grass, thatch
茇 centers on plant material used for covering: grass or thatch. The meanings are closely related, with thatch being a specific application of grass.
茇 combines 艹 (grass) with 犮, which likely contributes the sound. The character is associated with grass or thatch, but the exact historical development is uncertain.
The top 艹 is grass, and the bottom 犮 looks like a bundle of stalks tied together. Picture grass bundled and laid over a roof as thatch.
For ハツ, imagine a hat made of thatch: hat -> ハツ, and the grass top keeps your head covered.