Tragedy strikes: kabocha squash vine killed off by a cold snap. [Photographs: Lauren Rothman]

One thing I wasn't prepared for, when I started gardening seriously, was how incredibly obsessed I would become with my new hobby. Now, I have a bit of a compulsive streak, so I guess I shouldn't have been too shocked. Still, it's surprising how attached one can become to their plants. In the morning, as soon as I get dressed, I rush out to the garden to see how everything's doing, and in the evening, when I get home, I always spend a few minutes outisde checking on the plants and making little adjustments, before I even go inside.

When your plants are flourishing, it can be incredibly gratifying. After all, they are living things, and you, as the gardener, are responsible for their health—and, if you raised them from seed, for their very existence. On the flip side of this, if something goes wrong with your plants, it can be tough to deal with.


Human error + too-hot sun = tomato seedling burnt to a crisp.

There are so many complications that can befall a garden, and most of them are brought on by temperamental weather: issues like too-hot sunlight, super-strong winds, cold snaps, and drought. And though even mature plants are susceptible to damage by these forces, tiny seedlings, which are the plant equivalents of newborn babies, are much more vulnerable.

Seedlings raised indoors, or even in a cold frame or greenhouse, are utterly unprepared for outdoor life: it's up to you, as the gardener, to gradually get them used to their environment, so that they don't die of shock. To extend the newborn baby metaphor, a seedling raised inside and then transferred outdoors is like a baby suddenly being wrenched from the womb and thrust into the world: it's a pretty traumatizing experience.

That's why, for seedlings, a process called hardening off is absolutely essential. All it means is gradually exposing your seedlings to the outdoors: instead of just planting them out once the weather seems warm enough, and the plants seem big enough, you have to invest some time by slowly bringing them outdoors, for only a few hours at a time, over the course of several days, or longer. That way, they'll gradually adjust to outdoor conditions, making them that much stronger when faced with the problems I cited above.


More weather problems: strong winds snap a broccoli seedling's stem.

Hardening off requires patience. Fellow gardeners will recognize the feeling of urgency one gets once the spring weather has warmed: you just want to get those plants in the ground. But it's important to take the time required, or else the seedlings you've already spent weeks or months of energy caring for could die in one fell swoop.

It happened to me, folks: you'll see in these photos that a sudden cold snap a few weeks ago killed off one of my squash plants; a night of strong winds did in some of my broccoli plants, snapping their delicate stems right in half; and finally, due much to my own stupidity, I lost several tomato seedlings when I felt they were strong enough to withstand a few hours of direct sunlight (they weren't yet).

So here's a short list of tips for hardening off your seedings: with a little time and tender loving care, you should be able to minimize your losses when planting out your seedlings. Learn from my mistakes, folks!

Hardening Off Seedlings: Some Tips

  • Consider a cold frame: I didn't use a cold frame this year, because I'd assumed—without doing any actual research—that the cost of one would be prohibitive. Not true! Many garden centers sell low-cost cold frames that are essentially wire shelves protected by a clear, zippered vinyl cover. Using one of these will allow you to get your seedlings outdoors faster—it's safe to transfer your plants once night temperatures hold steady in the upper 30s / low 40s. On sunny, warm days, make sure to unzip the cover so your plants don't fry, and always make sure to zip it up tightly at night.
  • Take your time: As I stressed above, take your time acclimating your plants to the outdoors. Place all your seedlings on trays that are easily moved around, then start by keeping them in a sheltered, shady spot during the day (a covered porch, or under a sturdy shrub in the garden, will work well). Leave them out for only a few hours a day, and certainly remember to bring them in at night. After two or three days, you can start moving them into direct sunlight, but only for a few hours at a time—the strong light can kill the seedlings very quickly. Make sure to keep plants well watered. Continue taking plants inside at night. After another two or three days, you can leave the seedlings in the sunlight for longer periods of time. Once night temperatures hold steady in the upper 40s / low 50s, you can start leaving them outdoors overnight. This whole process should take you a week at the minimum, or possibly longer if the weather continues to fluctuate. Once it stabilizes, get those plants in the ground!
  • Know what you're buying: Even if you purchase large, healthy seedlings at a garden center or greenmarket, chances are they've been raised in a greenhouse and aren't yet exposed to outdoor conditions. Make sure to ask when you buy, as most of these plants will require hardening off—albeit a shorter period—too.
  • Watch that weather report: Once your plants are all planted out, continue to pay close attention to the weather forecast! There are measures you can take if things go wonky. If a cold snap, strong winds or intense rain is predicted, you can shelter plants by placing an upside-down pot over them, and removing it once the weather passes.

Thanks for the Feedback!

Your comments last week were absolutely fantastic—they really made me think, and I learned quite a few tips. Please, keep the ideas coming! Any other hardening off tips or tricks? Please share!




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