Why Your Spring Greens Bolt And How To Stop It

Gardeners all over the Northern Hemisphere are rubbing their hands together, just waiting for the moment the soil has warmed enough to get going.

And when it comes to early spring plantings, most of us turn to hardy greens – those plants that will germinate in cooler soils, grow happily in partial sun, and reward us with a big handful of leaves to saute or pop in a salad.

But wait! If you don’t want your early crop of greens to bolt before you get a chance to harvest, you need to know how to avoid a very common spring gardening mistake.

Ladies and gentlemen, put on your science hat and join me for this edition of Pretend To Be A Plant.

Certain plants are very sensitive to how much sleep they get – that is to say, how many hours of darkness they are exposed to every 24-hour-period. This sensitivity is called photoperiodism. From the Greek, photo = light and period= recurrence or cycle.

We used to think it was hours of daylight that controlled plant photoperiodism, so all the terminology is based on that. But we’ve since learned it’s actually hours of darkness that trigger photoperiodic responses in plants. Like me, these plants are deeply concerned with how much sleep they get.

Not all plants are photoperiodic, but for those that are, that day-to-night ratio has a huge influence on the plant’s development.  According to the University of Illinois:

Photoperiodism influences many activities in plants including growth, seed germination, flowering, fruit development, and the onset of winter dormancy. Some plants need a specific ratio of day to night length to initiate flowering. The time required can vary among plant species and plant varieties.

As a gardener, you’ve probably heard of long day and short day onions. But it’s not just onions that are sensitive to the number of daylight (or, again, more accurately, darkness) hours. Some of our most common spring vegetables are long day plants highly sensitive to hours of darkness.

Each plant has a different length critical photoperiod, or critical night length. Long-day plants flower when the night length falls below their critical photoperiod. These plants typically flower in the northern hemisphere during late spring or early summer as days are getting longer. (Wikipedia)

Now where I garden in Seattle, the change in daylength from winter to summer solstice is incredible. On December 22nd or thereabouts, there is less than 8-and-a-half hours between sunrise and sunset. That means over 15.5 hours of darkness.

On June 21st or thereabouts, we get almost exactly 16 hours of daylight, sunup to sundown. That means only 8 hours of darkness. Put another way, every 6 months we gain (and then lose) nearly a full 8-hour-workday worth of sunlight.

(Fun fact: Seattle is the northernmost major city in the Continental United States. And I know it seems like Maine is way further north than Washington State, but it’s actually more southerly. Crazy, huh?)

For us northerners, the rate of change in daylight hours during those spring months is fast. Through all of March and April, we gain over 3 minutes of daylight every single day. And for every 3 minutes we gain in day, we lose 3 minutes of night.

As a gardener, I can feel that change. That’s how noticeable it is. The plants can feel it too. And for many of them, the “trigger point” of sleep deprivation comes well before the Summer solstice. Decreasing hours of darkness trigger these long day plants to initiate flowering, or – as we gardeners call it – bolt.

Now the problem is that this trigger point of bolting often happens before the plants have grown large enough to provide much of a harvest. You’ve probably seen this happen around late May or early June with your arugula, spinach, or bok choy. What looked like a very promising stand of teenage greens turns, seemingly overnight, into a sea of bolting stems. And you look at your veg and you scream, “you’re not old enough to have babies! What could you possibly be thinking!?!”

It’s not their fault. They’re just sensitive.

Photoperiod-sensitive, long day plants include mustard greens, bok choy and similar Asian greens, and, to a lesser extent, arugula and spinach. It’s just difficult to grow these to full size in our cool springs. By the time it’s warmed enough for the seed to germinate and the plant to put on a few leaves, the hours of darkness have waned enough to trigger bolting.

And because the rate of day-to-night change is so rapid up here, it can seem like you just blink twice and all your plants are suddenly bolting.

But not all is lost!

First, most of these greens are actually more delicious and succulent at smaller sizes, so when you’re growing a crop of greens – harvest early and often! Don’t wait for your greens to hit “full size” before you harvest and enjoy them; they might not make it that far.

Second, by using a few simple season extension techniques like low tunnels and cloching, you can warm up the soil a bit in early spring. This allows for earlier seeding and more rapid germination. Getting a 2 or 3 week headstart with these plants can make all the difference in a harvestable yield.

Third – and this is the major thing – if you flip the year around and grow these plants under conditions of decreasing daylight (increasing darkness) and they become shockingly easy to grow to full-size, harvestable perfection. I once thought Chinese cabbage was nearly impossible to grow. Well, in the spring it is. But grow it in the late summer for a fall harvest and suddenly it’s as easy as lettuce.

So, for the easiest solution – just sow your photoperiodic greens at least several weeks after the summer solstice (or later if you live somewhere hot) and these plants will mature in the cool days of fall without ever hitting that “initiate flowering now” trigger that comes with shorter nights.

If you’ve had trouble growing “spring greens” in the spring because they bolt on you, try them as a fall crop instead. I bet you’ll find they’re far easier that way.

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