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Onions can be planted in a number of ways: from seeds, from small bulbs called "sets" that have been harvested when small for this purpose, and from onion seedlings. All three methods work well, but many people opt for onion seedlings, as onion seeds can be slow to germinate, and onion sets do not always grow properly, as they may be withered, damaged by mold, or simply dead. Planting onion seedlings can be accomplished with very little work and the simplest of tools.
To plant onion seedlings, first you must prepare the soil. Onions prefer rich, loose, well-drained soil, high in organic matter and with a minimum of rocks and gravel. They are fairly tolerant of poor soil conditions, however, and will grow in less than ideal soil. The soil for onions should be well cultivated to a depth of at least 8 inches and free from weeds. Rake the soil with a garden rake to level it off. Your local garden center or agricultural extension will know the best time of the year for planting seedlings.
Most onion seedlings come planted in clumps, in small containers. A container small enough to fit in the palm of your hand may contain as many as 50 seedlings, which are usually no thicker than a cocktail stirrer. Before planting, water the seedlings in their container. This will keep them moist during the planting.
The only tool you will need for planting is called a dibber. Many things can be used as a dibber, including a pencil, a length of dowel rod, or even a stick. A dibber should be bigger than the onion seedlings but less than a 0.5 inches (1.3 cm) in diameter. This is the simplest of garden tools and is used by simply pushing it into the soil to create a hole into which a seed or seedling is placed. If necessary, the dibber can be rotated to widen the hole slightly.
To plant the seedling, gently separate it from the rest of the clump, and use the dibber to make a hole wide enough to accept the seedling. The hole should be deep enough that when the seedling is placed in it, the point on the stem where the color changes from red or white to green is about 1 inch below soil surface. Place the seedling in the hole as gently as possible, and use the dibber or your hands to gently place soil around it. The planting distances may vary with the onion variety and should be included with the seedlings, but a general rule is to plant them in rows with the individual plants 6 inches (15 cm) apart and the rows 2 feet (60 cm) apart, or wide enough to walk down without treading on the onions.