The plant may be rootbound. P. nigra normally shoots early in the Spring. It will occasionally put up shoots later in the Summer and will put up smaller whip looking shoots throughtout the year.

Too much water can cause new shoots to die off, or as I mentioned, being rootbound can cause problems also.

Kt

My nigra acted the same way in a pot but recovered nicely when I put it in the ground. Just keep in mind that nigra is only hardy to 0 degrees f. and may need protection from the winter winds. I use row covers. I first put 4 bamboo stakes around it then wrap them with the row cover leaving the top exposed. This definetly helps all of my other not so hardy boos.