How to Grow an Onion in a Jar
Although an onion usually requires soil to produce another onion, you can grow an onion plant with nothing more than a jar and a little water. This method of growing plants in water is known as hydroponics. It is a fun experiment to try during the winter when it is too cold for planting outdoors, and the shoots that sprout from the onion are edible as well.
Select a firm onion that has a good root system. The root system or nib of the onion is located on the bottom of the onion.
Fill an empty pickle or Mason jar three-fourths full with water.
Stick one toothpick into each side of the onion. Insert each toothpick 1 inch into the middle part of the onion. The toothpicks hold the upper part of the onion out of the water.
Put the onion bottom into the water with the toothpicks resting on the edge of the jar. Place the jar in a warm location near a window where it receives indirect sunlight. Shoots from the top of the onion will appear in approximately five to seven days.
Grow Good Onion Heads
Loosen the soil with a hoe in a bed that gets full sun and has well-draining soil. Do this in spring after the soil dries and the average temperature is above 50 degrees Fahrenheit. Plant onion sets 1 inch deep. Transplant indoor-started seedlings at the same depth they were at previously. Too much water can cause the plants to rot, while too little stresses the onions and may cause the foliage to die back before full bulb formation. Harvest the onions only after the yellowing foliage begins to fall over.