Before discussing how to collect onion seeds, have to first define what type of bulbs you are collecting. Most of the bulbs collected from commercial seed production facilities are "hybrid" ones, meaning the bulbs are a cross between a hybrid parent variety chosen specifically for certain characteristics. If you are growing your own garden, be sure to be patient with these bulbs when you are getting started. Some bulbs will flower more profusely than others and some will flower more intensely than others. The key is in the time you spend observing each plant and deciding which one is most suitable for your garden needs.
For harvesting onions, you will want to put your attention primarily on the "big bulbs" which are usually sold in bunches. When you look at the flowers of these bulbs under a bright light, you will see an abundance of beautiful colors ranging from lavender to pink to red to yellow-green. Collect as many of these bulbs as you possibly can.
Once you have harvested all the bulbs from your chosen plants, then you will have to set them aside. It is very important that you only plant bulbs into a container designated for onion seeds. Do not plant bulbs into an open area where they could become exposed to harsh elements such as rain or strong winds. Keep in mind, too, that if you plant your bulbs into a container for planting on your deck, patio or in your yard, you must have a well-drained container to ensure that your bulbs don't dry out. Do not let your bulb dry out for very long or you will risk the possibility of losing them to frost.
When you have harvested all of your bulbs for planting, then it is time for you to begin sowing them. If you are planting single-celled onions, then you should leave the root system intact and simply pick the bulb(s) up and put them into your garden. For multi-celled onions, the root system will need to be removed and the bulbs disassembled. There is really nothing much different with regards to the sowing process; it's simply a matter of preparing the soil for the actual planting. After the soil has been prepared, you can then begin to place your bulbs in the holes and water them with a watering can. Once the bulbs have been planted, you will simply need to monitor their growth and harvest them at the right time.
Some onion seeds will be smaller than others. The ones that are smaller, usually referred to as mini-onions, will be able to grow faster than other types. If you're planning on harvesting these onions before they have a chance to sprout, then you'll want to wait until they are approximately four to five inches in height. Typically, this is about the size change that you see when you cut an onion open. Once the mini onion seeds are fully grown up, you can then remove them from the plant and place them in your garden.
Some onion seeds will come through the plants' leaves and grow down through the stems before setting seed in the ground. These larger varieties, also called full-grown sets, can sometimes take two or three years before they sprout. You should be prepared for this, because some of these sets may never produce any real fruit. On the other hand, most full-grown sets can usually produce several fruits during their first year, allowing you to easily pick your preferred varieties.
Once your chosen varieties have sprouted and established, it's time to move them to their proper potting soil. You can do this by either purchasing a potting soil that you can purchase in bulk, or by digging a hole that is three to four feet deep and scooping out the existing potting soil. It is important that you don't plant your seedlings directly into the cold, wet weather that typically accompanies winter in northern climates; it would be unwise to plant on such a weatherless site.
After planting your bulbs in the fall, you will have to keep them well watered until they begin to produce. Be sure to mulch around your seedlings once they appear; this will help protect them from pests. In addition, after your bulbs begin to sprout their first set of leaves, immediately place them into a shallow glass of water with the drainage hole turned away. The reason for this is to ensure that the plant has sufficient moisture in order to form a strong root system. It will probably take about four weeks before your bulbs develop their leaves; if you watch closely, you will even be able to watch them change color as they grow.