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How to Harden Off Seedlings Before Transplanting

How to Harden Off Seedlings Before Transplanting

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Plugging seedlings into ground soil without gradually acclimating them to the very diverse changes in their growing conditions will most likely result in transplant shock. Hardening off seedlings is the gardeners’ lingo for toughening your plant up to cope with all-weathers.

When to start

  • 1 to 2 weeks before you intend to transplant outdoors
  • … But, only when there are at least two true leaves on the plant.

How to tell the difference between seed leaves and true leaves?

The seed leaves are the first leaves to emerge. They’re usually small, oval and rounded. The true leaves grow above the seed leaves. When you have at least two true leaves above the seed leaves, your plant is “nearly” ready for transplanting. Harden the seedlings off first to avoid transplant shock.

A 7-day plan

  • Day 1: Put the plant in a location that’s sheltered and shaded for 2 to 3 hours. Then bring it back indoors. Early morning or late afternoon is ideal to start. Avoid the peak heat times between midday and 3 pm.
  • Days 2 and 3: Use the same location, and give the plant an extra hour than the day previous.
  • Days 4 and 5: Move the plant to a location with some filtered sunlight and partial shade. On day 4, start with the same time as day 1 – 2 to 3 hours. On day 5, increase the time by one hour.
  • Days 6 and 7: Move the plant to a spot that gets more direct sunlight and let it sunbathe for a couple of hours. Again, avoid the hottest part of the day. Two hours will be enough to start direct sun on day 6, then another hour on day 7.

By the end of the week, they’ll be resilient and ready for the next stage, which will be adapting to night conditions.

Introducing plants to overnighters outdoors

On the nights when the temperatures will remain above 50°F (10°C), leave the plants outdoors overnight. The first week only introduces some new elements. The night is different and darkness is just as important to plant care as sunlight.

Overnight, there’s no sunlight, it’s cooler, and there’s bound to be some creepy crawlies hovering around it. All things young plants need to be acclimated to.

Throughout the second week, leave the plant outdoors overnight – weather permitting.

Precautions

  • Aim to begin the hardening-off process after the last frost date has passed.
  • Before transplanting, check the forecast. You want the day and night temperatures to be constantly above 50°F (10°C) to avoid cold damage.
  • Light showers are fine; heavy downpours can snap young seedlings. Bring your plant indoors if rain is persistent.
  • Wind: Shelter the seedlings from strong winds. Ideally, expose them to a moderate breeze.
  • Check the soil moisture more regularly because outdoor conditions can cause the soil to dry out faster.

Don’t neglect soil temperature when transplanting

Hardening off seedlings before transplanting doesn’t do much for the roots other than moderate their water consumption in light of the faster drying out of the soil. When transplanting in the ground, the ground temperature is important.

Probe a digital thermometer into the ground to a depth of 4” to 6”. Warm-weather plants like tomatoes do well with soil temperatures in the range of 65°F to 70°F (18°C to 21°C). Cool season crops are fine with soil temperatures around 50°F (10°C).

Raised beds have warmer soil. More so if it’s metal.

The best time to transplant

Late afternoon on a cloudy day works best because there’s less sunlight so the leaves aren’t going to lose as much moisture from transpiration.

Know the word “overcast”?

That’s describing clouds “diffusing” the sunlight. There’s still plenty of sunlight to support plants’ vital functions. They just have to work a tad bit harder and that’s what you want. To make them work so it toughens them up.

Helping young plants transition with mulch

Adding mulch is good for plants. It makes transitioning after transplanting easier on your plants. It prevents weeds from competing for nutrients, protects roots from drought and moderates the soil temperature.

Two to three inches is all you need. Any more than that can limit the amount of water reaching the soil. Since that’s where the roots are buried, too much mulch will hinder the health of the plant. Go easy on it.

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