Buy RS3 Onion seed
Onion seeds are a low level Farming seed in RuneScape 3. You can plant them in any allotment patch once you reach level 5 Farming. Each patch needs 3 seeds, and the onions take about 40 minutes to grow.
Planting onion seeds gives 9.5 Farming XP, and each onion you pick gives 10.5 XP more. Every patch has at least 3 harvest lives, and compost can raise that to 6 with ultracompost. As your Farming level goes up, you also get a better chance to pick onions without using a harvest life.
You can keep your crop safe in two ways. Pay a farmer 1 sack of potatoes to watch the patch, or grow a marigold, white lily, or starbloom in a flower patch next to it. These plants stop your onions from getting disease.
Onion seeds are also used in the Garden of Tranquillity quest, so they are handy to keep around. The seeds are stackable and tradeable, so they take up little space in your bank. They are a simple and cheap way to train early Farming levels in RS3.