門松 (lit. gate pine)

Very tall kadomatsu at Gokoku-Jinja

simplified kadomatsu

Kadomatsu is the traditional decoration with pine, bamboo, plum
and other lucky plants placed on either side of the gate posts at New Year's.
They are put to welcome New Year's God called Toshigami-sama (歳神様).

Kadomatsu are put both sides of the gate

Since ancient times, the pine tree has been revered as a symbol of longevity,
because it's evergreen and has vital energies.
In old time, people went to the mountains to get pines for kadomatsu, and this habbit, called matsumukae,
had been done on December 13, the day of Kotohajime (the first day starting preparation of welcoming New Year's Day)
The period which they are displayed is called matsunouchi (松の内)*,
and the day they are taken down, matsuake(松明け).
That's usually January 15th , ko-shogatsu.
On that day, they are burned. We call this event dondo-yaki or sagicho.
Here is photos of Sagicho held in Minami-Chita-cho.

* In Kansai area (around Osaka ), Matsunouchi is until Jan.15.
In Kanto area (around Tokyo), it is until Jan. 7.