Day 11: Seed Starting, Part 3: Hardening Off

Once your seeds have germinated and the seedlings are growing, the next step in the process is transplanting them into a larger container—or not.

tomatoes hardening off

Tomatoes harden off in a part sun location.

If I’m growing lettuce under lights in a container, when the plants have grown a few inches above the top of the lid, I move the entire container outdoors into a cold frame to harden off, that is, to get used to an outside environment. Then, when it’s time to plant in the vegetable garden, I take the entire clump out of the container and divide it for planting by breaking off sections. Lettuce does just fine that way and there’s no need to transplant it to a secondary container. However, for tender plants like peppers and tomatoes that I’ve started in peat pellets, the seedlings need to make the transition into a larger pot where they’ll continue to grow under lights.

It’s time to transplant your seedlings when they’ve developed their first set of true leaves—not the initial two leaves that form when they first sprout, but the second set that look like their normal leaves. Transplanting is easy, but can be tedious, if you’ve started a lot of seeds. I use 3 inch pots for transplanting, which I’ve found to be an efficient size, especially for the hundreds of seedlings I give away each year. As mentioned earlier, I’ve started two seeds in each pellet, and usually both germinate and produce strong seedlings. If that’s the case, I remove and discard the netting over the pellet and carefully break it in two, planting each half in its own pot. If one of the seeds has sprouted too close to the other, I’ll keep the more robust plant and snip off the other one. The process is the same for a seedling I’ve started in a tray; I carefully dig out a section of potting mix where it’s growing, trying to disturb the root structure as little as possible.

hardening off

These plants roll in and out of the house on a cart while hardening off.

The seedlings are planted into potting mix with a little additional organic fertilizer. Consulting my seed notebook to make sure I’ve identified the correct variety, I label a plant tag for each seedling. The tags are made from vinyl mini blinds that I buy from a thrift store and cut into small pieces. I’ve tried using wooden plant tags but they quickly fade or rot in the soil. The plastic ones will fade too in time, so when I plant them in the garden I angle the writing side down so it’s not directly exposed to the sun.

After the seedlings are transplanted, I water them well and place them in aluminum foil pans, which are actually lasagna pans that I buy at the Dollar Store. They’re deep enough so that the pots can’t tip over when they fill the pan, and the seedlings seem to benefit from the light reflected from the foil. Then they go back under the lights for 12 to 16 hours a day.

How much longer the seedlings need to continue to grow under lights depends on how well I’ve timed starting the seeds and when it’s safe to plant outdoors. Obviously, that varies from year to year depending on the weather patterns. Lingering indoors for too long under lights can sap seedlings of momentum, but there are ways at this stage to stimulate their growth and toughen them up for their eventual home outside. Lightly brushing the tops of the plants with your hands on a daily basis simulates the action of the wind, and you can also place a small fan set on low near your plants to achieve the same effect. Doing this helps promote growth of stronger, more robust stems. And because individual seedlings are receiving varying amounts of light relative to their placement under the fixtures, and that affects their growth, I rotate all of the plants every other day: the ones directly under the lights are swapped with those nearer the edges.

When it’s warm enough the trays of seedlings are brought outside to harden off for increasing intervals of time, starting out in a shady area protected from any harsh winds. Gradually, I increase their time outdoors and exposure to direct sunlight. When all danger of frost has passed they can be planted in the vegetable garden, or traded or given away to fellow gardeners. I’ve found that people love to get a gift of a plant that someone they know has started from a seed, and I enjoy introducing friends to new vegetable varieties they might otherwise be unlikely to try.

 —Tom McKusick

Follow Notes from Northern Gardener throughout January as we offer our series on 31 Days to a Great Northern Vegetable Garden. Tomorrow: Cold Frames for Early Starts.




Leave a Comment