The Importance of Day Length When Selecting Onions

Onions are one of my favorite seeds to start indoors for the summer garden. They are fairly tough, easy to propagate, cheap to buy, and can produce amazing results. And despite our temps hitting record lows here in far Northern Idaho this weekend I am dreaming of spreading those funky little black specks over a freshly made pot of seedling mix. They’re one of the first varieties I start in the winter, often on or around my birthday in January.

My husband, our son and I bundled up and braved the subzero temperatures (minus 6 degrees when we left, and -12 by the time we returned home!) to gather some necessary supplies for a weekend holed up in the cabin, keeping snuggly warm and tending frequently to the animals in the extra-frigid weather. One of our stops was to a local hardware and farm supply for some tractor parts and I couldn’t contain my excitement when we walked in the automatic doors. After being blasted by a welcoming rush of warmth from the overhead heaters I looked up and there was a generous bank of seed racks already in place. “Oh my gosh babe, they have SEEDS!” I exclaimed, and the cashier closest to the door let out a little chuckle. I didn’t care in the least. There were a few things still on my list that I was just waiting for this exact moment to fulfill.

My excitement quickly turned to annoyance when I perused the onion varieties available on the shelf. I probably shouldn’t be so perturbed, being that most locally-sourced seed selections are sent out on a national basis and aren’t tailored to a particular growing zone. However I couldn’t help sending a snapshot of the offenders to my mom. How could they? I said. Setting people up for failure like that. What was the issue you ask? Three of the five seed packets in particular that I had found on the shelf in my area were either the incorrect day length for our region or didn’t state the day length at all. But what is day length, and what does it mean to us home gardeners?

Day length is the term for different durations of photoperiod response for daylight-sensitive plants. That sounds really scary and complicated but the point is this: an onion, in particular, will wait until the hours of sunlight in a day have reached a certain point before it starts to form a big, delicious bulb like we purchase from the grocery store. Long-day onions require a day length of more than 12 hours to form a bulb, while short-day onions require about 12 hours. Intermediate or day-neutral onions purportedly can work in either region; I haven’t tested that variety yet myself. As you can see on the map below, depending on your geographical location, you may need to purchase a specific day length onion in order to get good results. Being in Northern Idaho at the tippy-top of the United States, I couldn’t very well choose a short-day onion and expect to get a great harvest. The days here are so long that it would barely bulb before it peaked in growth and toppled over, ready for harvest at a modest or even laughable size.

*Photo courtesy of Johnny’s Selected Seeds*

I can’t even begin to tell you the number of conversations I’ve had with friends, family and fellow gardeners alike about the challenges of growing onions. I have been starting bulbing varieties from seed since the dawn of my time as a gardener so I couldn’t understand what the problem was. Even my grandparents, each lifelong and successful gardeners, both had trouble getting consistent results.

After really delving into the subject I was flabbergasted to find such a lack of information available when purchasing bulbs or bunches from local suppliers. Often these onion starts are grown on for a short period of time and then placed in cold storage for delivery across the country the following year. Most people purchase their bulbing onions in this manner and I can see two potential problems with this practice.

Improper Variety For Your Region

Whether purchasing seeds or starts for your onion crop, when shopping locally, it can be difficult or even impossible to determine the day length of the varieties available. I have visited a number of nurseries in the area in order to research this theory of mine and I was surprised to find that none of the sources I consulted with knew what variety the onions were, let alone the day length of what they were selling! Most bulbs or bunches come labeled in boxes with such descriptions as “Red Onion,” “White Onion,” and so on. Upon my request the managers were able to produce the original packaging which the onions were shipped in and neither the boxes nor the included pamphlets mentioned specific variety or day length. This is very concerning considering many of these starts come from out of state and may be completely ill-suited for a particular growing region. I have purchased a handful of each of these bulb and bunched starts for experimentation this year and will share my results in the Homestead Diary when I harvest mid-season. I suspect they won’t perform well based on the exclusivity of onion types, and will pale in comparison to my home-started seedlings.

Improper Stage for Transplanting

Another concern of mine when it comes to onion performance is improper stage of growth at the time of transplanting. Onions are biennial plants, which mean they are signaled biologically to produce seed in their second year of life. The results we are looking to get in our home gardens typically happen in that first season – when the onion is putting on rapid growth within the bulb. However, when you are yanking your baby transplants from their original birthplace (a greenhouse or farm likely far, far away) and placing them in cold storage for transport, the plants consider themselves having experienced “winter” and will therefore spend all their energy on reproducing when they finally get to your garden rather than growing you a lovely bulb.

After an onion flowers it then enters senescence, or the waning of the growth cycle. They flourish, produce seed, and then die. This is the life cycle of a plant. We, as gardeners, want to stop this process at a precise moment in time so we can maximize the harvest of our precious crops. I find this impossible when we’re being provided the improper type, size and variety of starts from local sources.

My Recommendations for a Killer Onion Harvest

Here are my top three recommendations for an awesome onion harvest this year:

Find your day length by location, and purchase the right variety of onion for your region
Reach out to local gardening forums or your University Extension office to ask for region-specific variety recommendations
Start your onions from seed for the best results

Some seed suppliers have really done us a solid by listing their onion varieties by day length. One of those companies is Botanical Interests, and another is Johnny’s. For your convenience I have linked a number of resources below depending on the type of onion you’re looking to purchase.