Biological Features Of Onions

2024 Author: Sebastian Paterson | [email protected]. Last modified: 2023-12-16 13:47

Growing onions in the Northwest region

Onions belong to the onion family. It is a biennial plant.

In the first year, it forms a bulb, in which reserve nutrients are accumulated, and in the second year, a flowering plant is formed from the bulb, giving seeds. Under the conditions of the Non-Black Earth Zone, onion plants produce seeds, as a rule, only in the third year, and in amateur vegetable growing it is difficult to obtain high-quality seeds.

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What are onions

Onion seeds have an irregular triangular shape and are covered with a black hard shell. 1 g contains 250-400 pieces. They sprout slowly. When sowing in the soil in spring - only on the 10-16th day, and under unfavorable conditions of temperature and soil moisture, onion shoots appear only after 20-30 days. Seedlings have the form of a loop formed by the cotyledons and the hypocotyledonous knee, part of which is submerged in the ground. If the sowing is done in heavy or floating, crusty soil, or the seeds are very deeply planted, then on the surface there may be not an unfolded cotyledon, but a root. Such plants die.

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At first, the onion develops very slowly. With the appearance of the first true leaf, the cotyledon dies off, and the crops at this time look yellow. This is a natural phenomenon and should not be scared. The root system of the onion is poorly developed. The roots are stringy, weakly branching, covered with a large number of the most delicate root hairs, which, when the plants are excavated, lose their turgor and dry out quickly. Most of the roots are located at a depth of 5-20 cm.

Figure: 1. The structure of onions

In fig. 1 shows the appearance of the plant, as well as the inflorescence, flower, fruit and onion seed. Onion leaves are tubular, covered with a waxy bloom. The base of the leaf covers the kidney and the part of the stem on which it has developed. Each subsequent leaf appears inside the previous one and leaves it at a certain height, forming a "false stem" by the sheaths.

After 8-10 leaves grow, nutrients begin to be deposited, as a result of which the leaf sheaths gradually thicken, forming the scales of the bulbs. The most severe thickening occurs in the middle of the leaf sheath. As the bulb grows and forms, the leaves die off, and with them the sheaths die off. Gradually drying out, they form a thin bulb neck.

Figure: 2. Development of onions

In fig. 2 shows the development of onions from germination to seed ripening. The earlier the neck dries, the better the bulb ripens. If it has not matured, then it is poorly stored. The onion bulb is well expressed, covered with 2-3 layers of dry scales on top, which, depending on the variety, can be of different colors.

Inside the bulb, at the bottom, vegetative and flower buds develop, from which either new bulbs or peduncles (arrows) with inflorescences develop later.

Table. Industry Standards for Onion Sets and Onion Picks

Depending on the number of vegetative buds, the bulb can be small or multi-bud. Budding is also a varietal characteristic of onions. The number of vegetative buds determines the little or many buds of the bulb. Bulbs larger than 4 cm in diameter are considered a turnip.

Small bulbs of small-nested varieties are divided into 3 groups, medium- and multi-nested varieties - into 4 groups. Onion sets and onion picks must meet the requirements specified in table. 3. Small sets, less than 1 cm in diameter, are called wild oats. They are allowed to purchase only in the spring months, since it is more difficult to save it.

Figure: 3. Requirements of onions to the optimum temperature in different phases of growth and development

Onions are a cold-resistant plant. It easily tolerates spring cold snaps, but in the loop phase, seedlings may die at -2 … -3 ° C. Although real leaves can tolerate frosts down to -3 … -6 ° C, the tops of the leaves then turn yellow and die off. The optimum temperature for leaf formation is + 15 … + 20 ° С. When the bulbs are stored at a temperature of about 7-10 (up to 15) ° C, flower buds form in them, and arrows appear in the spring.

Onion plants are able to withstand heat above 35 ° C, although the high temperature inhibits leaf growth and accelerates the maturation of the bulbs. Their fastest formation occurs at 20-25 ° C (Fig. 3). Rooted bulbs overwinter successfully in the open field.

Figure: 4. Requirements of onions for illumination and length of daylight hours

Onions are a light-loving plant. Low light will delay the formation of bulbs. It takes a long day (15-17 hours) to form the bulbs of the northern and middle lane varieties. If sowing is late, the formation of the bulbs is shifted to a shorter day.

