Boost the flavor of your favorite dinner dish with onions, a pungent and sweet herb that grows a large and juicy bulb underground. Encourage the underground bulb growth by keeping the onion's foliage trimmed. This helps divert the plant's energy and nutrients toward bulb development rather than growing large leaves or flowers. Trim regularly throughout the onion plant's lifetime until harvesting.
Set a ruler in a vertical position against the onion with one end resting on the base of the onion where the plant enters the soil.
Measure 4 inches from the onion's base and mentally mark where this is on the plant.
Grasp the onion leaves together. Use sharp scissors and trim the onion plant at the 4-inch mark. Discard the cut leaves or throw them into your compost pile.
Repeat the trimming process as needed to keep the onion plants at a height of 4 inches until harvesting. Onions are ripe and ready for harvest when 25 to 50 percent of its tops have wilted and the onion bulbs have grown a paperesque skin.