This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
藂 combines the grass radical 艹 with 聚 (gather, assemble). The exact historical development is uncertain, but it likely relates to plants growing in clusters.
Grass 艹 on top of 聚 (gather) suggests many plants clustered together, like a thicket or tuft.
For ソウ, imagine a dense clump of grass growing so thickly that you need to sow (ソウ) seeds to thin it out.