Why Are My Onions Splitting in the Ground?
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Why are my onions splitting in the ground? A split onion bulb is one where the bottom of the bulb opens up while the bulb is still growing. Two sources we opened address this problem directly: an Oregon State University field study on late-season water stress, and a bulb disorders page from India’s National Horticulture Board. They point to different causes, and neither one reduces the problem to a single fix. This guide sets out what each source reports and where the evidence is thin.
Quick answer
- Split bulbs are a bulb disorder in which the bottom of the bulb opens, and they can lower the marketable yield of a crop, according to the Oregon State study.
- The National Horticulture Board page lists adverse climatic conditions and an imbalanced nutrient supply as causes of splitting and doubling.
- Mechanical injury during hoeing and weeding can start new growth and cause splitting, according to the same Indian source.
- The Oregon trial found a late-season irrigation interruption had mixed effects, and it reported that split bulbs were very rare overall.
What split onion bulbs look like
The Oregon State study defines the problem plainly: “Split onion bulbs is a phenomenon where the bottom of the bulb splits open.” The study says split bulbs can reduce onion crop marketable yields. In its trial, split bulbs were counted and weighed separately from double bulbs and from bulbs with rot, so the researchers treated splitting as its own grading category.
Splitting is related to, but different from, doubling. The National Horticulture Board page groups them together, saying that “Adverse climatic conditions and imbalanced nutrient supply leads to splitting and doubling of bulbs.” The two are listed as disorders of bulb development, and the sources do not describe a shared mechanism beyond that.
Late-season water interruptions
Gardeners often suspect irrigation. The Oregon State study notes that “Anecdotal evidence suggests that late-season irrigation interruptions can cause bulb splitting.” The researchers set out to test that claim at the Malheur Experiment Station in Ontario, Oregon. They imposed a temporary water stress in late July and early August, by holding back irrigation until the soil water tension reached a set level, and then they resumed irrigation.
The result did not confirm a simple cause-and-effect pattern. The study reports that “Both the percentage and yield of split bulbs was very low in this trial.” For one variety at the 330,000 plants per acre population, the percentage of split bulbs rose under water stress. For another variety at that same population, both the percentage and yield of split bulbs fell under stress. So water stress can change splitting, but the direction of the change depended on the variety and the plant spacing.
For a basic explanation of how soil moisture moves and is held, the explainer on how capillary action moves water through soil describes why soil can dry unevenly around roots. For the light and bulb formation side of the same crop, see why onions need full sun.
The trial measured soil water in a practical way. The authors describe four granular matrix sensors installed at 8-inch depth in the center of each double row, with irrigation decisions made every 12 hours from the average reading of those sensors. The water stress period ended on August 12 for four plots and on August 17 for the last two. Those details matter when comparing the study with home gardens, because a bed watered by hand or by a timer has no equivalent sensor control.
Nutrients and climate
The National Horticulture Board text links splitting to climate and nutrition. It says that “Adverse climatic conditions and imbalanced nutrient supply leads to splitting and doubling of bulbs.” The page does not give a list of climate thresholds or nutrient ratios, so it cannot tell a gardener which nutrient is out of balance in a particular bed. A soil test is the usual way to find out whether the nutrient supply is off, and the local extension service can explain how to read the results for onions.
Climate is a broad cause. The same page does not say which weather pattern is most likely to split bulbs in a given region, and the sources here do not quantify any climate effect, so this guide does not give a number for it.
Injury from hoeing and weeding
Physical damage is the third cause that the National Horticulture Board page lists. It says that “Mechanical injury to plant during hoeing and weeding initiates new growth and causes splitting and doubling of bulbs.” The page says the injury is what starts the new growth, so the damage is treated as the trigger for splitting and doubling.
In practice this suggests weeding carefully near the bulbs and avoiding tools that strike the plant base. This is a precaution drawn from the Indian source, not a result from a trial, so it should be read as a general point rather than a measured effect.
What we could not verify
What the sources do not say
The Oregon State trial was run in one location, in one season, with a set of onion varieties and plant populations specific to that station. It does not say whether the results would hold in cooler or wetter regions, or in home gardens with different soil and spacing. The National Horticulture Board page does not give a numerical threshold for water, nitrogen or any other nutrient, and it does not describe how the disorder was scored.
The sources do not rank the causes of splitting for home gardens. They also do not say whether a particular onion variety is more prone to splitting than another, aside from the Oregon varieties in the trial. Gardeners who need a firm answer for their own bed should check with a local extension office, which can look at the soil, the variety and the weather history together.
How we chose
We selected the two sources because they address splitting directly. One is a field study from Oregon State University’s Malheur Experiment Station, and the other is an onion disorders page from India’s National Horticulture Board, a government body. We opened both documents on 2026-10-29 and used only statements found in the fetched text. We did not use retailer pages, seed catalogs or forum posts for this article.
Frequently asked questions
Is splitting the same thing as doubling?
No, although the National Horticulture Board groups them together as disorders of bulb development. The Oregon study counts split bulbs and double bulbs as separate grading categories.
Can late irrigation cause onions to split?
The Oregon State study says that anecdotal evidence links late-season irrigation interruptions to splitting, but its own trial found that split bulbs were very low and the effect changed with the variety and plant population. It does not prove a general rule.
Does weeding close to onions cause splitting?
The National Horticulture Board page says that mechanical injury during hoeing and weeding can start new growth and cause splitting and doubling. Shallow weeding away from the bulb base is the practical response that follows from that statement.
Do nutrients cause onion bulbs to split?
The Indian source lists an imbalanced nutrient supply as one cause. It does not say which nutrient, or how much, so the sources here cannot identify a specific deficiency or excess.
Does the Oregon study show one cause for splitting?
No. The study found that splitting was very low in its trial, and the effect of water stress depended on the variety and the plant population. It points to variation rather than a single cause.
