What meanings different plants have.
The rowan is a very common kind of holy tree on yards. Not a single twig was allowed to cut from such a rowan. In Kainuu, the roan and the spruce were the most common type of holy trees. Sacrifices were given to the yard's rowan. In South Savo, a rowan which grew from the crack of a stone, Pihlaskivi, was thought to be the dwelling place of a god.
A rowan branch with berries is a good decoration during weddings, and in grain drying and threshing cabins, because demons are afraid of rowans.
An old Finnish riddle states Pyhä on pihlaja mäellä, pyhä marja pihlajassa "The rowan on the hill is sacred, there is a holy berry in the rowan", the answer to which is a pregnant woman. Based on this, Martti Haavio suggested that the rowan is not holy at all. Uno Harva questioned this, because even if it would imply that any pregnant woman is holy, the rowan has still had a special meaning.
Some researchers have thought that rowans are related to a goddess named Rauni (from Old Norse *raunir "rowan"). However, there is no evidence of the existence of such a goddess. Rauni is related to Röönikkä, which appears as one of the names of the fertility god in runosongs. Forest Finns songs state that Ukko-Röönikkä-Visakanta tried to slay a giant pig, but got scared and "jumped into a tree". Karelian runosongs state that Ukko-Palvonen-Virokannas tried to slay a giant ox, but got scared and "jumped into" various trees such as a spruce. One version says that "Pivari jumped to the end of a rowan" or "Demon jumped to the end of a rowan", though this makes little sense as demons are afraid of the rowan. Demon, piru, might have only made its way into the runosong because it shares alliteration with pihlaja "rowan". Kaarle Krohn believed that "Pivari" was then this mysterious goddess, but I am still of the opinion that it's just a Karelian name for the fertility god.
Fun fact: Louhi is called Raana, Raani, Raino, Raano, Ranu, Runo, Rania "used item; useless person; tatter" in Karelian and Savonian runosongs. IF Rauni was the name of an earth related goddess, that goddess would have to be Louhi. (I still Ukko's wife is Päivätär).
Raunikki and raunukka in Finnish refer to the plants pepperwort and gypsophila, which has for some reason been dismissed in the common Rauni goddess theories.
Puffball mushroom is also called ukontuhnio, ukkosentuhne, and ukonsieni "Ukko's mushroom". The Sámi of Sweden have also referred to this mushroom in connection with their equivalent of Ukko, Áddjá. This mushroom's decayed dust has been used in magic.
Tower rockcress, treacle-mustard, wild radish and turnip rape, all being plants with yellow flowers, have also all been referred to as ukonnauris "Ukko's turnip", ukkosennauris "thunder's turnip", ukonistukas, ukonkaali "Ukko's cabbage", pitkäsenpalko "Pitkänen's pod", and pitkäsenkylvö "Pitkänen's planting". (Pitkänen being a name of Ukko and also meaning lightning.)
I have no source for this but I believe that ukontulikukka "Ukko's fire flower", or the great mullein with its yellow flowers, is a part of the same family.