July 16, 2020

Ask an Onion the Time of Day


Janine Pineo Photo | Can onions tell the time of day?

Back in July 1997, I heard from Paul Hepler, a retired University of Maine associate professor of horticulture in food science.

I’d written about root crops and he wanted to clarify a couple of points about turnip vs. rutabaga and then those wacky onions, which aren’t a true root crop at all.

“This is a funny one,” he said Paul Hepler as he started to tell the strange tale of turnip vs. rutabaga.

He said Mainers have the odd habit of calling anything that looks like a rutabaga a turnip. (I pleaded guilty, too.)

The difference, Hepler said, is simple to spot: The turnip has a rough-textured leaf, while a rutabaga has a smoother leaf.

There’s also a big difference in the number of chromosomes between the two, but I doubt many of us can check that as easily.

Then there is the onion, which Hepler says isn’t a true root crop at all because it’s a bulb made up of much-shortened leaf bases. The onion has a complex life cycle that responds to light (ever notice the short-day and long-day tags attached to onion descriptions?).

Those terms are linked to the photoperiod, or the number of daylight hours that best suit the growth and maturation of a plant.

Onions are long-day plants. During its formative stage — “when the factory is built,” Hepler says — the leaves contained in the bulb grow. They need long periods of daylight to do this, which is where the “long-day” designation comes in.

When the day begins to shorten, the products of photosynthesis are stored in the form of a bulb. The plant’s maturative growth is its “photo response” to the change in light, Hepler says.

So if you plant your onions late, you’ll likely get thick-necked “roots” and no bulb because there haven’t been enough long-day days for the onion to build its factory. Hepler recommends April planting for best results but even the first week in May is OK.

And forget about bending down onion tops to help the plant mature. That’s an “idiot statement,” Hepler says. The tops will die when the plant is fully grown.

– Janine Pineo