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Hardening and strengthening plants - here's how

20.01.2023  /  Reading time: 9 minutes

Hardening off plants is very important to ensure that your young plants grow well in the bed. In this article, we explain why and how you should harden off and strengthen your young plants before planting them out. There are also tips on planting out vegetable plants!

This article contains:

  1. Hardening off seedlings: why harden off plants?
  2. Getting plants used to the outdoors: it's that simple
  3. Hardening plants: a guide
  4. Hardening the tomato plant
  5. Planting out: from the windowsill to outdoors
  6. Protect plants from frost
  7. Frequently asked questions about hardening off plants

Quick Overview

Hardening plants

  • When? One to two weeks before the planned planting date
  • Why? To acclimatize the young plants to wind, cold, temperature fluctuations and direct sunlight
  • Which plants? All plants that are grown in advance, such as tomatoes, peppers, chili, pumpkin, zucchini, etc.

Instructions for hardening and strengthening plants

  • Start slowly by putting the plants outside for a few hours. First choose a shady spot protected from the wind. Increase the number of hours each day.
  • After about a week, get the plants used to the sun and slowly expose them to the sun's rays. After three to five days, the plant can then be placed in full sun.
  • Towards the end, leave it outside at night to acclimatize it to temperature fluctuations.

Hardening off seedlings: why harden off plants?

When growing vegetable plants, the next important step after pricking out your seedlings is to harden off the young plants. Before they are planted in the bed, you should introduce your protégés to life outdoors. On the windowsill, there is no wind, cold or strong temperature fluctuations between day and night. In addition, the small plants have never been exposed to direct sunlight. So that they don't have to get used to all these environmental stimuli when they are planted out, hardening off is essential to strengthen them beforehand. Hardening off stimulates certain metabolic processes in the plants and stores secondary plant substances for protection. They are planted out stronger and can put all their energy into their root system. The cuttings grow better and continue to grow robustly and vigorously.

Getting plants used to the outdoors: it's that simple

Only young plants that are grown in advance need to be hardened off. In our latitudes, these are mainly nightshade plants such as eggplants, tomatoes, tomatillos, peppers, chillies and pumpkin plants such as squash, melon, cucumber and zucchinis. These crops require particularly warm temperatures and can only be planted outdoors after the Ice Saints from mid-May. Start hardening off two weeks earlier. However, lettuce, kohlrabi, chard and herbs are also often grown in advance. As a general rule, all plants that have been grown on the windowsill need to be hardened off for one to two weeks. For crops that are planted out early in the year, you should acclimatize the young plants to cold temperatures in particular. However, you should protect them from extreme weather and cover them in the event of frost, for example.

Kohlrabi young plants hardening off
Before you plant out the kohlrabi seedlings, they need to get used to the frosty temperatures early in the year.

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Hardening plants: a guide

1) Strengthen vegetable plants against environmental influences

To make the tender seedlings more robust and stable, you need to expose them to wind and weather. However, start hardening off the young plants slowly, step by step. First, place them in a shady, wind-protected spot and leave them outside for a few hours. Now increase the number of hours your protégés are outside each day. After two or three days, you can also move them to an unprotected place and expose them to wind and minimal temperature fluctuations. Place them back in the sheltered house overnight.

2) Getting young plants used to the sun

Without getting used to sunlight, the young plants would get sunburn. Only when the plants harden off do they store so-called flavonoids in their surface, which protect them from harmful UV radiation.

How long should plants get used to the sun?

After you have acclimatized your young plants to the temperature and wind outside for a few days to a week, you can slowly start to place the plants in partial shade. Extend the time in the sun every day. After about three to five days, your young plant can now stand in full sun without burning itself.

3) Harden plants against the cold

During the first week, the seedling will gain stability and resistance. Now you can also leave it outside overnight. The plants need to get used to the temperature fluctuations between day and night. At first, cover them to protect them from the cold. After about two nights, the young plants will be hardened off and can spend the night outdoors without additional cover (unless there are spontaneous, severe late frosts!). They are then ready to be planted out.

