Tamarisks, species of the genus Tamarix are very common trees and shrubs in the Middle East, especially in soils with high salt concentration and are therefore the only trees found on the shores of the Dead Sea. There are several species of tamarisk.
The most commonly planted species and one which grows into a good sized tree is Tamarix aphylla. The tamarisk has small scale like leaves and small branches which give the tree a pine-like appearance.
During the heat of the day the tamarisk secretes salt, a process very wasteful of water. The salt dries. During the night the salt absorbs water from the air. In the morning the water evaporates creating a sort of natural air-conditioning. This cooling effect is another reason for its popularity as a shade tree.
Attractive pink or white flowers are produced during the winter, although a tree may flower any time during the year. The fruits are wind dispersed but the tamarisk is easily propagated by cuttings.
Two plants are mentioned in Genesis 21. The first is the shrub under which Hagar placed Ishmael (verse 15). The second is the tamarisk planted by Abraham (verse 33). The shrub could also easily be a tamarisk as this is one of the most common shrubs and trees in the vicinity of Beersheba or it could be the white broom.
Why did Abraham plant a tamarisk? Trees were often used as memorials for great men. It is therefore appropriate that Abraham should honor God by planting the tamarisk. It would be a permanent memorial of the covenant between the two.
Saul held court under a tamarisk in Gibeah (I Samuel 22:6). This dark green tree would be evident from the hilltop and provide a sort of "address" for visitors and could probably be seen for miles. However, this was a planted tamarisk as Gibeah is in the Mountains of Judea and now is a suburb of Jerusalem.
Tamarisks are not native here. In I Samuel 31:13 we read of the remains of Saul being buried under a tamarisk tree while in I Chronicles 10:12 the reference is to a pistacia (or terebinth) tree. This apparent discrepancy can be explained by the fact that the word for tamarisk can also be translated as "grove."