He cannot solve the problem. He can help lessen it. Aspen can grow a plant in bad soil, but he cannot make bad soil fertile. Just as well, the edge of Nova Terra could be in such a state that not even he can grow anything there. It’s another world with its own rules. 

In line with his story and the premise given about the world in the OP, Aspen would not just stay there at the edge. He seeks a new place to take root and become a world tree. Anywhere but as close to the center as possible will not suit him. Basically he agreed to help under the assumption doing so will bring him closer to the center. He has a very valid story reason to move forward. If the Silver Sun chooses to settle, he’d move on his own leaving them behind and with him any amendments he brought to their situation. I figured he be used like a utility, not a solution. The silver Sun is in a horrid state, having an apple tree won’t suddenly improve their situation. He’s not a one man forest (yet), there is only so much he can do.

I can understand the folly of my previous characters. But cherry picking powers so that no one can help a struggling civilization beyond fighting to get back to the top seems a bit ludicrous.

I’m waiting for what the GM has to say, but I have no problem removing his ability to revive plants, making it so he can only grow and cultivate living plants. This way if things are as bad as described they’d have to move forward before he can make any meaningful impact.