The 'hadas' is identified as the Myrtus communis - the common myrtle. 

The leaves of the common myrtle are used at the Feast of Tabernacles (Sukkot) as one of the four species which were ‘waved’ before the Lord.

Common myrtle

 

The common myrtle at Neot Kedumim Biblical Landscape Reserve Israel. 

The common myrtle is an evergreen shrub or small tree, which grows to around five metres tall. Its leaves are between three to five centimetres long and it produces a fragrant oil. Its flower is star shaped with five petals and sepals which are usually white and it has numerous stamens. The plant produces a round blue-black berry.

The common myrtle is widespread throughout the Mediterranean region. 

As stated already, it is used as one of the four varieties in the festival of Sukkot. Although not specifically mentioned, it is thought that the hadas were the "leafy branches" which are referred to in the following passage: "​
So beginning with the fifteenth day of the seventh month, after you have gathered the crops of the land, celebrate the festival to the Lord for seven days; the first day is a day of rest, and the eighth day also is a day of rest. On the first day you are to take choice fruit from the trees, and palm fronds, leafy branches and aravi (willows), and rejoice before the Lord your God for seven days." (Leviticus 23:39-40).

The hadas was one of the plants that were used to make the booths during the same festival, that of Sukkot: "...and that they should proclaim this word and spread it throughout their towns and in Jerusalem: 'Go out into the hill country and bring back branches from olive and wild olive trees, and from hadas (myrtles), palms and shade trees, to make booths' - as it is written." (Nehemiah 8:15)

The hadas was also mentioned in the promises made by God through the prophet Isaiah: "I will make rivers flow on barren heights, and springs within the valleys. I will turn the desert into pools of water, and the parched ground into springs. I will put in the desert the cedar (erez) and the acacia (shittah), the hadas (myrtle) and the olive (ets shemen). I will set pines (berosh) in the wasteland, the fir (tidhar) and the cypress (te’ashur) together." (Isaiah 41:18-19)

and

"You will go out in joy
and be led forth in peace;
the mountains and hills
will burst into song before you,
and all the trees of the field
will clap their hands.
Instead of the thorn bush (na’atsuts) will grow the pine tree (berosh)
and instead of briers (sirpad) the hadas (myrtle) will grow." (Isaiah 55:12-13)

The hadas is mentioned in the vision of the man on a red horse, which was made by the prophet Zechariah: "During the night I had a vision - and there before me was a man riding a red horse! He was standing among the hadas (myrtle) trees in a ravine. Behind him were red, brown and white horses. I asked, 'What are these, my lord?' The angel who was talking with me answered, 'I will show you what they are.' Then the man standing among the hadas (myrtle) trees explained, 'They are the ones the Lord has sent to go throughout the earth.' And they reported to the angel of the Lord, who was standing among the hadas (myrtle) trees, 'We have gone throughout the earth and found the whole world at rest and in peace.'" (Zechariah 1:8–11)

This is an edited excerpt from Alvin Johnson's iBook 'Biblical Flora', 2017. The book is available for free download on iTunes. A teacher's edition is also available for purchase.