Fresh scallions, otherwise called green onions or young onions, are an ideal addition to sauces, salads, soups and practically any other dish that you want to spice up a little with some flavor. They come in red, white or yellow, and are a great source of vitamins C, A and K, iron, potassium and calcium. Growing them fresh is a great way to have them year-round, as you can freeze them for future use.
Plant the scallions in the greenhouse in the fall, since they are perfect for winter vegetables in a greenhouse. This is because they cannot tolerate high temperatures. Cooler weather and sun shining into the greenhouse should create a temperature of around 70 to 80 degrees Fahrenheit within the greenhouse.
Buy a collection of scallions at a farmers market or grocery store. They can be whatever type you want, such as yellow, white or purple, and you can mix and match and buy as many as you want to plant in your greenhouse. Look for the freshest ones you can find, with strong healthy firm leaves that are dark green, hard, white bulbs and some roots hanging down if possible.
Prepare the soil and greenhouse containers. Fill them halfway up (containers should be around 6 inches deep) with rich quality soil that has a neutral pH of around 6. The greenhouse containers need to drain well.
Take the scallions you purchased, and cut off the top dark green leaves, only leaving the bulb and roots. Use these leaves for cooking, or freeze them in a plastic bag for future use.
Make a 1-inch deep hole in the soil and plant the first bulb with the roots facing down in the soil. Plant them about 2 inches apart for smaller bulbs (less then 3/4 an inch in diameter), and 4 inches for bulbs larger than that. Lightly sift more soil on top until the top of the bulbs are left exposed. You don't want the bulb to buried completely. Repeat this with the rest of your bulbs. Water the greenhouse planters to make the soil moist.
Set the containers in the greenhouse in direct sunlight where they will receive at least six hours of direct sunlight. If you are having problems with your greenhouse getting too hot, try putting your greenhouse vent fan on a timer, prop open the door and check the plants often to make sure they are not wilting (which means it is too hot for them).
Water the scallions regularly but not too much, as the bulbs will rot in too much water. A general rule to watering scallions is when the soil feels dry, water them, but not before that.
Harvest the scallions when the leaves are at least 3 inches tall. These are young leaves, and will taste delicate and light. The taller the leaves get, the stronger the onion flavors. Use sharp scissors to carefully cut off the leaves at an angle.
Make sure to keep an eye out for disease or insects, which can get transported around a greenhouse especially with a lot of different plants. If disease hits your scallions, trim the leaves, but you may have to get rid of the entire plant if trimming doesn't help. If you notice bugs such as aphids anywhere on the scallions, kill them and wash the plant.