“What’s light got to do with it?”

“What’s light got to do with it?”

  • By Cleve Campbell
  • /
  • April 2015 - Vol 1. No. 4

Onions.  Yes, light has something to do with onions.  And I have learned that it had something to do with MY onions.  There are so many varieties of onions —  Vidal, Walla Walla, Texas Super Sweet, just to name a few.  But for years the kind I longed to grow were the BIG onions. Man, I have tried, but have failed to achieve those softball-size bulbs found in the super markets and pictured in seed catalogs. I would even settle for a baseball-size onion. The best I could do was something between the size of a golf ball and a lime, until I became aware of an essential environmental element: photoperiod.  Here’s how it happened.

After many years of attempting to grow those “market size” onions, I came to the unhappy conclusion that perhaps it was time to admit failure and to put that “onion space” in the garden to better use.   Well, having been told on more than one occasion by my wife that I was “hard-headed” and just “plain ole’ stubborn,” I figured what the heck, just one more attempt. Humbly, I did the manly thing and admitted a little research was in order if I was ever going to be able to claim the bragging rights for growing the largest and finest onions in the neighborhood.

Living here in Central Virginia, naturally the first question that flashed across my mind was, “I wonder how Mr. Jefferson’s onion crop fared.”  A friend, knowing my interest in gardening and history, recently gave me Peter Hatch’s book, A Rich Spot of Earth. I wondered if Mr. Jefferson had experienced the same onion problem as I.  Maybe he had peeled back the proverbial onion and found a solution I might use to resolve my onion dilemma. According to Mr. Hatch,

“In the South, onions (Allium cepa) are a specialty item, grown in quantity under specific conditions. Mid-Atlantic home gardens usually contain only ‘green onions’ or scallions (Allium fistulosum), especially because onions are so cheap in super markets. Onions were recorded as being planted in the Monticello garden only nine times. Since onions were a staple in some of Jefferson’s favorite dishes and were purchased twenty-one times from the Washington market in 1806, one might conclude that Jefferson found other sources for this age-old but prized culinary treasure of the subterranean world.” (Hatch, p. 184).

 It appears that the onion plantings at Monticello were limited to Egyptian walking onion.  A little more about that later, but for now, the Monticello onion revelation was not much help in my quest for softball-size onions.

My next research stop was the Virginia Tech Publication 426-411 titled “Onions, Garlic and Shallots.”  Here are the environmental preferences of onions as listed in Pub. No. 426- 411:

  • Light:  sunny  (Check; my garden spot is in full sun and gets at least 8 hours of sun depending on the time of year).
  • Soil:   well-drained (Check;  my unsuccessful attempts were performed in a well-drained raised bed.
  • pH:  5.5- 7.0 (Check; a soil test revealed a pH of 6.6.
  • temperature: Cool (45-60º F) during development; medium to hot during bulbing and curing (60-75º F) (Check)
  • Moisture: moist but not too moist (Check)

Having run though the environmental checklist with flying colors, now what?  There must be something that I missed.   A little humbled, I decided to do the unthinkable:  I revisited the publication and proceeded to read whole thing. Well, you guessed it; I had indeed missed something important.  I learned that there are three different groups or categories of onions: “long-day” onions, “short-day” onions and “intermediate-day” onions (also called “day neutral”).  What was that all about?  Hmmm, perhaps something to do with daylight length? This reminded me about that old thermos bottle story — it can keep stuff cold or hot. How does it know which to do? You’re right. A little more research was required.

Most onions, as it turns out, are biennial plants, which is just a fancy way of saying that it takes two (2) growing seasons to complete the life cycle.  In its first year, the onion plant develops and produces a bulb that is used for food storage, and in the following year, it produces flowers and seeds.   In short, onions have three (3) “major” phases of growth:  the vegetative phase, when roots and leaves are produced by the plant,  the bulbing phase when bulbs are produced, and the reproductive phase, when flowers and seeds are produced. The bulbing stage and flowering stage are usually separated by a cold (vernalization) period. Onions grown for their bulbs are treated as annuals and harvested at the end of phase two (2). The three distinct phases are driven by temperature and length of day, also known as photoperiod. Since onion bulb development depends on the temperature and length of day, random planting during our growing season here in Virginia will result in limited success.

The size of the onion bulb is dependent upon the number and size of the green leaves or tops at the time of bulb maturity. For each leaf there will be a ring of onion; the larger the leaf, the larger the ring will be. The onion will first form the top, and then, depending on the onion variety and length of day, it will start to form the bulb. (Magruder). Herein lies the challenge: the requirements for leaf and foliage development and the requirements for bulb development are different. Since the onion is a cool weather crop, foliage development requires a temperature of 45-60º  and bulb formation requires a temperature requirement of 60-75º for optimum bulb size.

In addition to temperature, bulbing is initiated when the daylight length reaches the number of hours critical for that variety. In short, temperature and daylight length are the factors that determine when vegetative growth stops (phase 1) and bulb development (phase 2) begins (Lancaster).  There have been attempts to isolate the trigger that moves from leaf growth to onion development, but these have been unsuccessful. (Remember that how-does-it-know thermos story?), but some research suggests that hormones and growth regulators such as auxin, cytokinin, gibberellins, and ethylene have been implicated in bulbing (Brewster).  However, research conducted at Texas A&M and The U.S. Department of Agriculture concluded that no evidence could be found to support the claim that treating onions with growth regulators and hormones increased the size or weight of onion bulbs.

