I started a thread on this topic a week or two ago, with bascially the same reasoning. You might look back and see what was said. Concensus was that a modest application of a low-nitrogen (or no-nitrogen) fertilizer would be beneficial.

On the question of odd weather, I noticed that my plum and pear trees, had almost no production of fruit this year, while the previous year was a bumber crop. I attributed that to the weather, but didn't remember about the unseasonable rains until you mentioned it. I do remember clearly that many new culms were wind whipped (and broken) in those rainstorms. Every morning for about 4 days running, I would go out and sadly remove them, from my 4 black bamboo stands. As to fertilizer, I just added a thin layer of a half & half (mix) of "Steer-gro" + organic compost over some of my plants, and figure the rain will carry all the nutrients needed down to the roots all winter. The idea of adding a slow release fertilizer in the form of say, pellets, seems like overkill to me. This time of year, many stores put the (previously bagged) compost and "Steer-gro" on half-priced sale. But I'm for whatever works ...

Winter is root development time, not foliar development. When soil temperature is as low as 40F, bamboo roots grow. They need phosphorus, so fertilize with phosphate or bone meal, not nitrogen. If you use an N-P-K mix, make sure the P is the highest number, and that N is very low (like 2-8-2).

Woodinville is a nice place for bamboo. I've been there several times (my Japanese sword instructor has his school there) and always find time to go to Molbak's. I've brought plants I bought there, including 'boo, back on the plane.

Perhaps just a naive addition to this thread from a new bamboo addict....
I notice that the Bamboo Sourcery website recommends horse manure as the ideal fertilizer for bamboo. A 4" mulch spring and fall is all the fertilizer needed.
And the fellow I've been buying bamboo from says do not substitute steer manure for horse manure. It's all wrong for bamboos (sorry, no further explanation, but I assume they know more than I do about the subject).

Just my 2 cents here... nearly every article I've seen regarding fertilizing says to treat your bamboo the same way you would treat your lawn. I have not seen a single article the contradicts this. After-all its fancy GRASS right? ; - D
take care all
CJ

My concern about nitrogen fertilizers at this time of year is that they stimulate quick foliar growth that might not have a chance to harden off before a frost and freezing temperatures. Woodinville has a mild climate, so maybe it's not such an issue there as it is here in New England.

Manure compost is good to use any time of year, no matter what the climate, however, because the nitrogen and other nutrients are released more slowly and take longer to affect plant growth.

Lawns are a different situation. If tender new grass growth freezes, it's no problem. You just remove the thatch and mow in the spring. But bamboo is more vulnerable to serious damage.