Cultivars used for seedlings can also be intended for direct sowing, but growing them from seedlings ensures higher and more reliable yields. Seedlings can be grown as bare-root or with a clump of substrate, with the latter being preferable, as with other vegetables.
Container Cultivation of Seedlings in Protected Spaces
Seedlings can be grown on beds or in containers, with container cultivation offering several advantages, making it more commonly used.
Advantages of Container Cultivation of Seedlings:
- Seedlings grown in containers have a completely uniform vegetative space, ensuring consistent plant growth and high uniformity among seedlings.
- Ready-made substrates with good water-air ratios, high water retention, and sufficient nutrient content are used for seedling cultivation. These substrates are sterilized and free of disease pathogens, pests, and weed seeds, which are typically present in soil.
- Transplanting with a clump of substrate around the roots reduces stress for the plants when moving to field conditions compared to bare-root transplants, allowing continuous growth. They establish more easily and quickly, making replanting almost unnecessary. Such seedlings can also be transplanted into soil that is not at optimal moisture levels and can be planted throughout the day, optimizing the use of existing planting equipment and allowing for planting on larger areas at the optimal time.
- Seedlings grown in containers mature earlier for harvest and generally yield more than bare-root seedlings grown on beds.
- The container method is particularly suitable for the production of hybrid vegetables, where the high cost of seeds makes it crucial to produce quality seedlings from each seed.
Ready-made substrates with good water-air ratios, high water retention, and sufficient nutrient content are used for sowing in containers. These substrates are sterilized and free of disease pathogens, pests, and weed seeds.
Sowing can be done manually or with a pneumatic seeder.
For spring planting, sowing should be done in late January or early February, with seedlings grown in protected spaces. For autumn planting, sowing can be done outdoors in mid-August in continental areas or mid-September in Mediterranean areas.
Microclimatic Conditions for Seedling Cultivation
To ensure rapid germination, containers are placed in germination chambers where optimal temperature (around 28°C) and full air saturation with humidity are maintained using micro-sprinklers that create a fine mist. Containers remain in the germination chambers for a few days until the seeds germinate. After that, they are moved to heated protected spaces where a consistent optimal temperature is maintained day and night until the seeds fully emerge, and the substrate moisture is maintained. After the young plants emerge and develop their first cotyledon leaves, the temperature in the protected space should be lowered to around 17°C, with a night temperature higher than 5°C. Regular substrate moistening, ventilation, and active air circulation are maintained until the seedlings develop 3-4 leaves and the false stem reaches a thickness of 4 – 5 mm, at which point they are ready for transplanting. In conditions of excessively high temperatures after emergence, elongated and fragile seedlings of poor quality and lower productivity are obtained.
Method of Seedling Cultivation and Care of Emerging Plants
Containers in protected spaces should be elevated slightly above the ground, using bricks or stone blocks and supported by concrete rebar or wooden slats. This elevated position allows for faster and better heating, and the air space beneath the containers prevents roots from growing through the holes at the bottom of the seedling cells, making it easier to extract the seedlings during transplanting.
The substrate must be kept moderately moist throughout the entire seedling growing period. Due to the relatively small volume of the seedling cells in containers, watering should be done in the morning, and moisture should be maintained during the day by misting. It is desirable for the plants to remain dry in the evening and at night. The best water for irrigation is rainwater collected from structures, water from wells, or clean water from natural water sources. Direct watering with tap water is not ideal, as it is usually significantly colder, which can negatively affect seedling development. Watering seedlings is done using automatic irrigation systems with sprinklers that produce extremely fine droplets, which do not damage young plants or compact the substrate. If watering is done manually, watering cans with fine nozzles should be used to create very fine droplets. With an automatic irrigation system, foliar fertilizers such as Fertina P, Profert Mara, Kristalon, and others can be applied as needed. Foliar feeding is applied when there is a noticeable loss of dark green color, which often occurs if seedlings cannot be transplanted at the optimal development stage and must remain in containers for an extended period.
Using quality treated seeds and growing in sterilized substrate generally eliminates the need for disease protection for seedlings during container cultivation. However, before transplanting, it is advisable to treat the seedlings preventively with contact fungicides such as mancozeb at a concentration of 0.2% or copper-based fungicides at 0.2%.
A week before the planned transplanting date, protected spaces should be maximally opened to expose seedlings to direct sunlight, helping them better adapt to the full spectrum of sunlight they will experience after transplanting in the field. Before transplanting, the substrate must be moderately moist, which allows for easy extraction of seedlings with substrate attached to the roots. If the substrate is too wet, it falls off the roots during planting, and if it is too dry, the plants will establish poorly in the field.
Transplanting Seedlings into Open Fields
About 8 weeks after sowing onions in August (or September), when the seedlings have developed 3 – 4 leaves and the false stem is 4 – 5 mm thick, they are ready for transplanting. They are planted in beds or on flat surfaces in rows (the number of rows in a strip can vary, but strips with 5 or 6 rows are most common), with a row spacing of 20 – 30 cm and a plant spacing within the row of about 10 cm. When planting, the stem base should be buried 2 – 3 cm deep. For this type of cultivation, cultivars with a long vernalization stage and a longer juvenile phase are suitable, as in warm autumns and mild winters, the plants pass through the juvenile phase, vernalize in the second half of winter, and then flower in spring, losing their consumer value, which is the goal of cultivation.
Seedlings grown during the winter for spring planting are ready 10 – 12 weeks after sowing. The planting method is similar to that of autumn planting, except the spacing between plants is slightly larger (about 15 cm).
In both cultivation methods (autumn and spring planting), proper water application, nutrient supply, and protection from weeds, diseases, and pests are essential.
