When hoeing weeds and grass do not chop too deeply. You may be cutting onion roots. Always hand pull weeds when possible. When onion plants have 5-6 leaves, apply fertilizer again to help grow larger plants and bigger bulbs. Each leaf forms a ring in the onion bulb.
More leaves mean more rings and larger onion bulbs.
Use about 1.2 cup of fertilizer for each 10 feet of onion row. Scatter the fertilizer evenly between the rows. Water after adding the fertilizer. See figure 2. Insects and Diseases Onions do not have many insect problems.