I think you need to define what you mean by “intelligence.”
This one can be answered, however, taking “behavior” in a broad sense. Plants respond to stimuli such as light, gravity, water, nutrients, and damage to their tissues. These responses are the result of the production of plant hormones, growth factors, and other chemical signals.
Maybe what the OP is trying to get at is better expressed in terms of the amount of information in the DNA rather than the level of intelligence, although I suppose it depends on how you define intelligence.
You could say that trees have “behavior” in the sense of how they respond to changes in their environment.
Trees are smart, but they’re assholes. If you want it to grow, you have to make it think you want it to stay small. Even better, make it think you’re desperate for it to stay small.
Try this: when you water it, whisper things like “You’re so cute and small, please don’t ever change, never grow up, my lovely little tree”. But then, when you’re talking to your spouse/dog/invisible friend on the other side of the room, say something like this softly (but not too softly): “Thank god that tree is small… if the revolution comes, I can still take it hand-to-hand. But if it ever gets big, I’m worried about what will happen to this household!”.
That way the tree will think you’re afraid, and trees respond to nothing more than fear… and it will grow.
I find just the opposite as the maple treeis smart enough to form a symbiotic relationship with humans by producing a sweet substance that mankind desires and thus nurtures the tree to keep it healthy.
No tree (or cultivated plant) responds well to begging on the part of humans.
What may be useful is conveying an intent to cut down or dig up the poor performer if it doesn’t shape up. Or if it’s obviously dead, plant something else in that spot and watch it spring back to life.