a tree
• Small mimosa-family tree
• That blooming tree starts with a century at the double