Growing onions by direct sowing is possible only with an irrigation system in place. Soil preparation for sowing is described in the previous article (“Soil Preparation and Fertilization”).
Direct sowing in continental areas should be done in the second half of March or the first half of April, taking care to monitor soil and air temperatures. Sowing in soil temperatures below 7°C can result in slow and uneven germination, with some seeds failing to sprout, leading to thinning stands and reduced future yields. If negative temperatures (below -2°C) persist for more than 2 days during germination, the young plants will perish. Sowing should not be too late, either, as this does not leave enough time for the plants to develop sufficient leaf mass for proper bulb formation, resulting in thick-necked bulbs unsuitable for storage.
For direct sowing, it is recommended to use hybrid seeds that are calibrated, uniform in absolute weight, have a high germination rate, and are treated against diseases and pests. The amount of seed used depends on the sowing method and the desired plant density (number of plants per unit area). The following formula can be used to calculate the precise amount of seed needed:
[ \text{X} = \frac{1000 \times \text{A}}{\text{B} \times \text{C} \times \text{D}} ]
Where:
- X = seed (kg/ha)
- A = planned number of plants/m²
- B = number of seeds per gram
- C = % germination rate
- D = field factor
The field factor is 0.7 under favorable conditions and 0.5 for early sowing and unfavorable conditions.
For growing bulbs with a diameter of 5 – 7 cm, a density of 50-100 plants/m² is planned, while for larger bulbs, a density of 25 – 50 plants/m² is used.
The number of seeds per gram is usually provided by seed companies on the packaging.
Sowing is done with a precision, usually pneumatic, seeder for small-seeded vegetable crops, at a depth of 1.5 – 3 cm. Sowing is done in five- or six-row strips with a row spacing of 25 – 30 cm and a strip spacing of 50 – 60 cm.
For uniform germination, the sowing layer must be evenly moist. If this cannot be ensured naturally, irrigation should be carried out, preferably with light sprinkler irrigation, to avoid crust formation on the fine seedbed layer.
Onions should not be deprived of water during the growing season. In case of drought, irrigation must be provided. While onions can tolerate drought, a lack of water will halt growth and development, resulting in reduced yields even if growth resumes with water supply. The greatest need for water and nutrients occurs during the phase of intensive leaf mass growth and during bulb formation. Irrigation should be stopped three weeks before the planned harvest to prevent the outer dry leaves from splitting due to the subsequent growth of the bulbs.
In areas with moderate winters, onions can be sown in mid-August using the same method as in spring sowing. Moisture supply is critical during germination and the early growth phase, as it is desirable for the plants to develop enough to withstand possible low temperatures without undergoing vernalization. Cultivars intended for this type of cultivation are bred to withstand temperatures down to -10°C, provided the false stem diameter is 5 – 7 mm.
The use of herbicides is essential throughout the entire growing season of onions.
