I've had a few trees that did not want to grow. PPK x 2 trees, and 1 grafted sweet tart. No other ppk trees but I have other grafted sweet tarts that grow fine.
In your picture I see a few barren spots in the yard/field. In my yard I have moles and I suspect they formed cavities under some of my smaller trees stunting their growth. Just a guess for your tree also. You might want to try stomping down the dirt around the trunk just in case air pockets are drying the roots too much.
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I'll try anything to help it grow. I do see moles in my yard but I have several
other mangos in the same proximity and they grow fine?
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As Cookie alluded to earlier, I bet your sweet tart is being influenced by the rootstock. I have an Edward mango tree that does the same thing. Been in ground for about 6 years, is only about 6 feet tall. I try to feed it and get it to grow but it refuses. Each year it tries to set a ton of mangoes and I end up with 10-15. I was about to get rid of it because I wanted a large tree in that spot, but then I thought "who else has a dwarf producing Edward mango tree?". If nothing else it's a good conversation piece. Every so often, there will be rootstock variance in the turpentine seedlings. Even Chris at Truly Tropical has a tiny Keitt that won't grow and produces a couple mangoes each year. They should be easy to move if you ever needed to...
E.