Onions (Allium cepa var. cepa) are a cool-season vegetable that you can grow directly from seed or from onion sets in the spring. Knowing how to store onion sets over the winter ensures that the onions will grow and that you will have a successful harvest the following year.
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How to Store Onion Sets
Start onion sets the summer before you plan to plant them by sowing onion seeds spaced about 1/2 inch apart in a sunny location. Water them regularly to keep the soil moist and do not fertilize them. When the bulbs are 1/2 to 3/4 of an inch in diameter, you can remove them from the soil. Discard any that are smaller or larger, as they aren't as suitable for planting.
Place the sets you plan to keep in a sunny location to dry for about 10 days. When the tops of the onion sets are dried out, you can remove them. You can store onion sets for planting in much the same conditions as you would store those you plan to eat.
One option for how to store onion bulbs for next year is to place the sets together in a mesh bag, or you can tie the tops of the onions and hang them. The storage location should be cool and dry but not freezing, as this can damage the sets. Also, avoid warmer temperatures, as the sets may begin to sprout. Can you store onion sets in the fridge? They will last for as many as seven months when stored at a temperature of 32 degrees Fahrenheit; just make sure that there isn't moisture in the refrigerator that can cause the sets to rot.
How to Plant Onion Sets
Plant your onion sets in your garden in the spring about two to four weeks before the last frost. Select a location where the onions will get full sun and have rich, well-drained soil. Aim for a soil pH between 6.2 and 6.8. It is a good idea to conduct a soil test before planting and amend the soil as needed to add nutrients, organic material and lime or sulfur to correct the soil pH.
The area should be free of weeds that will compete with the onions for moisture and nutrients. Put the sets in the ground at a depth of about 1 inch. Make sure the pointed end of the set faces upward. Space the bulbs 2 to 4 inches apart.
Growing and Harvesting Onions
Monitor the bed and remove any new weeds right away. Water regularly to keep the soil moist throughout the growing season. Don't allow the soil to dry out, as the onion's root system is shallow, and they need moisture to produce a good harvest. The soil should be rich with nutrients before planting. You can use a transplant starter solution to side-dress the onions to supply additional nutrients to these heavy feeders.
The time to harvest can vary by cultivar, but you can generally expect to start harvesting onions about 60 to 80 days after planting your sets. Some cultivars may take up to 110 days before they are ready.