High mowing suggests trimming onion seedlings when they reach 5" to make them grow thicker and stronger. We did not remember to trim them while they were growing under the lights, but decided to try a side-by-side comparison with some plants trimmed just after transplanting them in the garden.
Many of our transplanted walla-walla onions were very close to the recommended 5" height for trimming. I selected a 5×5 grid of the onions (out of the 14×5 grid of walla wallas planted) and trimmed the tops.
Below is a picture ot the allium bed before and after the trimming experiment — rows 3-7 from the back/right edge are the walla walla onions. Most of the walla wallas near the bottom of this picture were close to 5", so the grid of 5×5 walla wallas on the bottom was selected for the trimming experiment.
 Before trimming |
 After trimming |
Close up of one of the walla wallas before and after trimming — the leaf was trimmed from 7" down to a little over 1".
A few of the walla wallas in the 5×5 grid were almost double the height recommended for trimming because they had been seeded a week earlier. For the sake of uniformity, I trimmed these onions too, but left close to 3" of the onion top as opposed to 1-2" for the smaller ones. Below is an example of one of the larger walla wallas before and after trimming:
The trimmed onion tops caught up with the untrimmed tops quickly. By week 4, they were similar in size and it was difficult to tell them apart.
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Trimmed onions, Week 4 (May 20) |
Untrimmed onions, Week 4 (May 20) |
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Trimmed onions, Week 8 (June 17) |
Untrimmed onions, Week 8 (June 17) |
We harvested the onions on July 22. There still was no strong noticeable difference between the trimmed and untrimmed onions. Both conditions grew well and had good sized onions, but the trimming did not seem to give much, if any, advantage to the onions.
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Trimmed onions, harvest day (July 22) |
Untrimmed onions, harvest day (July 22) |
