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The process of hardening off: Texas farmer readies transplants for spring planting

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Photo of seedlings being watered
Transplant seedlings must not dry out when they're hardening off. Photo by Marshall Hinsley
Photo of Marshall Hinsley and seedlings
Marshall Hinsley moves his transplant seedling out of the greenhouse to begin hardening them off. Photo by Allee Brand
Photo of seedlings on a picnic table
Seedlings are kept off the ground to keep them out of reach of rabbits. Photo by Marshall Hinsley
Photo of seedlings being watered
Photo of Marshall Hinsley and seedlings
Photo of seedlings on a picnic table
Marshall Hinsley

From Austin to North Texas, the last frost for the season usually takes place by mid-March, which means it's safe to plant just about everything in the garden. But before planting the young seedlings I started last January, I must acclimate them to the outdoors.

Seedlings started indoors or in a greenhouse tend to be spindly and fragile. They've spent the first three months of their lives in a controlled environment, with mild temperatures and little more than a light breeze from a fan. To ready them for wind, intense sunlight, and wide temperature swings between day and night, the transplants must undergo a process known as hardening off.

 To ready them for wind, intense sunlight, and wide temperature swings between day and night, the transplants must undergo a process known as hardening off.

I begin by moving them outdoors in direct sunlight sometime in the the afternoon, on a day when the wind is not too strong and the temperature is in the 60s. There's no complexity to it; I simply move all the seedlings outside, positioned on a table to keep them safe from rabbits or any other herbivore who might like a tender snack.

There's little soil around their roots, so I give them a generous sprinkle of water so the wind and sunlight don't dry them out. After three hours, I move them back indoors before the sun sets. They usually look a little beaten after the first day of hardening off, but they'll rebound by the next morning.

The next day I set them out again, water them and leave them outside a little longer, perhaps five hours, then move them back inside again before sunset. Unless there's a high wind, the seedlings will look much more robust by the second day. Leaves will start to thicken, and stalks will add girth. Tomato seedlings will turn a darker green, while colorful plants such as red Swiss chard or purple basil will become more vibrant.

I continue the process for several more days, leaving the plants outside a little longer each day. I also allow the soil to become dryer between waterings so the plants will acclimate to variations in moisture. I don't apply any fertilizer during the hardening off process because I do not wish to spur rapid growth.

Cloud cover, full sunlight, light breezes and even high winds are welcome. The seedlings must develop the framework to hold up to the harsh realities of the great outdoors, which they will do if exposed to these conditions in light doses.

After five days, I leave the plants out all night, as long as the temperature is forecast for 50 or above. I watch weather forecasts closely while my plants are outdoors. If the temperature is predicted to dip below 40, I move them back into the greenhouse.

I learned this the hard way. Once, when the nighttime low was predicted to be 38 — well above freezing —  I woke the next morning to find that the temperature outside was 30. All my plants outside were killed.

After the seedlings have spent several consecutive days outside all night and all day, they will look noticeably stronger, greener and filled out. There's something about the exposure to the full spectrum of sunlight, versus the filtered light of a window or greenhouse glazing, that infuses a plant with vitality. At this point, they will be ready for transplanting into the garden, sometime in late March or early April.

Soon, I will have a variety of produce to pick from my garden, which is a luxury that can't come soon enough. This frigid winter has been unkind to my kale, collard greens, rutabagas and turnips. What's more, it killed off my Swiss chard. I long for squash, melons and vine-ripe tomatoes — anything but greens. Holding fast to my experiment in food independence, I'll simply have to wait, be patient, and carefully tend my crops until I can reap the reward.

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