Even though the word is not originated from foreign word, names of daily necessities, vegetables, fruits, plants and animals are often written in katakana. Especially when its kanji is relatively difficult and takes time to write. Still it's completely fine to use kanjij or hiragana.
Example:
rose=バラ, ばら, or 薔薇
salmon＝サケ、さけ、鮭
This is more like a custom than a rule. I don't know the exact reason.
So the explanation below is merely my opinion with no evidence.
When we read Japanese, we unconsciously recognize hiragana parts(such as particles, verb endings etc.) as less important because they carry only grammar, not actual tangible meaning; and kanji parts as more important because they carry actual meaning.
If a word, let's say 茸（きのこ） is written in hiragana, it's hard to recognize the word in one look because it's 'buried' onto other hiragana parts. That's why katakana is used instead of kanji, I think. Katakana stands out among hiragana.
Note: In academic research papers, plants and animals names have to be written in katakana. This is a solid rule.
