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  • Writer's pictureTony Taurone

Hardening Indoor Seedlings

Updated: Mar 25, 2023

Q: What is hardening plants?

A: Hardening plants that were started indoors is the process of gradually introducing them to sunlight and outdoor temperatures before planting them in the garden.

Q: How long do you harden plants?

A: 1 to 2 weeks of daily hardening is needed to build the seedling tolerance to sunlight UV and cold.

Starting garden plants indoors is a great way to get a head start on the growing season, but it's important to remember that these plants need to be "hardened off" before they can be transplanted outside. Hardening off is the process of gradually acclimating the plants to outdoor conditions so they can adjust to the wind, sun, and temperature changes they'll encounter outdoor in the garden.

Indoor-grown plants that have been started under LED lighting may not have developed UV resistance because LEDs emit very little ultraviolet (UV) light, which is an essential part of the spectrum of natural sunlight. Without UV exposure, plants may be more susceptible to sunburn, dehydration, and other stresses when they are moved outside into full sun.

UV radiation affects the way that plants grow, and exposure to UV light can trigger physiological changes that help them tolerate harsh outdoor conditions. For example, plants that are exposed to UV light produce more protective pigments such as flavonoids and anthocyanins, which can help shield them from UV damage and other environmental stresses.

If you've started your plants indoors under LED lighting, it's important to gradually expose them to natural sunlight during the hardening-off process. By gradually increasing their exposure to UV radiation, you can help them develop the necessary UV resistance to thrive in the outdoor environment.

Here are some tips for hardening off your indoor-grown plants:

Start early: Begin the hardening-off process at least one week before you plan to transplant the plants outside. If possible, start even earlier to give your plants more time to adjust.

Gradually increase sunlight exposure: Start by placing your plants in a shady spot outside for a few hours each day. Gradually increase the amount of time they spend outside and the amount of sunlight they're exposed to until they're getting full sun for most of the day.

Increase wind exposure: Similarly, begin by placing your plants in a sheltered spot outside, such as against a wall or fence. Gradually expose them to more wind by moving them to a more open location each day.

Watch the temperature: Keep an eye on the weather forecast and bring your plants inside if there's a risk of frost or other extreme weather conditions. If possible, gradually expose your plants to cooler nighttime temperatures as well, as this will help them adjust to the outdoor climate.

Water regularly: Plants that are exposed to wind and sun will dry out more quickly than those kept indoors, so be sure to water your plants regularly during the hardening-off process. Keep the soil moist but not waterlogged.

Transplant carefully: Once your plants are fully acclimated to outdoor conditions, it's time to transplant them into the garden. Be sure to choose a mild, overcast day for transplanting, and water the plants well before and after transplanting.

By following these tips, you can successfully harden off your indoor-grown garden plants and give them the best possible start in their new outdoor home. With a little patience and care, your plants will thrive in the garden.

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