Help: Weeping Cherry not doing well - growth maybe?

I have a fairly large weeping cherry approx 10 years old 8' high by 12' wide that appears to be not healthy. I live in the NE USA NYC area. The tree sits in full sun and in a 12' to 15" planted area next to my driveway. The soil is not very drained, nugget mulched, and soil is hard and clay like and some roots appear to making their way to the surface.
For the past two springs I have pruned this over the years to keep the weeping growth more like an umbrella shape. I may have over-cut a branch or two that was more like one of the main branches rather than shoots. Last spring, the tree blossomed but not as full or robust as usual. This spring same thing. Last late spring & summer the leaves appeared not robust with 25% dead branches. I suspect this year same thing after bloom. There's no question the tree is much alive but just unheathy looking based on earlier years.
I noticed at the crown where all the main branches stem from there to be a hard bulge not too normal looking with a gap in the center. The size of the main trunk is about 7" to 8" inches. It appeared last summer there were very small ants traveling in and out the crown and into the tree.
I want to understand what this growth could be and why/how insects penetrated it. I want to save it and seal that gap to stop the insect penetration. Can you give advise on what this can be - disease or stress, how to treat it, and how to seal the gap (what material / where to get). Also, am I doing all the right things to keep it healthy?
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The knob is the graft area (weeping cherry trees are grafted) These trees need good drainage and compacted soil is not good- you can plunge a spading fork into the ground and pry back and forth to help ease compaction. And you could add sifted compost or used coffee grounds to the areas you disturb. Grounds are a nice mild fertilizer and source of organic material for the soil. Sealants are not usually reccomended but you may want to clean any debris out of the crown as the branches may form a bowl that never quite dries out. An arborist might insert a drain tube drilled into this area but it is generally beyond home gardener skills.
Go online and read up on weeping cherries - look for websites ending in .edu for answers that have been researched.
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wrote:

Find out which type of ants, probably a kind of carpenter ant. It sounds like the ants are feeding at the graft, the ants are chewing at the tree and the tree is attempting to heal itself, ergo a bole would form. Unless you kill the ants the ants will eventually win. Carefully inspect the entire trunk from the ground up, you will likely find a small round hole, that will be the queen's chamber entry. Get yourself a can of pressurized insecticide that does in ants, fill any holes and spray liberally at the graft. I would also create drainage around the tree... dig a trench all around at the drip line, at least a foot wide and as deeply as you can. If you're fortunate you will break through the clay strata... fill the trench with a mixture of sand and compost. You may need to dig a trench at the low side radiating away from the tree.
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