Under these conditions, the period of bulb formation is extended or they are not formed at all, since a short day, low temperature and high soil moisture enhance the growth of leaves and increase the growing season without bulging. Southern varieties mature better on short days, and an increase in day length only leads to intensive leaf growth (Fig. 4).

Figure: 5. Requirements of onions for moisture in different phases of growth and development

Onions are very picky about moisture (Fig. 5). The soils should be sufficiently moist, as onions do not grow on dry soils. In the first 2-3 weeks after sowing and 2-3 weeks after germination, during the active growth of the root system, onions must be watered.

In the second half of summer - late July - early August - onions need dry and hot weather. When planting bulbs, it is less demanding on growing conditions than when growing from seeds, since the bulb has a reserve of moisture for the initial growth of roots and leaves. Onions do not tolerate waterlogging of the soil.

The best for onions are rich in organic matter, light loamy and sandy loam soils. Onion grows well on floodplain soils and drained peatlands, but its ripening is delayed. Heavy clay soils are not very suitable for it, especially when sowing with seeds, since a crust forms on them, which prevents the emergence of seedlings.

Onions have a weak root system, therefore they are highly demanding on the diet. On acidic, floating soils, it has small light green leaves with yellow tops. Such plants in the conditions of the North-West of Russia are earlier affected by downy mildew and suffer more from diseases.

Soil liming promotes root growth, improves the absorption of phosphorus, sulfur, boron and copper. With a lack of calcium, onion leaves die off and eventually the plants die. Timely application of lime enhances their growth. It is inappropriate to apply manure directly under the onion, since it causes an increased growth of vegetative mass and delays the ripening of the bulbs.

With a sufficient amount of nitrogen, the leaves become dark green in color and have a powerful waxy coating. With an excess of nitrogen, the growing period is lengthened, the yield of bulbs decreases, the vegetative mass increases, the sugar content decreases, the amount of water in the tissues increases, they become friable, and the use of irrigation causes a decrease in the weight of one plant, the total yield and delays the ripening of onions.

An acute lack of nitrogen in the soil reduces the content of this element in the plant, limiting its growth and productivity, while the content of dry matter, sugars, and vitamin C increases. Plants develop weak, with light green leaves. It is best to apply nitrogen fertilizers under the onion in several steps and at the time when the plant needs it most of all, i.e. in the first half of the growing season.

Phosphorus is one of the protein components of cells and tissues, accelerates the development and maturation of plants, the formation of bulbs, increases resistance to adverse environmental factors: low temperatures, frost, increases resistance to fungal diseases, promotes rapid root growth, especially in the early phases of growth, balances the action of excess nitrogen. Without phosphorus, nitrogen is not assimilated, especially if there is not enough of it in the soil, therefore, a lack of phosphorus causes nitrogen starvation.

Plants need potassium fertilizers for photosynthesis. Potassium increases the dry matter content. With a lack of it, growth and productivity are limited. Potassium deficiency leads to the gradual death of the leaves. Most of all, plants need it in the second half of development. The use of magnesium sulfate and manganese sulfate increases the yield of onions.

Copper and zinc contribute to an increase in leaf growth rate and chlorophyll content. They are necessary for onion plants at the very beginning of development.

Onion leaves accumulate more nitrogen and potassium, bulbs - phosphorus. It is noticed that an increase in the dose of one of the nutrients leads to intensive growth of the leaf apparatus to the detriment of the yield of bulbs. Individual nutrients have different effects on the chemical composition of the onion. Increasing the dose of potassium increases the disaccharide content, and the pungent taste (from glycosides) increases under the influence of nitrogen. Chloride fertilizers reduce the level of essential oils and glycosides.

Under the influence of sulfur, the content of essential oil increases. The introduction of nitrogen fertilizers into podzolic soils increases the amount of vitamins B1, B2, B6 in green onions. Under the influence of increasing doses of nitrogen and magnesium, the carotene content increases. Potassium helps to increase the content of ascorbic acid in onions.

Read the next part. "Interesting varieties of onions" →

All parts of the article "Growing onions in the North-West region"

• Part 1. Biological characteristics of onions
• Part 2. Interesting varieties of onions
• Part 3. Preparing the soil for planting onions
• Part 4. Growing onions through the set
• Part 5. Growing onions from seeds
• Part 6. Vegetative propagation of onions
• Part 7. Growing green onions