Planting a seed in a plant pot requires the appropriate Farming level required to plant the sapling in the relevant tree patch. You cannot assist a player to help them plant a seedling that is higher than their Farming level, although temporary skill boosts like a Garden pie do work.
Seedlings may be deposited into a bank and grow into saplings while stored. If unable to find a seedling that you have deposited, check Tab 1. Saplings revert to the top tab if that type of sapling isn't already present elsewhere in the bank, and stack in the bank although not in inventory.