There are many types of onions you can grow, but the most common is the “bulb” onion, sometimes called a storage onion. Grown from seeds, sets, or plants, bulb onions can range from sweet and mild (Vidalia, Walla Walla, Sweet Spanish), to pungent (Stuttgarter, Yellow Globe, Copra).
If you plan to grow your onions by seed in Pennsylvania, you will need to start them indoors by late January in order for them to reach a decent size for outdoor planting. Or, you can buy onion sets, and onion plants either online, or at your local nursery, to plant straight in your garden. Onions do great in the cold so you can plant them outside as early as March here, depending on the weather that year. As soon as you can work the ground, go for it.
As they continue to form their leaves or tops, a specific combination of daylight, darkness, and temperature is necessary to generate the bulb. The size of the mature onion bulb is dependent upon the number and size of the green leaves or tops. For each leaf, there will be a ring of onion; the larger the leaf, the larger the ring will be.
The confusing part is that each variety needs a particular combination of help. You don’t want the day length to trigger the bulb until there are many leaves of good size! For instance, a variety that needs many hours of summer light will not perform well in an area that receives fewer hours of light. To simplify this, onion growers have categorized onions in one of three ways; Short Day, Intermediate Day, and Long Day. Here in Pennsylvania, we can grow both Long Day and Intermediate Day onions successfully.
Failure to plant the right variety in your area can lead to disappointing results. One common problem for bulb onions is they sometimes send up flower stalks. In other words, they are going to seed. This can be caused by several things but usually the most prevalent is temperature fluctuation.
For example: An onion is classed as a biennial, which means that it takes 2 years to go from seed to seed. Temperature is a controlling or triggering factor in this process. If an onion plant is exposed to alternating cold and warm temperatures resulting in the plant going dormant, resuming growth, going dormant and resuming growth again, the onion bulbs can prematurely flower or bolt. The onion is deceived into thinking it has completed 2 years of growth in its biennial life cycle so it finalizes the cycle by blooming. This can be controlled by planting the right variety at the right time for our area.
Onions prefer loose, well drained soil that is high in fertility, slightly acidic, adequately irrigated, and exposed to full sunlight. The looser the composition of your soil, the larger your bulbs will grow. Animal manures and plant based composts make great additions. Also, adding some blood meal and bone meal to your soil will ensure they will start off with enough nitrogen (for leaf growth), and phosphorus (for root growth).
An onion typically generates a new leaf every 2-3 weeks. Remember, the size of the onion bulb is dependent upon the number and size of the green leaves or tops at the time of bulbing. For each leaf there will be a ring of onion. The larger the leaf, the larger the ring will be when the carbohydrates from the leaves are transferred to the rings of the bulb. The perfect onion has 13 rings, so the key is to generate as many large leaves as you can before the onion starts bulbing.
Contrary to what people may think, onions really are heavy feeders. Once they have become established (3 weeks after planting), they will require additional or extra nitrogen for leaf formation. Feeding or side dressing onions every 2-3 weeks with a good source of nitrogen (blood meal) is essential. Water after every application, because the only way for onions to take up the nitrogen is through the root system. When the bulbs start forming, stop the additional use of nitrogen, and switch to a naturally adequate supply of phosphorus (bone meal). This is important for good bulb formation. Fish Emulsion or Fish Meal, and Seaweeds are also great all purpose fertilizers for onions. They can be used occasionally in addition to the extra Blood Meal and Bone Meal. This will supply your onions with a small, but constant supply of Nitrogen, Phosphorus, and Potassium.
Weed suppression is critical for onions to grow properly – you can grow weeds, or you can grow onions, but not BOTH in the same spot! Mulching will help keep the weeds down and maintain good soil moisture during hot weather, but be careful during extra rainy days in PA, you don’t want the soil to become overly soaked. You can always pull your mulch back to dry it out a little.
There is never a time when onions aren’t ready for harvesting. They can be picked and eaten at any stage. However, it is really nice to have a supply of bigger onions stored away for the fall and winter months. They are fully mature when you see the leaves start to loose their color, turning a yellowish brown. The tops will weaken and flop over. This is natures plan – the leaves have put the last of their energy into the bulb.
It is important to ”Cure” or dry your onions before storing them. Pull or dig them out and simply lay them back down on the dirt to allow the onions to dry in the hot sun. Usually this takes 2-3 days. If the weather is calling for rain, place them in a sheltered spot – you want them to dry, not get wet. After they soak up some sun and dry out for a few days, cut off the roots, and the tops, and move your onions to a WARM, covered, dry location (porch, sun-room, etc.) with good air circulation. Spread them out on an open rack and allow them to dry for a second, longer drying, or curing process. The longer you cure onions, the longer they will keep.
They are now ready to be stored over the winter. Keeping onions in the refrigerator is NOT recommended because humidity, moisture, and other factors such as gasses set off by other vegetables can become an issue. Onions should never be kept with potatoes because of this! They should be stored in mesh bags or anything airy and open such as a wire or plastic basket. Check each onion to make sure you did not miss a wet spot – it will spoil quickly and take others with it. Move them to a basement, root cellar, or garage where they can stay dry, COOL, and away from light. They will keep this way for several months.