The Gemara on our daf discusses a variety of plants, some of which cannot be used as skhakh – roofing – once they have begun to be processed. Flax, for example, is fine for skhakh as long as it has not begun the process turning it into linen. Once that process has begun it is no longer raw material and becomes unfit for use on the sukka.
Flax – Linum usitatissimum. It is an erect annual plant growing between 30 and 100cm tall, with slender stems. The flowers are pure pale blue, 1.5-2.5cm diameter, with five petals. The fruit is a round, dry capsule from which oils are derived. Flax is one of the oldest cultivated crops on record; its growth is mentioned in ancient Egypt. Today it is cultivated mainly in tropical areas.
The main product of flax is the fibers from which linen is made. Flax fiber is extracted from the bast or skin of the stem of flax plant. Flax fiber is soft, lustrous and flexible. It is stronger than cotton fiber but less elastic. It is removed via a lengthy process whereby the plant is dried out and then soaked until almost rotten. At that point they are once again dried out and the fibers combed out.
While all agree that shushei can be used on the sukka, there is a disagreement between Rav Yehuda and Abaye regarding shvatzrei, as Abaye is concerned that the strong smell will drive people from the sukka.
Shushei is, apparently, from the glycyrrhiza family, whose sweet roots are the source of licorice today. These are short, annual plants with leaves and bluish flowers. This plant grows in Israel and Babylonia in wet areas and is used both in medicines and confectionaries.
Shvatzrei has been identified as artemisia, also known as wormwood, which are shrubs that have small, hair-like leaves that are a whitish-grey color. The wormwood has a bitter taste and a strong smell. It is used in medicines, as well as being an ingredient in certain types of wine. It can be easily understood how the smell of this plant could cause discomfort to people sitting in a sukka that has it as skhakh.