Onions require a long growing season and are usually started indoors 10-12 weeks before they will be set out in the garden in early spring, after the threat of frost. Onions need abundant sunlight, which determines the size and development of the bulb, and are categorized by the amount of daylight needed for proper bulb maturity. Short day onion varieties need around 12 hours, while long day varieties require 14-16 hours of sunlight to develop. Intermediate onion varieties fall in the middle, needing 12-14 hours. Onions may also be planted directly in the garden after the threat of frost by using onion sets, small dried onion bulbs, but the varieties are usually limited. I prefer to grow my onions from seed for this reason.
One of my favorite heirloom onion varieties, Australian Brown, was originally introduced by W. Atlee Burpee in 1897. It was the first onion I attempted to grow from seed in my garden. Australian Browns are a reliable intermediate day variety that develop medium to large sized bulbs, depending on growing conditions, and are known to be good keepers. My first planting yielded robust, medium sized onions that stored exceptionally well, lasting through the winter. Australian Brown onions are a classic style cooking onion, bearing a sharp, eye watering bite, and are suitable for use in many cooking preparations.
Red of Florence is an Italian heirloom dating back to the 1800s. They have an oblong reddish purple tinted bulb and have a milder, sweeter flavor. I like to dice them fresh to use in salads, or chop with vine ripened tomatoes and basil for a delicious bruschetta. Red of Florence onions are a good substitute for shallots in cooking recipes, and work well when sautéed as part of a roux base for sauces. Red of Florence is a long day onion variety. They take up little space in the garden, requiring only a few inches to accommodate their small, oblong roots.
There different methods and techniques for starting onion seeds indoors, and I have tried many of them, with varying degrees of success. The most important thing is to get the roots healthy and established while still indoors, to where they are strong enough to withstand being set out in the garden. I have found the most success by using a two tiered tray method, which requires less work in the beginning and more time when transplanting, and by using an individual potting container method, which is more tedious in the initial planting process but smoother on the transplant side.
Two Tiered Tray Method
For the two tiered tray method, I use a flat 11” x 22” x 5”tray set. The top tray contains drip holes to allow for drainage, and sits inside of a solid bottom catch tray. I purchased my set specifically for my grow rack because it fits nicely in the frame, but any flat container with holes to allow excess water to drain into a slightly larger container will work. Plastic clam shells can be used when seeds are sown in the base after piercing holes, then cut the top off to use as the catch tray.
The seeds are surface sown, then light covered in seed started mix in the top tier tray. Water carefully so as not to disturb the delicate seeds. Sprouts will start to emerge within 7-10 days in temperatures of 70 degrees F or warmer. The tray may be covered and placed indirectly near a floor heating vent to help the process along, then move the tray under grow lights when sprouts appear.
Thin plants as needed to allow enough room for the roots to develop and keep moist. I like to use a spray bottle for misting so that I don’t disturb the fragile roots as they are grow. If more than 14 days go by without a sprout, it’s time to try again.
When it comes time to transplant, the tedious work sets in. I use a fork to break loose and separate each tiny onion by the root, then gently place it into its designated spot in the garden. This process can be time consuming, depending on how many onion seedlings you are working with, but extra care when transplant each little plant will help yield healthy, well-formed onions come harvest time.
Individual Potting Container Method
Another method I have used to start onions from seed is to sow seeds in individual planters that can be transplanted along with the onion when they are moved to the garden. I have purchased mini biodegradable pots in the past, but found that they didn’t decompose fully once the onions were transplanted. I was left with underdeveloped onion bulbs, still tasty and usable, but disappointing in that they were not allowed to grow to their true potential.
The best onion seed planter pot I have discovered is made by repurposing toilet paper tubes. I start collecting them in the summer, and by the time February rolls around, I have toilet paper roll planters to spare. To make planters, cut each tube in half, then cut four slits on one side of each half. The slits are then folded into each other to make a solid base. One tube makes two planters. The planters may be set in a large solid bottom tray, then fill each tube with seed starter soil. Carefully sow 1-2 onion seeds per planter tube then water until moist. Cover the seed tray and keep in a warm place until sprouts appear, then move to grow lights until the plants are ready to be set out in the garden.
Transplanting onions is less tedious when working with individual seed planters since they are already separated into perfect little planting packages. By now, after 10-12 weeks, the tubes should be starting to decompose. If not, gently spread apart the folded bottoms when putting the seedlings into the ground to ensure the cardboard will not inhibit bulb growth.
Once transplanted after the threat of frost in the spring, keep the onion bed free of weeds and wait for the tops to fall over to indicate they are ready for harvest. Cure onions for ten to fourteen days before storing.Photo credit: Rebecca Anne Cole