How Spacing Directly Controls Onion Bulb Size
For most vegetables, spacing is about airflow and avoiding root competition. For onions, it's also a direct sizing dial: University of Maryland Extension's tiered guidance -- 2 in. for green onions, 4 in. for moderate bulbs, 8 in. for large bulbs -- means you can genuinely target a harvest size by choosing a spacing distance, not just by choosing a variety.
But wider isn't simply "better." Cornell Cooperative Extension field research comparing narrow (4 in.) to standard (6-8 in.) in-row spacing on plastic-mulch beds found that "wide plant spacing produces big bushy plants with more leaves, thicker necks, delayed maturity, and bigger bulbs" -- and that those bigger bulbs also had a higher rate of bacterial bulb rot in storage. Their narrower-spacing plots produced fewer of the biggest "colossal" bulbs but significantly more healthy, storable jumbo-sized ones. If you're growing onions to store rather than to win a size contest, that's worth knowing before you space for maximum bulb size.
Green onions (scallions) are a genuinely different crop from a spacing standpoint, not just a smaller version of the same one. University of Minnesota Extension gives scallions their own figure -- thin to one plant per inch -- tighter than any bulb-onion tier, since the whole plant is harvested young before bulbing matters.
Why Spacing Matters for Onions
Because onion spacing is a sizing lever, getting it wrong doesn't just crowd the bed -- it works against whatever you're actually growing the onions for. Space for large bulbs when you wanted a steady supply of scallions, and you'll get far fewer plants than you needed; space for green onions when you wanted big storage bulbs for winter, and none of them will size up.
The rot-risk tradeoff from Cornell's research adds a second layer most spacing charts don't mention: the widest spacing that produces the single biggest onions isn't automatically the spacing that produces the best storage crop.
