Onions are a common site in many home gardens, and they can be grown both in the ground, or inside a greenhouse. Although many gardeners start the onions in a greenhouse and move them outdoors later, you can simply increase the pot sizes and keep them in the greenhouse continuously. The most important part of growing the onions indoors is to monitor their size regularly so that the roots do not become crowded in small pots.
Wait until the temperature in the greenhouse rises to at least 17.8 degrees C before you plant the onions. This is the minimum temperature required for germination.
Fill a compartmented seed tray 3/4 full of compost or a soilless seed starting mix. An appropriate tray has compartments that are 1 1/4 inch across and 2 inches deep.
Water the compost until it is completely wet and water runs out of the bottom of the tray.
Place three to five onion seeds into the top of each compartment and press them down into the soil. Cover the seeds with 1/4 inch of vermiculite until all of the soil and seeds are covered.
Place the tray inside the greenhouse in a location that receives sunlight exposure for most of the day. Do not place them near taller plants that may shade them out. The trays must be elevated off of the ground to prevent root rot from developing.
Water the trays daily in the morning until the compost and vermiculite are completely wet. If the vermiculite feels damp, postpone watering until the next day. The onion seeds will germinate in approximately five days.
Wait until the onion sprouts reach 3 to 5 inches tall. Clip the stems back to 3 inches with hand pruners. Continue clipping them each time the sprouts reach 5 inches until the onions are 10 weeks old.
Transplant the onions into 1 gallon or larger pots once they reach 10 weeks old. Fill the pot 3/4 full of an all-purpose potting soil with fertiliser mixed in. Plant the onions only as deep as the bottom of the green stem. Place the pots back in an area of the greenhouse that receives bright light and keep them watered until harvest.
Although the time varies by variety, most onions are ready for harvest 80 to 150 days after planting.