Skip to Content

Why Do My Onions Have Flat Bottoms

Gardeners growing their own fruits and vegetables know that sometimes you get a few veggies that do not look picture perfect. It is normal to have a few plants that look a bit strange. Onion plants are no exception to this rule, and occasionally gardeners will wonder: why do my onions have flat bottoms? 

Overcrowding, lack of nutrients, and soil that is too dense can all potentially cause onions to produce flat bottoms. The variety fo onion is also a factor.

In this article, we will describe all the potential causes of flat bottoms in onions that are supposed to be round. Additionally, we will also be explaining what onions have flat bottoms naturally. Alright, let’s jump right into it! 

Are There Certain Types Of Onions That Are Naturally Flat Bottomed

Like with most other types of plants, different species of onions come in various shapes, colors, and sizes, which means that there are onion varieties that are actually supposed to have bulbs with flat bottoms. Here are some of the most common types of onions that can have flat bulbs. 

The Stuttgarter Giant Onion Tends To Be Flat At One End

The Stuttgarter Giant onion is a large onion that will often have one side of its bulb that is flattened and the other side that is slightly domed shaped. This is a popular variety of sweet onions due to their mild flavor and how easy it is to grow. Stuttgarter Giants tend to do best in full sun with well-draining soil that also has a neutral pH. 

The Cipollini Onion Has A Very Flat Shape 

Cipollini onions hail from Italy and have a unique bulb shape. The bulb of the Cipollini onion has a flat top and a flat bottom, with rounded sides. These onions almost appear like someone took a round onion and squashed it down. These onions are small, fairly easy to grow, and have a lovely sweet flavor. Cipollini onions grow best in moist, mildly acidic soil and full sun. 

The Vidalia Onion Can Appear Flatter Than Usual At The Ends 

Vidalia onions will often have a part of their bulb that is flattened, and the other end is slightly domed-shaped. The Vidalia onion is famous for its sweet flavor, making it a favorite among chefs throughout the United States. Interestingly, Vidalia onions can only be grown in certain areas within the state of Georgia because it is an area that uniquely has no sulfur in the soil. 

Causes Of Flat Bottoms In Onions

Several factors could potentially lead to flat bottoms in onions that shouldn’t be flat. These things mainly include issues with soil, fertilizer, or overcrowding. Here are the three possible reasons why your onions came out of the ground with flat bottoms. 

Your Onion Plants Could Have A Nutrient Deficiency If They Are Flat

Nutrient deficiencies in the soil can lead to onions having growing problems. Unfortunately, this also means that onions can grow bulbs that are smaller and flatter in shape than they should be. The best fertilizers for onions are those rich in nitrogen, phosphorus, and potassium. 

Your Onions Could Become Deformed Due To Overcrowding 

If your onions have little space to grow properly, then it is likely that you will get onions that appear to be unusually flat and small. These types of deformities occur because there is no room for the onions to grow out their bulbs properly. For most onion types, it is recommended to plant your onion plants six inches apart to prevent these kinds of issues. 

Onions Can Become Deformed If The Soil Around Them Is Packed Too Tightly

Like overcrowding, planting your onions in soil that is too dense could lead to flat onion bulbs because onions cannot grow as easily in dense soil as they can in looser soil. This leaves the onions with a similar squashed and small appearance to overcrowded onions. 

Why Do My Onions Not Bulb Up

In addition to factors that can cause deformities in onions, some things could potentially cause bulbing problems in onions altogether. This could leave you with an onion bulb that is very tiny and flat or an onion plant that will not bulb at all. Here are some factors that could cause onion plants not to bulb up as they should. 

Onions Planted In The Wrong Climate Will Not Bulb Up 

The most common reason onions are not bulbing is that they are planted in the wrong climate. As a result, taking the time to ensure that a species of onion will thrive in your climate will end up saving you a lot of stress. Not to mention, you will likely get a better harvest as well. 

Issues With Planting Can Cause An Onion Plant To Not Bulb

Another possible cause for onions not bulbing is that the onion seedlings are not planted in the ground deep enough. When this happens, the plant will not have enough room to form a bulb, especially if the soil is very dense. It is best to plant onions about one inch deep into the soil. 

Pests Can Cause Issues With Bulbing In Onion Plants

Pests can also cause growth and development problems in onion plants, which can lead them to not bulbing properly. This is especially true when pests like worms, larvae, and mites eat your onion plants. This can prevent bulbs from growing at all if they haven’t gotten the chance to develop yet, and damage from pests can also lead to stunted growth in onion plants.