Onions (Allium cepa) are available all year long at the supermarket, but if you grow your own, it's unnecessary to go out to buy them. Onions can be grown outdoors in U.S. Department of Agriculture plant hardiness zones 3 through 9, but with little more than a plastic bottle, anyone can grow onions right in their kitchen.
Onions exist in a variety of sizes, shapes and flavors. Common onions grow sprouts and can be harvested young, as green onions or scallions, and the bottom portion of these onions work well for planting in a bottle. Perennial bunching onions (Allium fistulosum), hardy in USDA zones 6 through 9, also work. Keep at least 2 inches of the white bulb portion of green or spring onions for planting in a bottle.
Find a clean clear plastic bottle and cut off the top third. Remove the lid from the bottle top, turn it upside down, and place it into the top of the bottle. If it slips down, poke a hole into each side at the base of the bottle top and insert a pencil or a thin dowel through both holes so you can rest it across the top of the bottle. Fill the bottle with water so the upside-down piece contains about 1/4 inch of liquid. Place an onion into the bottle top so the base is in the water and leave it to grow. If the water level falls below the onion base, add more water. In a couple of days the green stalks should start growing and you can cut them off as you need them.
Grow spring onions and other sprouting varieties vertically in a bottle. Clean out a plastic bottle and cut off its neck. Cut several small holes on all sides of the bottle from the top to the bottom. Make the holes large enough so the onion bulbs will poke through but won’t fall out. Add garden soil to the bottle until you reach the lowest holes. Insert an onion bulb (with the top facing out) into each hole and add more soil until you reach the next holes. Insert bulbs into these holes and add soil on top. Continue layering until you’ve filled the bottle. Add water to thoroughly moisten the soil and set the bottle on a windowsill.
Onions thrive in well-draining soil rich in organic material with a pH or between 6.2 and 6.8. They do not grow well in highly acidic soil. Mix compost into the garden soil prior to placing it into the bottle to ensure onions will thrive. You can also add a complete water-soluble fertilizer with a nitrogen, phosphorous, potassium ratio of 1-2-2, such as 5-10-10. Onions generally require about 1 inch of water per week. They grow best when consistently moist, so if you’re using the soil method, don’t let the soil dry out completely. If you’re using the water method, try to keep the water above the lid portion of the upside-down bottle top. Place the bottle in a windowsill where the onions will receive full sunlight.
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