Winter onions (Allium proliferum) grow prolifically on their top, middle and bottom. Alternate names for them include Egyptian walking onions, top onions, tree onions, Egyptian tree onions and perennial onions. After planted, winter onions sprout edible, green leaves, and the onion sets multiply underground. Each leaf forms a small, brown area that grows into a capsule holding many small bulbils, or top sets. When the weight of the top sets increases, the leaves bend over and deposit the top sets on the soil. The top sets produce new plants. Winter onions are hardy in U.S. Department of Agriculture plant hardiness zones 3 through 9.
The optimum time to plant winter onions is fall. The onions sprout, grow leafstalks and establish a good root system before they die back in winter. Their underground bulbs store energy during winter for new, vigorous growth in early spring.
Soil Conditions and Spacing
Winter onions require loose, well-draining soil and full to partial sunlight for their optimal growth. A planting bed that receives full sunlight yields taller winter onion plants and more bulbils on their top sets than a planting bed that receives less sunlight. Because the onions multiply and are perennials, plant them on the edge of a vegetable garden so you do not disturb them when planting the remainder of your garden. Winter onions also can grow in large pots or other containers.
Plant each winter onion set about 2 inches deep in moist, well-draining soil. Space the sets about 6 to 10 inches apart, and space rows of onions about 1 foot apart. Those distances provide room for the leaves to lean over and allow the bulbils to root in soil.
A nitrogen-rich fertilizer, such as blood meal, should be applied as a side-dressing four to six weeks after planting time and again after an additional four weeks. About 1/4 cup of the fertilizer is used per every 9 square feet of soil. Applying a side-dressing requires making a shallow trench in the soil beside each plant or row, putting the fertilizer in the trench and then covering the fertilizer with soil. Water the site so the fertilizer can dissolve into the soil.
Mulching winter onions helps to hold moisture and warmth in their soil. Add about a 3-inch layer of hardwood mulch around the plants' shoots before the first frost. Provide the plants with about 1 inch of water per week in winter only when little precipitation falls. Onions do not like “wet feet” and rot in wet ground. Bottom bulbs that rot do not produce leaves or top sets in spring.
You may harvest and eat winter onions' green leaves any time they grow. Examine the leaves to choose which to harvest. Because a leaf with a bump in it will produce top sets, avoid cutting such a leaf so the plant can multiply. After you select a green leaf to harvest, snip it about 2 inches from the ground with scissors. The leaf will regrow.
If you want to plant winter onions' top sets elsewhere, then harvest the top sets in late summer or autumn when the plants' leaves turn brown. Store the top sets in a dry area for planting at a later date. An option is to let the top sets take root in the ground naturally as the leaves bend over.
The underground onion bulbs multiply and are ready to harvest for eating or separating in late summer and fall when the plants' leaves start to brown. Gently lift the bulbs from the ground by using a spading fork. The bulbs need to be stored in a dark, well-ventilated spot for a few weeks to cure. Place them in a refrigerator for extended storage.
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