QUESTION: There are a number of species of willows in our area and we are not certain which one should be used for the lulav? (Terry Osman, Philadelphia PA)
ANSWER: The Bible along with other ancient texts is not very specific about the nature of plants. For that matter, up to the time of Linneus plant identification was quite uncertain and popular names in various countries as well as various sections of countries often were used to identify entirely different plants.
All indications point to the willow species (Salix Alba) as the willow of the Biblical period. However, it might also have been Salis Acmophylla. The difference between the two trees is that one grows in a cooler climate and the other in the hot climate of the Jordan River Valley as well some of the oases. We shall also note that the tree mentioned in Psalm 137.2 which is sometimes translated as willow probably refers instead to the Euphrates poplar (Populus Ephratica which is a totally different species also mentioned in the Book of Ezekiel (17.5).
As the Jews moved into various other lands, we simply used the local species of willow tree and this varied from country to country and was never questioned. Probably one reason for this is that fact that in contrast to the palm branch and the etrog it would not be possible to transport a willow branch any distance and keep it looking green.
Any willow, therefore, is acceptable for use as the lulav. One need not be particular about obtaining the Israeli species.