Blackberry won't grow in miracle gro pot?
 

I have a blackberry vine planted in a 5 gal pot, the two large
branches are (although much is dead wood now) about 18 in tall. The
soil is miracle gro. I measured the ph of this stuff, and it is a bit
acidic. The pot is well drained, and I put lots of rocks at the
bottom to keep the soil from going out the large holes. It is in full
sun, and the days lately have been in 70s and 80s. There are only
green leaves left at the bottom of the plant now, it is dying from the
top down. I did not damage the root ball when planted, it had a big
healthy root ball.
Why isn't it growing? I am in the northern edge of zone 10. Black
berry vines grow wild here. But this one was purchased at supermarket.
I keep it just slightly moist.
Any ideas?