Hello Tom,

Trimming onion tops during the growing season can have the following effects:

Encourages stronger root systems when onions develop their first genuine leaf.

Makes plants stockier and more vigorous at the time of planting.

Allows for harvesting bulbs when the neck dries and the tops fall over.

Curing is essential for storing onions.

Avoid damaging plant roots by removing the tops and roots.

Extension resources at the links below:

Yard and Garden: Planting and Growing Onions | News

2021 Considerations: Optimizing Onions

Planting, growing and harvesting onions | MU Extension

Growing onions in home gardens | UMN Extension

how-to-grow-onions-guide.pdf

You're asking a great question—and you're not alone in wondering whether trimming onion tops is helpful or harmful. The short answer? It depends on the timing and purpose.

Trimming Seedlings Indoors (Before Transplanting) According to the University of Minnesota Extension, trimming onion seedlings indoors to about 4 inches when they develop their first genuine leaf can:

Encourage stronger root systems

Prevent legginess or spindly growth

Make transplanting easier

This practice is common among both commercial and home growers, especially when starting onions from seed indoors. More information at this link:

2021 Considerations: Optimizing Onions

Trimming During the Outdoor Growing Season: Once onions are transplanted and growing in the garden, routine trimming of the tops is generally not recommended by university extension services. Here's why:

Photosynthesis: As you rightly pointed out, the green tops are the plant’s solar panels. Cutting them reduces energy production, which can limit bulb development.

Disease Risk: Fresh cuts can indeed become entry points for pathogens, especially in humid or wet conditions.

Bulb Size: While some anecdotal sources claim trimming encourages larger bulbs, research-based guidance doesn’t support this as a consistent or necessary practice.