Grass Seeds trip people up because chopping regular grass gives you nothing: you're meant to look up. They drop from one specific plant, and you'll need them for the Omni-Tool shovel. Here's exactly what to harvest and where.
Seeds come from Grass Flowers only, special stalks that stand taller than the surrounding grass, with a distinct pointed seed cluster at the very tip. Once you've spotted one you'll see them everywhere; the habit to build is looking up, not down.
The most reliable early source is right outside the Snackbar Ranger Outpost, convenient since you'll be passing through it anyway.
Grass Flowers spawn throughout the map wherever there's tall grass. Once you can recognize the seed-head silhouette, you'll never be short.
Two main jobs:
Kitting out the Omni-Tool? Here's the rest of the backyard lookup list.