Hardening Off Plants Started Indoors

Congratulations! You’ve done the hard part. Nurturing tiny seeds into vibrant, green life. But before you rush those pampered seedlings into the garden, there is one final, essential step to ensure their survival: Hardening them off.

Hardening off is a necessary transition period for seedlings grown indoors. In a controlled home environment, plants are not exposed to wind, temperature swings, or as intense sunlight or UV radiation. If moved outside suddenly, these plants will not be ready for the change and often suffer from transplant shock. This can manifest as sudden droopy plants, wilted stems or scorched leaves for example.

By introducing them to the outdoors gradually, you allow the plants to get a chance of developing more resilient leaves and stems. This change ensures they can withstand the environmental stresses of the garden, and thrive in it once fully acclimated.

How to Acclimate Your Plants

Check the Temperature

Before starting, verify that local temperatures are appropriate for your specific crops. Most seedlings should stay indoors at least until the danger of frost has passed. For heat-loving plants such as peppers, melons and eggplants, wait until nighttime temperatures are consistently at least 50-55°F (10-13°C) .

Phase 1: Indirect Exposure

On the first day, place your plants in a sheltered outdoor spot with only indirect light and protection from the wind for one hour. Increase this time by roughly one hour each day for one week. If you do not have a naturally sheltered area, you can use a row cover, horticultural fleece or something similar to filter the harshest direct sunlight.

Phase 2: Direct Sunlight

During the second week, allow the plants to stay in their sheltered spot for the full day. For plants that prefer part sun or full sun exposure later on, you can now begin moving them into direct sunlight for one hour at a time. Gradually increase this direct exposure throughout the week.

Transplanting

Once the plants have adjusted to the preferred light and stable overnight temperatures, they are ready to be moved into their permanent location in the ground or a large container.

Note that you might need to adapt the duration of the acclimation process. If you see signs of stress, such as droopy leaves, pause further exposure and let the plants adjust to that level before moving on.

What to Expect

Even with a careful transition, older leaves grown indoors may show minor signs of stress or pale patches, known as sunscald. These blemishes are often unavoidable and are generally not a cause for concern. The most important indicator of success is the new growth. Any growth that develop after the plant has been moved outdoors will be naturally adapted to the sunlight and wind.

Alternative Method: Using a Cover

For less sensitive plants, an alternative approach is to transplant them directly into their final location and immediately protect them with a row cover or shade cloth for one to two weeks. This fabric provides a barrier against harsh wind and direct sunlight, allowing the plant to establish its roots while gradually adapting to the outdoor environment.