The harvest chance mechanics are tied directly to your alchemy skill value but theres something interesting about the timing - even at max skill youll never get guaranteed harvests from regular plants in the wild. Watch for the "find nothing of use" message, which triggers based on a hidden roll against your skill level especially during early game gathering runs. For better consistency, you might want to focus initially around frostcrag spire's garden where the plants actually regrow on a timer, making repeat harvests more reliable even with the standard success chance rolls.

The special plants like nirnroot work differently though - these glowing guys are always harvestable when you spot them but they're one-time grabs and dont respawn. Worth noting that the timing of your harvest attempts doesnt matter at all, the success roll happens instantly when you activate the plant. Your best approach early on is focusing herb gathering around cities and settlements, where plant density is higher, letting you maximize successful harvests per time spent even with the occasional fails. Once your alchemy hits around 50, the success rate becomes much more manageable but remember, those random fails will still happen no matter what.