April 29, 2025, 12:32 pm | Read time: 4 minutes
Tomato, pepper, and zucchini plants often grow into seedlings on the windowsills of many hobby gardeners. myHOMEBOOK offers tips on how to best prepare them for the move outdoors.
In May, it’s finally time: The last frost nights leave a delicate frost over the garden. After that, you can plant and sow to your heart’s content, without the threat of frost damage. Even those who have painstakingly grown their own vegetable plants from seeds can now safely transplant them to their final location. However, this move should not happen in a single day. Why you should harden off vegetable plants is explained in the following lines.
Overview
Why You Should Harden off Vegetable Plants
Even the strongest young plants, which have grown from tiny seeds into already stately plants over several weeks on the windowsill or in the greenhouse, do not handle an abrupt move from their nursery well. Outside conditions are quite different from those indoors. Wind and weather can present the young plants with the following problems:
Instability
Apart from a little draft during occasional ventilation, it is windless in the house and apartment. The young plants grow without the influence of wind, and consequently, the stems of the young plants may be less stable. Especially in very bright and warm places like a south-facing window, tomatoes tend to grow rapidly. The stems are then particularly thin. If these young plants were moved directly into the bed from one day to the next, they could simply snap at the first gust of wind.
Cold and Heat Tolerance
In the living room, temperatures are more or less constant. The young plants grown there are accustomed to this. Even though there are no more night frosts after the Ice Saints, temperatures below five degrees can be challenging for heat-loving nightshade plants like tomatoes or peppers. During the day, however, the sun shines directly on the leaves of the plants without protective window glass. These extreme temperature differences are simply not what the young plants are used to.
Rain and Humidity
Rain and increased humidity can also lead to adjustment difficulties for young plants after a sudden move outdoors. Still delicate, unstable plants can bend under a heavy downpour. High humidity can promote diseases like fungal infections in the still less resistant plants.
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How to Harden off Vegetable Plants
When moving from the windowsill or greenhouse to the open field, it is primarily the changed climatic conditions that make hardening off vegetable plants necessary. Otherwise, the painstaking process of growing from seed to young plant can quickly be in vain. Therefore, gradually hardening off the vegetable plants is the right way to successfully transition from indoors to outdoors. The following steps should be taken:
- Two-week period: Hobby gardeners should allow about two weeks for the move of the plants. A time after the Ice Saints is ideal, when there is no longer a threat of frost.
- Initially only for a few hours: At the beginning of hardening off, place the vegetable plants for only a few hours in a wind-protected, warm spot in partial shade. It should not be pouring rain or have strong gusts of wind. Afterward, the plants return to the windowsill.
- Gradually increase to half days: After a few days of hourly hardening off, the number of hours can be gradually increased.
- Allow more climatic influences: After a week, the young vegetable plants can also withstand a rain shower, some wind, or stronger sun. They can now stay outside all day and only need to be brought in at night.
- Moving to the open field: Finally, after about two weeks, the hardened-off vegetable plants can be fully moved to their final location. If cool nights are still expected, a protective fleece can provide warmth at night.