Roots tend to form in the fall. You may also notice a few sprouts in the fall if you live in warmer climates, but most stems and buds form in the spring.
Most alliums can also grow in partial sun to light shade, but the stems might become weaker with less sunlight, and weak stems may not be able to support the weight of the flowers once they bloom. To determine whether or not the soil drains well, pay attention to it after a heavy rain. If you can spot puddles five to six hours after the last drop of rain, the soil is too compact and does not drain adequately well.
Dig this material into the top 12 to 18 inches (30 to 45 cm) of soil to ensure that the soil near the bulb will drain well. As a general rule, you do not need to amend the soil for the sake of adding nutritional content. Allium bulbs can grow in soils with poor nutritional quality, but they will rot in soggy soils.
When planting multiple allium bulbs, the bulbs should be spaced 6 to 8 inches (15 to 20 cm) apart. Consider spreading a 2-inch (5-cm) layer of agricultural grit or spent compost at the bottom of the hole before laying the bulb inside. Doing so can improve drainage conditions.
You may want to consider keeping the containers in a cool location during the winter. Allium bulbs actually thrive when they are allowed to cool down during the dormant season. You do not need to keep the containers outside, but storing them in a garage, basement, root cellar, or shed works well.
There should be at least four moderately sized drainage holes at the bottom of your container. Use a drill to create holes if necessary. Allium bulbs will rot if they are forced to sit in waterlogged soil.
Keep in mind that allium bulbs do not have many nutritional needs, but they do need well-draining soil. Dense soils that retain moisture can cause the bulbs to rot.
Large amounts of sunlight will help the alliums develop strong stems, and strong stems are more likely to support large blooms. If your container is too large to move, make sure that you position it in your desired location before adding the soil and planting the bulbs. If you are able to move your container, move it to different areas of the house or garden throughout the day so that it receives as much sunlight as possible.
If you plant more than one bulb in each container, make sure that the bulbs are 6 inches (15 cm) apart and 6 inches (15 cm) away from the side of the container. Dig a hole that is two to three times the diameter of the bulb when planting it. Consider sprinkling in 2 inches (5 cm) of compost before you place the bulb in the hole for improved drainage and nutrition.
You only need to water the bulbs during their active growth periods—spring, summer, and early fall. Once the plant enters dormancy, you can stop watering it.
Since alliums are so good at discouraging garden pests, they are often used as a border plant. Consider positioning alliums around the perimeter of your garden to protect more tempting flowers lying further in.
Allium flowers can be used in bouquets and similar decorations, just like any other garden flower. Some stalks can have a slight onion scent, but in most cases, the scent isn’t noticeable to the human nose.
Do not cut off the foliage after the blooms drop. The leaves still need to gather sunlight to produce energy so that the bulb can be strengthened for next season.
For average to good soils, this really isn’t necessary. Consider doing it if your soil condition is poor, though. Potash feed will encourage root and bulb formation. It refers to any potassium-rich fertilizer.
This especially applies to smaller bulbs. Many large bulbs are self-propagating and take care of the process themselves. Divide the bulbs after they enter dormancy but before the ground freezes. Dig out the clump of bulbs using a garden fork, working carefully to avoid cutting into the bulbs. After you remove the clump of bulbs, gently shake off excess dirt before carefully twisting the bulbs apart one at a time. Allow the roots to untangle themselves and do not tear them apart.
