Abarim Publications' online Biblical Hebrew Dictionary
The masculine noun הדס (hadash) means myrtle (-tree). This noun has no obvious root, but it occurs in several related languages. It appears to be a relatively recently developed word.
The myrtle is a perennial shrub with white flowers that were used for perfume. Since this plant is quite common in Palestine, the Bible uses the myrtle a few times symbolically (Isaiah 41:19, 55:13). Most notable is the Zechariahan scene of the horsemen who stand among the myrtle trees (Zechariah 1:8).
The myrtle is also mentioned as one of the plants from which the returned Israelites made their huts for the feast of booths (Nehemiah 8:15).