When I repotted, I found two thick long rhizomes circling around the pot. And I decided to cut off the tips of rhizomes by few inches. I loose up some dirt around the roots little bit.
What I suspect is layer of gravel(1 inch) at the bottom is holding some perched water.

Sam,
Put alot more holes in that pot.
Bamboo HATES wet feet.
You may not even have to take it out.
Just drill more holes in the bottom very carefully, and mabey even a couple on the bottom, on the sides of the pot.
Usually, black bamboo will actually loose almost all of it's leaves in the spring too. That is normal.
Good luck growing it in a pot.

Thanks Kudzu9 and butterfly4u for their advices. Today I unpotted the black bamboo and put it into the ground.
When I dug out the planting mix to pull out the plant and reached a level of standing water in the container. I smelled some rotten eggs from the soil. This was caused by lack of drainage.
I could have drilled more holes to improve drainage but doing so would put the plant at risk. Then I decided to leave the plant in the ground for a few years to restore vigor of the plant. When I bought it the condition of soil in the old pot wasn't very good. Those mushroom would grow on the top indicated the soil (shredded wood chips) were spoiled.
I dug a 7 inch trench around the plant without plastic barrier. And I hope I can contain rhizomes from spreading out.

sam89-
Sounds like you fixed your problem. Your bamboo should recover in a few months.

When it does send out runners -- probably not for at least a year or two -- a 7" (deep?) ditch won't be enough. You probably want to go at least 18". If you don't put a barrier in the ditch, you can simply fill it in with something soft like sand or bark, and then just use the pick end of a pickaxe to probe for runners. In your temperate climate, this is probably something you should do every 2 months. When you find strays, cut them off with a sharp shovel or pruner. Chopping them off usually kills anything beyond the cut point. However, it's best to extract them from the ground because, if they make it across the ditch and into the soil they may just have enough residual energy to get established and start a new plant where you may not want it.

Thanks all!

It looks like my black bamboo has survived the winter. The winter was very windy with northwest dry wind with RH down to 20% here. Today I found a few shoots and their sizes were thicker than previous generation.

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a closeup view of few shoots. In general, their leaves aren't that pretty at all. They are quite yellowish and I'm quite disappointed. It may I have it in a sunny spot and SF weather is quite dry.

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