Harden off young plants
Slowly introduce your young plants to their move outdoors. However, they should be protected and shaded for the first few days!

While you are hardening off your plants, you should keep the soil evenly moist. It is also advisable to fertilize the seedlings once they have reached a certain size and time in the pot. It's best to use organic liquid fertilizer such as nettle slurry or horsetail broth.

Make sure that your plants look healthy during the individual steps. As soon as leaves dry out, die or droop, this is a sign of stress or damage. Then find out what the cause is (e.g. sun or cold) and shorten the time with the respective environmental influence so that the plants can recover. Then you can slowly start hardening off again.

Hardening the tomato plant

Tomatoes are a very popular crop. Due to their enormous variety, they are often grown and planted themselves. Due to their tropical origin, nightshade plants like it warm and are very sensitive to cold. When hardening off tomatoes, make sure that there are no late frosts. If they do, be sure to bring the plants indoors, as they are never frost-hardy! The same applies to other nightshade plants such as peppers, chili peppers, eggplants and pumpkins.

When should tomatoes be put out?

You can start hardening off the tomatoesabout two weeks before the planned planting date. They should already be around two months old and should only be placed outside when the temperature is constant between 12 and 18 degrees. In the event of severe and prolonged cold spells, be sure to put the tomato plants inside! They can only withstand such extreme temperatures for a short time and will then die.

How long to harden off tomatoes?

Tomato plants should also be hardened off for one to two weeks. For plants that will be grown outdoors, it is better to choose two weeks. This will ensure that they are strong and vigorous enough. Only one week is usually enough for greenhouse tomatoes. The influence of wind and weather is not as strong here.

Planting out: from the windowsill to outdoors

After hardening off the young plants, they are planted out. Now they are ready for life outdoors. Most crops are planted out in mid-May, after the last frosts. This is especially true for heat-loving plants such as tomatoes, peppers, etc. Frost-hardy plants can also be planted out earlier, e.g. kohlrabi, various winter lettuces and cabbage. Depending on the crop, you should find out in advance when it is possible to plant them out. For a good start, choose a dry, windless day without extreme temperatures.

Freshly planted kohlrabi seedlings
After planting out, the leaves of the young plants tend to droop until the roots have grown properly.

Protect plants from frost

In general, you should make sure to water your young plants well when planting out (unless frost is forecast for that day). If frost is forecast, it is also advisable to protect the seedlings with fleece for the first few days. As an alternative to classic fleece, you can also reuse old blankets or newspapers. Coconut or hemp mats and old jute sacks are also suitable. Once the young plants are well established, they will eventually be able to withstand the frost. Until then, they will appreciate some protection. In general, you should protect all freshly planted young plants from frost. Frost can damage young plants in particular, which can cause them to die.

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Marie studied agricultural science at the University of Hohenheim. Her main focus is on ecological agriculture and permaculture. She writes articles for Fryd to educate people about ecological interrelationships and alternatives to current land use. Our current economic systems, especially in agriculture, have numerous negative effects on nature and destabilize our ecosystems. We need a great diversity in our gardens and beds again to counteract the extinction of species. Every gardener can contribute to creating and maintaining habitats and food for a wide variety of creatures. With her articles, she would like to pass on her experience in dealing with natural systems and give people the opportunity to contribute to a stable ecosystem and thus also to securing our livelihood.

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Plants must be hardened off to acclimatize them to wind, cold, temperature fluctuations and direct sunlight before they are planted out. This strengthens their resilience.

Hardening off should begin one to two weeks before the planned planting out in order to gradually adapt the young plants to the outdoor conditions.

After a few days of acclimatization to temperature and wind, the plants should slowly be exposed to sunlight for longer periods of time, starting with shady spots and working up to full sun.

Your tomato plants should be around two months old and should only be placed outside when the temperature is constant between 12 and 18 degrees. To harden them off at night, the temperatures should always be above 8 degrees.

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