evergreen oak, (kokuji)
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樫 centers on a specific type of oak tree: the evergreen oak. It is a kokuji created in Japan to name this tree.
evergreen oak
Quercus gilva (species of oak)
cork oak
ubame oak (Quercus phillyraeoides)
ubame oak (Quercus phillyraeoides)
oak flour (esp. as animal food); acorn flour
large evergreen oak
quercus myrsinifolia; bamboo Leaved Oak; quercus myrsinaefolia
ring-cupped oak (Quercus glauca); Japanese blue oak
Quercus sessilifolia (species of oak)
Japanese evergreen oak (Quercus acuta)
black oak
樫 is a kokuji, a character created in Japan. It combines 木 (tree) with 堅 (hard, firm) to represent the hard wood of the evergreen oak.
The tree radical 木 beside 堅 (hard) points to a tree with hard wood: the evergreen oak.
For かし, imagine a sturdy oak tree that you can 'cash' in for its valuable hard wood: cash -> かし.