I am a Rank 2 gardener (almost Rank 3) and my Giving Tree needs sun. I have given it all the Rank 1 sun (which only covers a small area) that I can and it isn't helping. I can't get the spell to cover a medium area until I am rank 3. Meanwhile my giving tree is wilting and progressing to dying and there is nothing I can do to stop it.
It seems to me that if the tree can be planted by a rank 2 gardener, it should be able to be grown by a rank 2 gardener. Challenging is fine. Impossible is unrealistic.
First, let me assure you that the single plot sunlight spell will work just fine on your Giving Tree.
However, perhaps you are casting the wrong spell. Some gardeners mistake the Magic ( a tiny yellow sphere) gardening icon for the Sunlight (just a quarter of a larger sphere, as if most of the sun is cut off) icon.
Confidentially, I've made that mistake before myself!
Many of the bushes want Magic more than Sunlight; maybe that might be what happened?