lotus tree, nettle tree, hackberry
榎 refers to a type of deciduous tree, specifically the lotus tree, nettle tree, or hackberry. The core idea is a particular tree species, not a general concept.
榎 combines 木 (tree) with 夏, which likely contributes the sound カ. The character was created to name a specific tree, the hackberry or nettle tree.
A tree (木) that is associated with summer (夏) because its leaves provide shade during the hot season: the hackberry or lotus tree.
For カ, imagine a car (カ) parked under a large hackberry tree for shade in summer: car -> カ.