The selection of the correct varieties and planting times are critical for success.   Choose a short-day variety when day length is 11-12 hours (latitudes less than 35 degrees);  choose intermediate-day types when day length exceeds 13-14 hours (middle latitudes) and remember that long-day types require greater that 16 hours of day, (latitudes greater than 39 degrees). (Brewster). Our approximate latitude in central Virginia is 38 degrees, so we just happen to be between the short day and long day categories. Our longest daylight day is 14 hours and 48 minutes  — obviously too short for long-day onions. But wait, 14 hours is more than enough time for short-day onions.   But since the length of day is what triggers bulb development, in our area that 12-hour trigger is March 17.  That’s because the size of the bulb depends on the top or foliage development, and we do not usually reach the optimum temperature ranges for that (45-65 degrees) until March 17.  That’s just a roundabout way of saying that around here, the bulb starts to form before there is sufficient leaf development to grow a large onion bulb. In the fall it’s just the opposite;  we have the correct temperature for top development but lack the 75-85 degree temperatures for good bulb development. So cutting through all that mumbo jumbo, the area we live in is not ideal for growing short- or long-day onions. Therefore, we are limited to intermediate-day onions. The chart below depicts the growing ranges for the various types of onions:

Source: Waldeneffect.org

Source: Waldeneffect.org

Seed catalogs often provide the length of day requirements for the onions they offer, and in a few instances, provide information such as “adaptation 35 degrees-45 degrees latitude.” In short, to increase the probability of having a successful bulb onion crop in our area, one needs to be cognizant of the category of onion that grows best here. In our area, short-day and long-day onions will have limited success, whereas the intermediate-day onions  — such as Candy F1, Candy (Red) F1, Crystal, Gladstone or Super Star — will have greater success .

Many of these varieties are available from various seed catalogs as seeds or starts. If you want to start onions indoors and transplant them to the garden, they need to be started in January. Seedlings or plants can be ordered from various catalogs.  Several of our local garden centers often carry intermediate-day onions in the early spring.

In our area onion plants should be set out as soon as the ground is workable and up into mid-April.  As for the bulk small onion bulbs (labeled: yellow, white and red) on sale in garden centers and big box stores or at our local hardware stores, their variety and length of day requirements are unknown; however, if your goal is to grow spring onions for their green foliage — not for a big “bulb” onion — they will work in our area for that purpose.

Once I figured out all that long-day, short-day and intermediate-day stuff, the actual cultivation of onions was relatively easy.   Va. Tech Publication 426-411 offers detailed instruction on the cultivation requirements.

Wow, I remember that first softball candy onion I grew. I couldn’t wait to cut into that sucker. Well, let me tell you, I didn’t know I was capable of so many tears, and when I took a bite, it was strong.  It turns out that onions have a defense mechanism;  when you cut in into an onion and damage the cell walls, it releases a compound called Syn-propamethial gas, which is a sulfate derivative.  In general, clay soils and organic matter have a relatively high level of sulfates (Brewster). I have clay soil, which has been amended with organic matter, which means that because of these high sulfate levels, I’ll never be able to grow that sweet onion that can be eaten like an apple.

Now the only thing left to figure out is when to harvest the onions. As a general rule the onion tops will turn slightly yellow and “fall over.” The onions should be harvested shortly after the tops have fallen over. Don’t leave the onion bulbs in the ground longer than a week or two after they have fallen because the bend in the onion stalk is a weak point and can provide a roadway for organisms to enter the plant. These organisms may cause the onion to rot prematurely. Naturally onions may be harvested before they fall over to be used fresh in the kitchen. Also, intermediate-day onions are not considered long storage onions or good keepers, and my experience is they can be stored for 3-4 months before they begin to go bad.

Now back to Mr. Jefferson and Monticello.  What is now known about photoperiod and its impact on onions had not yet been discovered in Jefferson’s time.  That didn’t happen until 1920. Therefore, the importance of selecting the correct length of day variety may have been an unknown in Jefferson’s day. In addition,  the soil at Monticello is clay, and in all probability has a high sulfur content, which will increase the pungency of the onions, and may have led to the perception that “northern” onions were sweeter and of better quality. Or it may be, as Mr. Hatch suggests in his book, bulb onions were so cheap that the space in the vegetable garden at Monticello could be put to better use.

In summary, onions are a cool weather crop. The development and size of an onion bulb is dependent on two environmental factors: temperature and length of day. There are three (3) categories of onions: short-day, intermediate-day (or day-neutral) and long-day. In Central Virginia, because of our location we can expect to be successful with the selection of an intermediate-day type onion such as Candy F1.

The heat or pungency in an onion is directly correlated to the level of sulfur in the soil; therefore, because our clay soils have a tendency to have moderately high levels of sulfur, one would expect the onions we grow to be on the pungent or hot side.   I was pleasantly surprised at how successful I was when growing the intermediate-day type onion, and hopefully you too will be successful.



Hatch, Peter J.,  A Rich Spot of Earth: Thomas Jefferson’s Revolutionary Garden at Monticello, Yale University Press, (2012).

Brewster, James L., Onions and Other Vegetable Alliums, 2nd ed. (2008)

Magruder, R., R.E. Webster, H.A. Jones, T.E. Randall, G.B. Snyder, H.D. Brown,  L.R. Hawthorn,  and A.L. Wilson, Description of Types of Principle American Varieties of Onion,  Miscellaneous Publication 435 USDA, Washington D.C. (1941). http://usdasearch.usda.gov/search?utf8=✓&sc=0&query=WEBSTER+MAGRUDER+PUBLICATION+435&m=&affiliate=usda&commit=Search

Garner, W.W. and H.A. Allard, “Effect of relative length of day and night and other factors of the environment on growth and reproduction in Plants,” Journal Of Agricultural Research 18 553-606 (1920).


The National Onion Association, http://www.onions-usa.org/