whether on may 14-22 in chengdu and Jiuzhaigou ?

The Spring starts from around the April and May for the valley and every plant turn green in that time.
Since you go there in the middle month, it is suggested to take thin sweaters, thin coat and rain gear. Besides, the sunscream is suggested. The temperature difference is big in that time.
From Chengdu, it is suggested to take the direct bus from Xinnanmen Bus Station to the valley directly. If you want to hire a van, you can ask the hotel for arranging one. Or find the local tour agency for help.
Upon reaching there, if you want to hire a car, you can hire a van to Yellow Dragon. This is my opinion.