What causes onions to have a poor appearance?

Abnormal onion growth leads to a poor appearance, and a poor appearance affects onion sales. Therefore, when abnormal onion growth occurs, attention should be paid to it and it should be solved as early as possible. Abnormal onion growth is generally related to climate, seed selection, nutrient imbalance, pests and diseases, and other reasons. Below is a detailed analysis of how these causes lead to abnormal onion growth.

1. Main causes of abnormal onion growth

1.1 Climatic causes. The onion is a typical biennial plant that undergoes vernalization as a seedling. Only after the seedling grows to a certain size (once the stem diameter reaches about 0.7 cm, it enters the critical period for responding to low temperatures) can it respond to low temperatures. At this time, if cold air intrudes or the temperature is low, the vegetative growth of the seedling will shift to reproductive growth. If the low temperature lasts too long, vernalization occurs and the plant bolts and flowers. If the temperature is low, incomplete vernalization occurs and abnormal differentiation of scale buds is caused. A “twin” or “multiple” phenomenon appears. For example, in 1998, the Huaguoshan onion production area suffered from low temperatures, excessive rain, and the intrusion of cold air, resulting in abnormal growth and almost total crop failure.

1.2 Seed causes. Because the environmental conditions of the seed's place of origin differ from those of the introduced area, non-local seeds respond differently to the low temperatures and day length of the introduced area. Weak-winter varieties are often sensitive to low temperatures, and seedlings bolt when they encounter even slight low temperatures. Strong-winter varieties can only sense low temperatures and bolt and flower after the seedlings are slightly larger, and they are less adaptable than local seeds.

1.3 Improper sowing time. Onion is a green-plant vernalization type plant. Sowing too early results in an overly long seedling age and overly large seedlings, exceeding the critical size required for green-plant vernalization. This makes vernalization or incomplete vernalization likely to occur, causing abnormal bulb growth.

1.4 Nutrient imbalance. Too much irrigation and excessive nitrogen fertilizer will cause the seedlings to be too large, delay or inhibit bulb formation, and in low-temperature seasons also cause bolting and flowering. Insufficient nitrogen fertilizer and soil drought cause the seedlings to grow too weakly, increasing the ratio of carbohydrates to nitrogen in the plant, leading to early entry into reproductive growth and the appearance of bolting and flowering.

1.5 Damage from root maggots. Severe damage from root maggots causes the onion to grow too weakly and delays bulb growth.

1.6 Planting depth. During planting, planting too deep or too shallow often leads to poor bulb growth and development. If planted too deep, leaf growth becomes excessive, the bulb becomes thicker, and the bulb is too small and prone to deformity. If planted too shallow, the plant is prone to lodging, the bulb is exposed, and after sun exposure it turns green and cracks.

2. Solutions for abnormal onion growth

The purpose of onion cultivation is to obtain enlarged bulbs. You should choose varieties suited to the local sunshine and temperature conditions, master the appropriate sowing period, and increase planting density so as to better prevent and control the occurrence of abnormal growth.

2.1 Strictly select varieties. In production, it is best to choose local excellent varieties. If introduction is needed, according to the variety's response to low temperature and day length, try to choose excellent varieties with strict low-temperature requirements. In this region, the commonly used variety is the Ningxia red-skinned round onion or the local red-skinned onion.

2.2 Determine the appropriate sowing period. The sowing period should be correctly determined according to variety characteristics and climatic features. In the Ili region, autumn sowing is from late September to early October; spring sowing is from late March to early April.

2.3 Strengthen field management

1) Fertilizer and water management. Pay attention to the application of phosphorus and potassium fertilizers, and avoid excessive nitrogen fertilizer, so as not to cause excessive aboveground growth and bolting or delayed bulb expansion. For spring-sown onions, keep the soil moist before and after seedling emergence, water frequently, and hoe frequently. The peak leaf growth stage and bulb expansion stage are the periods of greatest fertilizer and water demand for onions, so fertilizer and water should be applied in a timely manner. For 667 sq m, apply 15 kg of diammonium phosphate, 20 kg of diammonium phosphate, and 3--5 kg of potassium sulfate respectively.

2) Pest control. Mainly control the damage of root maggots. Dimethoate and dichlorvos can be applied with irrigation water. The use of raw manure as fertilizer is prohibited because root maggots are attracted to raw manure. Fully decomposed organic fertilizer should be used, applied evenly and deeply, and seeds should be separated from fertilizer. In plots where root maggots have already occurred, irrigate frequently and, if necessary, use flood irrigation, but irrigation should be coordinated with onion growth. Chemical control: 50% dimethoate 1000-fold solution or 80% dichlorvos emulsifiable concentrate 1000-fold solution can be applied with irrigation water.

2.4 Appropriate planting depth. For transplanted onion seedlings, attention should be paid to planting depth, to the extent that after planting the soil covering can bury the small bulb and, after watering, the seedlings neither fall over nor float.