Bunching onion

Baja Verde

A bunching onion variety with very early maturity. The tops are strong, upright and dark green in color.

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A bunching onion variety with very early maturity. A cross between the Allium cepa and Allium fistulosum types allowing a much earlier production of green tops than in Allium fistulosum.

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Botanical and biological features

Bunching onion (spring onion; Allium fistulosum L.) is a perennial herbaceous plant grown for early spring greens. When grown in the open field, it yields a high quality crop from April through November.

Bunching onion is undemanding in terms of growing conditions and merits attention as a valuable green crop producing crisp, succulent tops of excellent flavor and taste in early spring.

Young tops are fairly tender, juicy and less pungent than bulb onion. Crisp tops of bunching onion contain: 9,5-10,5% soluble solids; 2,6-3,0% sugar; 2,2% protein; 0,35% fat; 0,7 mg% provitamin A; 0,33% vitamin B1; 0,02 – B2; 0,16 – PP and 40-80 mg% of ascorbic acid (vitamin C).

Bunching onion does not form a regular bulb of the common-onion type but produces a slightly thickened pseudobulb. During the second year, it produces side shoots and acquires a bush-like plant habit. The root system is extensive, with roots in clusters, which are highly frost resistant and penetrate deep into the soil. The growing plants are capable of withstanding spring frosts of down to 10°C below zero.

This crop thrives and produces green tops in cold wind-protected bottomland areas with fairly fertile and well-tilled soil. It responds well to application of organic and mineral fertilizers.

Bunching onion propagates by seeds and by division of the bush (vegetative propagation). For commercial production, propagation by seeds in a single cropping (one-year culture) is predominantly used.

Bunching onion is a cold- and frost-resistant plant. Germination occurs at +2-3°С, but the optimum germination temperature is +18-20°С. The tops grow at temperatures not lower than +1°С. The optimum temperature for the forcing of green tops is +15-22°С in autumn-winter and +16-24°С in spring-summer period, the maximum temperature being +25°С. Higher temperatures result in leaf growth inhibition.

In mild winter conditions, bunching onion emerges from under the snow. Regrowth of tops was occasionally noticed to occur in December-February. The rooting occurs at +1-3°С.

Winter hardiness of bunching onion depends to a great extent on the presence of reserve material in the plant. The bunching onion plants low in reserve material have little or no chance of successful overwintering. Young plants (with one or two true leaves), adult plants weakened by frequent and late-season cuttings and the plants that have not been transplanted and have not rooted in time all exhibit poor winter hardiness. In the absence of snow, the bunching onion plants that have become well established do not get winterkilled and are capable of withstanding temperatures down to 25°С below zero.

The crop management practice

Bunching onion is grown both as an annual and perennial (2 to 5-year) crop.One-year cropping is preferable because whole plants (with blanched pseudobulbs) are a more marketable product than the cut tops alone.

Bunching onion grows on almost any well-aerated soil. The best preceding crops are winter cereals, grain legumes, early potato, cucumber and fallow. Adequate autumn cultivation is a key to successful bunching onion production.

Spring, summer and autumn sowing dates are normally used. Direct seeding in spring should be performed as soon as possible – at the first opportunity of going out to the field. The seeding rate is 20-25 g per 10 м2. Close sowing results in better quality green tops, easier harvesting and higher yielding capacity of the crop. The depth of sowing is 1,5-3,0 cm, depending on soil type and local growing conditions. The seeding is by the wide-strip method (the distance between strip centers is 25-30 cm) or the line method (the plant spacing is 12-15 cm).

For early production (12-15 days earlier than usual), late fall or underwinter sowings are performed, with the seeding rate increased by 15-20%. The field from which the preceding crop has been harvested is carefully tilled and the sowing is performed in late autumn when cold weather conditions settle and soil temperature drops to +3-4°С. For continuous production of green tops, an unbroken string of sowings and harvestings performed at 2-week intervals can be set up.

One way of growing bunching onion for green tops is to sow it in summer and then, without harvesting the tops, allow it to overwinter for extra early production of green tops in spring. With this method of sowing, bunching onion is slow to enter the bolting stage next spring, resulting in extended harvest of superior quality tops. Covering the bunching onion plants that have overwintered in the open field with plastic cover in spring makes the crop harvestable 10-14 days earlier (shortens the time-to-harvest by 10-14 days) and results in more tender and succulent tops. Spring sowing under multiyear cropping results in earlier production of tops next spring, but it also leads to earlier bolting. With spring sowing of bunching onion, the green tops are ready to harvest in 50-60 days after sowing. With summer sowing, the green tops reach standard size (25-30 cm in height) in 45-50 days.

Forcing of bunching onion tops

Bunching onion plants from a multiyear plantation are used for the forcing of green tops in greenhouses. It is easier and more profitable to force bunching onion tops than bulb onion tops since the former grow faster and become harvestable sooner than the latter in both spring and autumn-winter sowings. For forcing purposes, the planting material is obtained from 3-4-year fields in autumn. The plants are lifted, placed in boxes, sprinkled with earth or peat and stored until planting at around 0°С. When planted in greenhouses, bunching onion plants are arranged in a pavement-like pattern (one plant next to the other) at a density of 500 plants per 1 m2. The transplants are covered with peat or dried manure. During the first seven days, the daytime temperature in the greenhouse is maintained at +12-15°С. After two weeks, the temperature is raised to +20-24°С in the daytime and to +14-15°С at night. Where plants show poor development, complex fertilizer (N 50, P2O5 25, K2O 40 kg of active substance per hectare) can be applied after the plants become established. The forcing period is approximately 25-35 days in winter and 15-20 days in spring.

In Japan and China, the following growing technique is commonly used: the transplants are placed into deep furrows and hilled up to form ridges 10-15 cm high. This results in blanched plants with delicate flavor.

To ensure heavy yields of bunching onion, it is advisable to apply fertilizers at the following rates: 450 g ammonium nitrate, 400 g superphosphate and 300g potassium salt per 10 m2 (N 150, P2O5 80, K2O 120 kg of active substance per hectare). Good results are achieved with autumn application of 20-30 ton/ha of fermented manure. Since bunching onion is an extremely moisture-loving crop, it should be given 4-5 irrigations during the growing season with a water application rate of 350-400 l per 10 m2 (350-400 m3/ha). The most preferable method of irrigation is drip irrigation.

Bunching onion is suitable for the year-round production of green tops in greenhouses and in the open field. When grown for green tops, bunching onion is harvested at the stage of 25-30 cm-tall tops, but not taller than 35-40 cm. At later harvesting times, the yield is heavier but the quality of tops is lower. In one-year culture, the output of green tops can be as large as 40 ton/ha.