Henna Lawsonia inermis grew wild in the oases by the Dead Sea in Old Testament times. It is also known as “Egyptian privet” and is common in the hot, drier parts of Africa, southern Asia, and northern Australia. Although henna was, and is, primarily used because of its coloring properties derived from the dried leaves, in Song of Songs 1:14 the writer mentions its flowers, which are indeed pretty and fragrant. They are in a “cluster” (’eshkol in Hebrew), which could be taken as a woven object parallel to the little bag of myrrh in verse 13 or as a garland, or simply as a clump of flowers in the cultivated terraces above En-Gedi. The reference to henna in Song of Songs 4:13 is strange since it is in the plural form in Hebrew (kefarim), parallel to the yet stranger Hebrew word neradim (from nerd; see spikenard). Neither of these plants has significant fruit, so the verse can be taken as a glorious concoction of romantic images of spices and flowers.
Henna is a shrub that reaches 2‑3 meters (7‑10 feet) in height. It has opposite leaves and many fragrant white flowers. The leaves yield a red juice that people in many countries use for coloring the skin and hair.
The context is clearly metaphorical in both references to henna, so there is the possibility of substituting locally known equivalents. In places where henna is known only as an agent for coloring the hair, skin or fingernails, it may be important to substitute, or to create a note stating that the smell and beauty of the flowers are in focus. In Song of Songs 4:13 most translations ignore the plural, perhaps assuming that it is influenced by the word “fruits” earlier. Alternatively, a transliteration can be used based on a major language. The recent resurgence in skin decoration (“mehndi”) using henna may make it easier to find a known word.
Source: Each According to its Kind: Plants and Trees in the Bible (UBS Helps for Translators)