exuberant and vigorous foliage
芊 centers on lush, thriving plant growth: foliage that is exuberant, vigorous, and densely flourishing.
芊 combines the grass radical 艹 with 千, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound related to plants.
The grass radical 艹 sits above 千 (thousand). Imagine a thousand blades of grass growing together in a dense, vigorous patch of foliage.
For セン, picture a thousand (千, せん) lush plants spreading across a field: 千 -> セン, and the foliage is so thick it looks like a thousand leaves.