Sansai are natural and unimproved plants. Therefore, many of them have peculiar smell. And they have harsh taste(Aku[灰汁（アク）] in Japanese) for alkaloid, so it is need to remove them in advance. This process is called "Aku-nuki[灰汁抜き（アクぬき）]" in Japanese.
In spite of them, many Japanese like the unique flavor and enjoy the season by eating Sansai. Now, they are also cultivated and sold at supermarkets and department stores in the season but natural Sansai is sold at high price. Some people go taking Sansai in the mountain during the season of them and there are some people that are lost and die. And some species resemble poisonous plants, therefore it is need to be careful.
There are many species and many of them grow in spring.
Fukinotō[ふきのとう] is one of the earliest harvested Sansai. In the mountains, they appear in the period that snow starts melting. Fukinotō has a little bitter taste. It is a bud of Fuki[蕗（フキ）] and the stem is also one of Sansai.
From the end of April to May, you can harvest Kogomi[コゴミ]. Kogomi, which is suitable for eating, is a thin stem and top of that is spiraled. Kogomi does not have peculiar smell and has few harsh tastes, so, not to need Akunuki. Kogomi is eaten as Tempura or boiled that and dressed with seasonings.
After the season of Kogomi, you can harvest Zenmai[ゼンマイ] & Warabi[蕨（ワラビ）] which are the most popular Sansai.
Zenmai has spiraled stem like Kogomi. Spiral spring is the same name "Zenmai" in Japanese, it derive from this plant because of similar shapes. Zenmai is simmered with soy sauce, Sake, Mirin[味醂（みりん）](Sweet Sake).
Warabi is one of the representative Sansai. Warabi is eaten same as Zenmai that I mentioned above. And its pickle is also popular.
Udo[ウド] is popular and is often cultivated in the dug hole under the ground(Tokyo Udo which is cultivated in the suburb of Tokyo is major brand.). It is strong and unique smell which is loved by some and hated by others. It is eaten as tempura or boiled that with vinegar & Miso[味噌（みそ）].
Taranome[タラの芽（タラのめ）] is a bud of Taranoki[タラの木（タラのき）]. It is a spiny tree. Therefore, it is need to be careful when you pick them. Taranome is eaten as Tempura. It's a little bitter and it goes well with beer.
Nemagar-dake[ネマガリダケ] is Takenoko[筍（タケノコ）] of Chisima-zasa[千島笹（チシマザサ）]. it's crispy & good texture. It is used the topping of Sansai-soba[山菜蕎麦（さんさいそば）](buckwheat noodle with Sansai) with Warabi & Nameko[ナメコ](It is a kind of mushroom).
The bud of Koshiabura[コシアブラ] is not major than other Sansai but it is sometimes called "princess of Sansai". Koshiabura is a plant of the family Araliaceae like Udo and Taranoki. It has fresh scent like Mitsuba[三つ葉（ミツバ）](Japanese honewort). That is mainy made into Tempura.