What spacing and irrigation does onion (pyaaz) need for good bulb size?

Use close, uniform spacing of about 15 cm between rows and 10 cm between plants for marketable medium to large bulbs; too wide gives oversized bolters and doubles, too close gives small bulbs. Onion has shallow roots, so it needs frequent light irrigation rather than heavy flooding. Drip irrigation gives the best results, saves water, keeps the surface moist, and reduces disease compared with flooding. Critical stages for water are establishment, the rapid leaf growth phase, and bulb development. Most important, stop irrigation 10 to 15 days before harvest so the necks fall and bulbs cure for storage. Over irrigation, especially late, causes thick necks, splits, and poor keeping quality.