我国北方农村的建筑用材，多以杨、柳、榆等速生树种为主，但这些木材易发生腐朽与虫蛀。

Most structural timber species now used in the North China's rural areas are poplar, willow, elm etc, which are fast growing trees but whose woods are very susceptible to biological deterioration.

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人们因为禁火，需要重新用榆柳取火，所以就在门口插柳。

As fire was forbidden, people had to return to the elm willow for starting fire. Therefore, they planted willow trees in front of their gates.

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