Onion Bulb Size Response to Plant Population for Four Varieties under Three Irrigation Systems

Changing market opportunities for smaller size onion bulbs and the availability of new onion varieties, necessitate evaluations of yield and bulb size response to plant population for onion production in the Treasure Valley of eastern Oregon and southwestern Idaho. These evaluations can aid growers in making planting rate decisions.  The objective of this trial was to evaluate the response of four onion varieties to four plant populations under “conventional” drip irrigation, “intense bed” drip irrigation, and furrow irrigation. Conventional drip irrigation had 4 onion double rows and two drip tapes on a 2.2 m bed.  Intense bed drip had 6 onion double rows and three drip tapes on a 2.2 m bed.  Furrow irrigation had 4 onion double rows and two furrows on a 2.2 m bed.  Four onion variety split plots were planted in each irrigation plot.  Each variety split plot was thinned to four plant population split-split plots (296,000, 395,000, 494,000, and 593,000 plants per hectare). Irrigation system was not a statistically significant factor in the response of bulb size to plant population. Averaged over irrigation systems and varieties, marketable yield increased with increasing plant population over the full range of populations tested. Averaged over irrigation systems and varieties, yield of bulbs larger than 10 cm in diameter decreased and yield of bulbs smaller than 7.6 cm in diameter increased with increasing plant population. Yield of bulbs 7.6 to 10 cm in diameter increased or decreased with plant population according to the variety. Optimal population varied with growers’ marketing opportunities, with lower populations best for onion rings and populations with 400,000 plants or more per hectare when a mix of medium and jumbo onions are to be marketed.