I'm unsure growth wise, but it's not recommended to fertilize trees in the first two years after planting. You want them to grow a deep and expansive root system and fertilizing will promote top growth (think leaves and whatnot) while sacrificing that good root system you want.
Especially with a 'Green Giant.' They can get 50-60 feet tall; you wouldn't want a weaker root system causing that to fall down one day.
