How to properly hardening off seedlings

How to properly hardening off seedlings
How to properly hardening off seedlings

Germination cultivation methods for vegetables and other vegetable crops are related to our climatic conditions. In the vast majority of U.S. regions, the average daily temperature of 50-59°F (10-15°C) has a frost-free period of 110-140 days per year, much less than most vegetable crops with long growing periods (from 130 to 200 days or more).
Sowing and growing plants in the open field can start in March-April – the period when solar radiation is high enough. However, the frost-free period begins from February to March by region. Weather conditions have been created that limit the normal development of plants.

Under these conditions, a 30-60 day greenhouse period is a good time saver for thermophilic crops that do not have enough short summers to form and mature crops. This article answers the question of why is it necessary to hardening off seedlings?


Seedlings in our apartments and greenhouses are grown at artificially created optimum temperatures of 64-86°F (18-30°C). When grown in the open, sudden changes in temperature and humidity conditions negatively affect the seedlings.
In addition, any disturbance to the plant’s natural environment, including transplanting, can lead to disease. When transplanting, the root system is affected. A period is needed to restore the normal water supply to the above-ground plant organs.

During this recovery period, a gentle environmental impact on the seedlings is necessary. Idle root systems, mismatched light intensities, and temperature conditions lead to stagnation of the plant’s metabolic and growth processes. To shorten the adaptation period to the new environment, which will help seedlings recover faster, it is necessary to gradually acclimatize or prepare seedlings for the new conditions. This is the main essence of seedling hardening.


By means of sprouts, you can grow almost all vegetable crops whose development period is longer than the warm season in the region if you want to get an earlier harvest of vegetables in open spaces. Such crops include tomatoes, bell, and bitter peppers, eggplant, cucumbers, zucchini, courgettes, squash, watermelons, melons, all types of cabbage, and other crops.

To obtain healthy, well-developed seedlings, they should be hardened indoors (greenhouses, greenhouses, windowsills at home, etc.) throughout the growth and development period until they are planted in the open. Seedlings gradually get used to living in open spaces.

Temperature hardening regime

The first hardening of seedlings is carried out 2-4 days after sprouting. Indoor temperatures are reduced from 62-77°F (17-25°C) to °F (8-16°C) during the day and from 50-59°F (10-15°C) to 44-53°F (7-12°C) at night, depending on the culture (Tables 1 and 2), over a period of 4-7 days, which prevents seedling pulling.

Further reductions or sharp increases in temperature on hot days will result in slowing down the developmental process of seedlings and their disease. The temperature regime is maintained from 2 weeks of age until the seedlings begin to harden, gradually tightening the environmental conditions.

Ventilate the room on hot sunny days and do not ventilate it. Open windows or transoms from 5-15 minutes to 2-4 hours a day. During the growing season in the greenhouse, it is necessary to constantly monitor not only the air temperature but also the soil temperature. Delicate roots entering the open ground will not be able to withstand temperature changes and may become sick, resulting in plant death.

Table 1

Air temperature, °С4-7 days from germination of seedlings4-7 days from germination of seedlingsFrom day 8 from hardening seedlings to hardening seedlingsFrom day 8 from hardening seedlings to hardening seedlingsFrom day 8 from hardening seedlings to hardening seedlings
Name of cultureIn the afternoonAt nightin the afternoon (Mainly cloudy)in the afternoon (sunny)at night (sunny)
Sweet and bitter pepper14-178-1018-2025-2711-13
White cabbage early8-107-913-1515-177-9
Zucchini, squash20-2215-1718-2020-2516-17
Table 1

Table 2

Soil temperature, °С
Name of culture12-15 days from germination of seedlings12-15 days from germination of seedlingsFrom day 16 from hardening seedlings to hardening seedlingsFrom day 16 from hardening seedlings to hardening seedlings
in the afternoonat nightin the afternoonat night
Sweet and bitter pepper20-2417-1820-2215-16
White cabbage early15-1711-1214-1610-11
Zucchini, squash20-2317-2020-2415-17
Table 2

Sunlight exposure

Seedlings of all seedling crops are intolerant of direct sunlight during the first few days and young leaves can be badly burned. Therefore, shade seedlings from the first 3-4 days of germination, leaving them in the sun for 15-20 minutes a day, from 10 to 11 or from 14 to 15 hours. The duration of sunlight exposure is gradually increased, and by 2 weeks of age, seedlings can be open all day.

