green thumb

(US) A person with this skill.

Example: 1987, Lyall Watson, The dreams of dragons: riddles of natural history‎,
  Everyone knows someone who has a "green thumb," who seems to be able to get plants to grow well just by being around, by talking or even playing music...
2003, Barbara Damrosch, The Garden Primer,
  ...they leave gardening to those who presumably have lots of time to read and better yet, have a "green thumb."
2006, Des Kennedy, The Passionate Gardener: Adventures of an Ardent Green Thumb‎,
  Oh, I'll never have a green thumb, no matter how long I stay at it. Nothing ever seems to work for me. Everything I plant dies.
  I should ask the green thumb next door what she recommends for my droopy daffodils.