Growing onions from seed requires early sowing

Lee sowing onions in a miniature “field” of his own making (photo by Lee Reich)

I missed my deadline by four days, sowing onion seeds on February 5 rather than the planned February 1. That date isn’t fixed in stone, but the important thing is to plant onions early.

Onions are photoperiod-sensitive: that is, they respond to day length (actually, night length, but researchers originally thought that the response was to light rather than darkness, so the phrase “day length-sensitive” stuck). Once days get long enough, sometime in June, leaf formation comes screeching to a halt and the plants put their energies into making bulbs. The more leaves before that begins, the bigger the bulbs.

Plants from seeds sown outdoors – towards the end of March – won’t have as many leaves as plants given a jump-start indoors. I like big bulbs; hence the early-February sowing.

First step on my way to oniondom is to get fresh seeds. Onion seeds are relatively short-lived, and I want to give the plants plenty of time to grow. I don’t risk delays from poor germination and the replanting of old seed.

Seeds get sown in a miniature “field”: a plastic tub 18 inches by 12 inches, with drainage holes drilled in its bottom and filled four inches deep with potting soil. Some weed seeds are unavoidably lurking in the garden soil and compost in my homemade potting mix, so I top the potting mix with a one-inch depth of a weed-free 1:1 mix of peat moss and perlite.

The edge of a board pressed into the firmed soil mix in the tub makes furrows, six of them equally spaced and about a half-inch deep within the tub. Into each furrow go onion seeds, sprinkled at the rate of about seven seeds per inch. Once the furrows are closed in over the seeds, I water thoroughly and (to avoid washing away seeds) gently.

Covered with a clear pane of glass and warmed to 70 to 75 degrees Fahrenheit, the seeds should appear as grassy sprouts above the soil mix within a couple of weeks. From then on, my goal is to keep the plants happy with abundant light and water as needed. They get a haircut, their leaves snipped down to four inches, whenever they get too floppy. The compost and alfalfa meal in the potting mix should provide sufficient nourishment to the seedlings until they are ready for the great outdoors. That deadline is April 15, weather permitting.

Onions won’t be alone on the seedling bench in the greenhouse. I’m also now sowing seeds of celery, celeriac and leek. All, like onion, need a long period of growth before they’re ready for the outdoors.

These seeds get sown in furrows in small seedflats, from which the seedlings, once they have two leaves, are gingerly lifted and cozied into waiting holes poked into the potting mix filling seed trays with individual cells. Little growing space is needed, because a single seedflat can be home to a few kinds of seeds and the celled trays in which the seedlings grow until planted outdoors can house about two dozen plants in a square foot.

I’m sowing lettuce in a similar manner. In contrast to celery and company, lettuce grows quickly. It’s needed to fill in gaps opened up from winter harvests of kale, lettuce, mâche, claytonia, celery and parsley in the greenhouse, and should be ready to eat in April.

Last year’s onions were abundant, large, sweet and juicy. Anticipating their not keeping well, we ate them quickly, pulling the last ones from their hanging braid in the basement sometime in November. These were so-called European-type onions: varieties such as Ailsa Craig and Sweet Spanish.

Next year we should have fresh onions for soups and stews on into winter, because I’m growing some American types: New York Early and Copra. American-type onions are actually sweeter than European-type onions, but their sweetness is masked by their increased pungency. That pungency comes from sulfur compounds, which are vaporized during cooking. Those sulfur compounds are also what help these onions keep longer.

Soil enters the picture when it comes to onion flavor and storability. Sulfur is an essential plant nutrient, and the more sulfur in the soil (within limits), the more sulfur in the onions. Sulfur is a key component of organic matter, so my compost-rich soil (with a whopping 15 percent organic matter) should have plenty of sulfur.

Still, I’m thinking about spreading sulfur – the same pelletized sulfur that I use to maintain soil acidity beneath my blueberry bushes – on half my onion beds to see if flavor or storability are noticeably affected.


Any gardening questions? E-mail Lee at [email protected] and he’ll try answering them directly or in his Almanac Weekly column. To read Lee’s previous “Gardener’s Notebook” columns, visit our website at



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