Growing Onions From SeedMany gardeners grow their onions from sets rather than from seed. Others gain the deep satisfaction of scratching that gardening itch by starting their onions from seeds sown indoors. One advantage to using seeds is that you have a greater selection of varieties than those available as onion sets. More importantly, onions grown from seed perform better than those grown from sets. They are less prone to disease and bolting, size up their bulbs faster, and store better!
How many onions should you grow? Ask yourself how many onions you eat in a year? Dakota Tears onions store exceptionally well and should keep you in onions year around, if you plan for your usage. If you eat an onion a day, consider planting about 400 seedlings. Planting in a double row 8 inches between the rows with 3-4 inches between plants, you should be able to grow 6 onions per row foot. 400 onions will take up about 67 row feet of garden space. If you only eat two onions a week, plant about 130 seedlings, taking up 22 row feet.
SOIL MIXLet’s start with the soil. This soil mix is based on the mix used by the Rodale Center.
- 4 parts screened compost*
- 1 part perlite
- 1 part vermiculite
- (optional: for added moisture retention) 2 parts coir (ground up coconut husks)**
**Why coir and not peat? Coir’s pH is near neutral and it retains more water than peat. In addition, it is much more sustainable than mining peat bogs, which releases carbon into the atmosphere.
This mix aids in moisture retention and drainage, keys to starting healthy seedlings. It’s easy for the soil to stay too wet, and that can lead to damping-off, a fungal disease that causes newly germinated seedlings to rot at the soil line and die.
STARTING ONION SEEDS IN FLATSWe start our Dakota Tears onions the first week of April; this gives us 6 weeks to grow good sized onions before transplanting in the middle of May. We plant the seeds in propagation trays with 72 round cells per tray. We singulate the onion seed, using a homemade vacuum seeder, sowing 1-2 seeds per cell. You can also use a simple seeder, such as a or, to help singulate and dispense your seed.
Fill your tray with your starter mix and screed the excess dirt off the top with a straight edge so each cell is level full. Use another filled tray to place on top of the newly filled cells, pressing down to GENTLY firm the soil in each of the cells at once. Place the seed in each cell and cover with ~1/8 inch of your soil mixture. Gently pat to firm and water thoroughly and evenly. Tag your tray with the variety name and date planted.
We have also grown onion seedlings much like wheat grass, planting them in milk cartons tipped on their side with the top side panel removed and the spout firmly stapled. Poke holes in the bottom side for drainage. Fill the carton 2/3 full with your soil mix. We sprinkle the onion seed thicker in order to select against plants prone to damping off and to select for seedling vigor. Cover with a thin layer (~1/8 inch) of soil mix. Again, gently pat to firm, water thoroughly and evenly, and label your carton with the variety name and date planted.
KEEP THEM COZYPlace the flats near a heat source or on a heat mat made especially for seed starting. Onions take about a week to germinate at 70 degrees F.
KEEP THEM WARMMist the tray with a spray bottle as needed to keep them moist. You can cover the flats with plastic wrap to keep the environment moist and humid. However, if you use this method, keep the trays out of direct sunlight, to avoid getting them too warm.
BRIGHTEN AND LIGHTENAt the first signs of sprouting, uncover and make sure the containers are in a bright spot—a sunny window, a greenhouse, or beneath a couple of grow lights. If using grow lights, suspend the lights just 2 inches above the plants and gradually raise them as the seedlings mature. Plants will become weak and spindly if they have to stretch or lean toward the light. To turn the lights on and off at the same time each day, hook them up to an electric timer, mimicking the natural day length.
We transplant late in the day, usually after 5 pm, avoiding the heat of the day to reduce transplant shock. Onions need soil that is high in organic matter, fertile, loose and well drained, with no standing water after a rain. If your soil fertility and organic matter is low, apply at least two inches of composted soil and work it in to a depth of eight inches. Work the row to create a nice loose seedbed free of dirt lumps and rocks.
When the seedbed is prepared, carefully remove the plug from the cell and transplant each plug, placing them level with the soil surface and lightly covering the surface of the plug with fresh soil so the edges of the plug are covered.
If using the milk carton method, undo the spout and open up the end, laying it flat. Gently scoop your hand underneath the soil and gently separate a small section, removing it from the carton. Try to minimize touching the roots as much as possible. Handling the plants by the leaves, do not squeeze the stems, gently tug and loosen the soil to separate the roots of individual plants. Select the largest most vigorous seedlings and lay them in the prepared row, spacing approximately where they will be planted. Lay out about a half a dozen seedlings at a time and transplant to minimize the roots’ exposure to light or work with a partner to separate and transplant as you go.
Onion seedlings do not have much of an enlargement at the base of the stem. Look for the point where they change from the white base above the roots to the green stem. You want that white base submerged, so you have about a half inch to an inch of soil above where the root starts. Make sure to displace the soil deep enough to be able to accommodate the roots. Placing the base of the seedling just deep enough to cover the white stem end so that no white is showing. Actual depth of the hole will of course depend on the length of the roots. Make sure the roots are all in the ground and covered; if the roots curl some when transplanting, they will water in and anchor downward.
SPACING AND WATERING
Water and nutrients are an important part of onion growing. Onions are heavy feeders, especially as the bulbs begin to swell. Make sure your soil fertility is good.
The bulb-enlarging stage is the most critical. If it’s too dry and the plants are stressed, the bulbs will be smaller and have a stronger flavor. Our field onions are grown under dry land conditions. Dakota Tears onions are fairly drought tolerant but moisture stress will reduce bulb size.
Our family garden is under a deep mulch system, which conserves moisture. If using mulch, be sure to keep the mulch away from bulbs to avoid disease and rot. Onions should look as if they are sitting on top of the soil. This aids in the drying process by keeping moist soil away from the papery skins. As the onions start to mature, discontinue watering to encourage dormancy.
HARVESTING AND CURING
During a dry spell, when most of the onion tops have toppled and started to brown, and the bulbs have developed skins, gently pull the onions out of the ground and place them in a warm, dry, airy location to cure out of the sun, heat and rain. Curing onions in hot summer sun will bleach and soften the outside tissue and allow disease to enter. This is especially true for the large sweet onions, which have the greatest water content. We use a rack with triple stacked screens on wheels that can be moved in and out of the garage as needed and kept in the shade. Be sure to bring onions in before snow, rain or freezing temperatures.