Vegetable production: Establishing onions using sets


By Digital team | 20 December 2018
sets
Photo: Charl van Rooyen

Direct seeding, sets and transplants can be used to establish onions.

  • Direct seeding, using precision planters, is the preferred system for larger plantings.
  • Transplants are less popular because of the high labour cost.
  • Therefore, although expensive, planting sets can be useful for extending the harvesting season.

The information given in this article is aimed at production in summer rainfall areas (where a short day-length occurs). Planting dates should be adapted for production in other areas.

STAGE 1:
ESTABLISHING

  • The process is carried out in 2 stages.
  • In the initial stage, the seeds are planted or sown out of season (July to September) at a density of about 1 200 seeds/m².
  • Because of their size, sets form more robust plants at emergence than seeds and therefore do not need such meticulous seedbed preparation.
  • After emergence, the seedlings undergo a short vegetative phase and when favoured by the prevailing conditions of increasing day-length and temperature, bulb formation is induced.
  • The crop is grown with a relative low level of N (nitrogen) fertiliser to encourage thin necks.
  • Since the seedlings immediately induce bulbs, leaf growth is limited and the bulbs (called sets) remain small.
  • Irrigation is terminated a few weeks before the initial harvest towards the end of November, when the leaves start drying from the tips after a cycle of about 80 to 90 days.

INITIAL HARVEST

  • The sets are harvested in the normal way for onions, dried at 20ºC to 25ºC with good ventilation and stored in a dry place.
  • Dry sets can be stored in bulk up to 3 m deep until February of the following year.
  • The storage temperature and humidity requirements for onion sets are essentially the same as those for large dry onions (0ºC to 3ºC and 65% to70% relative humidity).
  • At humidity levels of more than 70%, combined with warmer temperatures, the sets may sprout, develop roots and decay.
  • In general, larger sets (>25 mm) are more prone to bolting and a higher storage temperature (25 ºC to 28ºC) may be desirable to inhibit bolting (flowering).
  • According to some researchers, high-temperature storage may also increase vigour, delay maturity and increase yield.
  • Relative humidity (RH) must be between 60% and 70% to prevent excessive moisture loss during high-temperature storage.
  • A weight loss of 10% to 20% during high-temperature storage is quite normal.
  • Onion sets should be stacked to allow for good air circulation.
  • A storage life of more than 6 months is possible for good-quality sets.

STAGE 2:
PLANTING

  • In the second stage, the sets are planted during the first 2 weeks of February at the usual spacing for seedling transplants.
  • Sets should be about 25 mm or less in diameter.
  • Some of the sets can be either smaller or larger than 25 mm in diameter, but are normally also used for transplanting.
  • It is important, how¬ever, to mention that sets having a diameter of 25 mm to 35 mm usually result in a high percentage of split bulbs, as well as unwanted bolting.
  • Smaller sets generally do not bolt because they are not large enough to pick up the bolting stimulus.
  • The sets will start sprouting after a couple of days and the young plants will grow rapidly because there are ample reserves stored in the bulbs.
  • Since sets produce more robust plants at emergence compared to seed, they can also tolerate less favourable conditions.
  • On the other hand, a relatively large percentage of the sets usually initiates more than 1 plant (split) and, because of the high density, the shape of the bulbs can sometimes be malformed.
  • Other defects such as thick necks, uneven maturing and unmarketable splits are mainly related to cultivar differences and the size of the sets planted.

FINAL HARVEST

  • Sets have a shorter growing season compared to plants grown from seed and mature between the end of May and mid-June, a period when onions usually fetch higher prices on local markets.
  • Not all cultivars are suitable for this production method and only a few of them have adapted to production, storage and planting of sets.
  • Cultivars used to produce sets include some of the older ones such as Pyramid, Granex 33, Dessex and Radium.
  • Another disadvantage of planting sets is the high production cost, because of storage and the long growing season (lasting from September to June).
  • The higher prices obtained during this period (late May to mid-June) and the possibility of extending the marketing season usually overrule these disadvantages.

Also read: The art of cultivating onions

  • This article was written by Dr. Ian du Plooy from the South African Agricultural Research Council.