Onions are terrestrial plants, but they can grow in water. The outcome depends entirely on the method used. A simple glass of water is effective for a quick, short-term harvest of greens from kitchen scraps. Growing a full, mature onion bulb, however, requires a specialized, long-term approach known as hydroponics. The simple water method cannot provide the necessary nutrients for substantial bulb formation, meaning the feasibility of growing onions without soil ranges from an easy kitchen experiment to a controlled horticultural endeavor.
Quick Regrowth: Harvesting Green Scallions from Scraps
The simplest way to grow onions in water is by using the root end of a scallion or green onion scrap. This technique relies on the plant’s natural ability to regenerate leaves from stored energy, not on creating a new, large bulb. Select the bottom one to two inches of the scallion, ensuring the root plate is intact. Place these scraps root-end down in a shallow container with just enough water to submerge the roots and the bottom of the white section. Placing the container on a sunny windowsill provides the light necessary for photosynthesis, and new green tops will often sprout within a few days.
This process is a temporary propagation method, as the plant has a finite amount of stored energy in the scrap. While greens can be harvested multiple times, regrowth will eventually slow down, and the flavor will diminish. This occurs because the plant lacks a continuous source of external nutrients. The goal is a fast, temporary yield of green tops, not the development of a full, mature onion bulb.
Advanced Hydroponics: Developing Full Onion Bulbs
To successfully grow a full, mature onion bulb in water, a simple jar setup is insufficient; the process requires advanced hydroponic techniques. Bulb development is a long-term project that demands a controlled environment and specific nutrient delivery. Systems like Deep Water Culture (DWC) or Nutrient Film Technique (NFT) are commonly used to suspend the onion sets or seedlings above a reservoir.
The necessity of a balanced nutrient solution is the primary difference from the scrap method. Water alone cannot sustain the plant through the 80 to 90 days required for full bulb maturity. The solution must contain elements like phosphorus, nitrogen, and potassium, with nitrogen levels managed to promote bulb growth over excessive foliage. Onions also have a precise light requirement, known as photoperiodism, which triggers bulb formation.
Onions are classified as short-day, intermediate-day, or long-day types, requiring 10 to 14 or more hours of light per day for a bulb to form. This light must be provided consistently using specialized grow lights in an indoor setup. The bulb itself must be kept dry and above the water line, with only the roots submerged in the nutrient solution, to prevent rotting and encourage proper development.
Maintaining Health and Preventing Water Rot
A continuous supply of oxygen to the roots is necessary for any onion grown in a water-based system, as stagnant water leads to oxygen deprivation and root rot. This condition, often caused by water molds like Pythium, presents as slimy, brown, or foul-smelling roots and can lead to plant death. Preventing this failure requires diligence, whether using a simple jar or a complex hydroponic system.
In the simple scrap method, changing the water every two to three days is necessary to replenish the dissolved oxygen and flush out any decaying matter or bacteria. Keeping the container clean and free of algae also helps to maintain a healthy environment for the roots. For advanced hydroponic setups, aeration is achieved by using an air pump and air stone to continuously bubble oxygen into the nutrient reservoir.
The temperature of the water is a factor in preventing rot, as warmer water holds less dissolved oxygen and encourages the growth of harmful pathogens. Maintaining the water temperature below 75°F, ideally between 65–70°F, helps prevent rot. Regular monitoring and sanitation of the growing environment ensure the onion’s roots remain white, healthy, and functional.