BITTER HERBS (מְרֹרִ֖ים). At the Passover, the children of Israel were invited to eat the meal with merôrîm, which really means “bitter,” the word “herbs” being understood (Exod 12:8). There is a similar reference in Numbers 9:11. These “herbs” as tr. are prob. not culinary herbs at all, but salad greens like chicory, endive, sorrel, dandelion, and even old types of lettuces. All these were found widely distributed in their natural state in Egypt. Watercress is included by some experts as a bitter herb. In Europe, at modern Jewish Passover feasts, horseradish is allowed, but this certainly could not have been used in the time of Moses.
Botanists point out that endive (Cichorium endivia) was an Indian plant, and so not known in Egypt in Bible days. But in some countries, endive is called chicory (Cichorium intybus).
The original lettuces (Lactuca sativa) were quite bitter to eat when unblanched, and are quite different from the crisp, ready-to-eat lettuces of today.
The dandelion used as a salad is Taraxacum officinale, and the sorrel, Rumex acetosella.
Orthodox Jews say that it is necessary to eat five different kinds of bitter herbs with the lamb at the Passover. Today, these five could be—chicory, endive, sorrel, lettuce and mint. As a matter of fact, some commentators have insisted that the bitter herbs mentioned in Exodus and Numbers are mints and mints only.
Ancient writings show that the Egyptians in the days of the pharaohs regularly ate salads or bitter herbs with their meat.
International Standard Bible Encyclopedia (1915)
hurbs, or urbs (merorim): Originally in the primitive Passover (Ex 12:8; Nu 9:11) these were probably merely salads, the simplest and quickest prepared form of vegetable accompaniment to the roasted lamb. Such salads have always been favorites in the Orient. Cucumbers, lettuce, water-cress, parsley and endive are some of those commonly used. Later on the Passover ritual (as it does today) laid emphasis on the idea of "bitterness" as symbolical of Israel’s lot in Egypt. In modern Palestine the Jews use chiefly lettuce and endive for the "bitter herbs" of their Passover. In La 3:15 the same word is used: "He hath filled me with bitterness merorim, he hath sated me with wormwood." Here the parallelism with "wormwood" suggests some plant more distinctly bitter than the mild salads mentioned above, such, for example, as the colocynth (Citrullus colocynthus) or the violently irritating squirting cucumber (Ecballium elaterium).