Need for increased lighting for seedlings

During winter and spring, seedlings clearly lack the intensity of natural light, and long-day plants require additional lighting. The additional light period for tomatoes is 14-16 hours per day. Eggplant and pepper have a light period of 14-16 hours at the stage of growing to 4 true leaves and 10-12 hours thereafter. For cruciferous plants, the additional light period varies within 10-12 hours. Pumpkins are short-day plants and do not require additional light.

When growing seedlings of several crops with different light periods in a greenhouse, use cover materials that do not let light through. When growing seedlings of several cultures in rooms with different lengths of daylight, move the containers with the plants to a dark and cool room after 10-12 hours of light-time and put them back the next day.


Regardless of where they are cultivated (at home, in a greenhouse, in a greenhouse, under a temporary cover of film or spindle fabric), seedlings must necessarily be hardened before planting. 1-2 weeks (no more) before planting seedlings, night air temperature is lowered to 53-57°F (12-14°C) for tomatoes, eggplants, bell peppers, squash, and for more hardy (cabbage, lettuce) – to 42-46°F (6-8°C).

If the active hardening period is increased to 3 weeks and more, even at further lower temperatures, the plants will retard growth and subsequently reduce the yield of the culture, sometimes by up to 30%.

Lower the temperature 3-5 days before planting to bring it to the ambient temperature of the open space. To do this, seedlings grown indoors are brought to a closed balcony and left there day and night. It is better to close the windows and fans at night so that there will be no sharp night frosts. If the seedlings are raised in a greenhouse or greenhouse, the beams are raised so that the temperature gradually equals the street temperature.

While the above-ground parts harden, the seedlings’ root systems become accustomed to lower and harsher conditions. As the air temperature decreases, the number of water decreases. Irrigation rates did not change. Only the interval between waterings was increased. Longer drying periods helped the soil to dry out. The soil remained moist in the root zone but dried out in the upper part.

This regime stopped the growth of young plants. They become “thicker,” the root system grows intensively, the leaf organs develop, and the cabbage leaves are covered with a waxy coating. During this period, it is very important not to allow the soil to dry out excessively.
The shoots will begin to drop, and the tension of the leaves will decrease to a painful state. Generally speaking, the vitality of the plant will decline.

Hardened feeding is done 1-2 days before planting to provide basic nutrition for the plants. Some gardeners perform this procedure 10-12 days after harvesting. You can feed the plants with ammonium nitrate, calcium superphosphate, and potassium sulfate solutions – 10, 40, and 60 grams per 2.5 Gal (10 liters) of water, respectively) or 60-70 grams per 2.5 Gal (10 liters) of water of nitroglycerin. For fertilization, you can use safe and reliable fertilizers or other mineral fertilizers containing nitrogen, phosphorus, and potassium. Feeding will shorten the rooting period and increase the number of rooted plants by up to 100%.

The last days of the seedlings should be in an open space under the canopy or on an open terrace around the clock. If there is a threat of frost, seedlings should be covered overnight with spun-bond or other mulching cloth. Covering with the film is less comfortable for the plants.

Hardened and nourished seedlings will more easily survive stressful situations and aggressively continue to develop further when transplanted into field conditions. If transplant preparation is not done properly, seedlings will delay development for 5-10 days or more.

More related information about hardening off

We will be happy to hear your thoughts

      Leave a reply

      4 + ten =!
      Compare items
      • Total (0)