Growing organic onions

GROWING ORGANIC

ONIONS

Words by Mike Nichols & Martyn Callaghan

Martyn Callaghan with onions grown from sets, early January, Pukekohe. Direct seeded already harvested

Though production costs for organic onions can be higher than conventional onion growing, there is a considerable price premium for the product due to consumer demand.

Organic onions can fetch prices double that of the conventionally grown product at times. However, the use of set grown onion production has the potential to reduce organic onion growing costs.The profit margin for growing onions conventionally is considered to be only about $NZ2.00 per bag, which on a per hectare crop of 50 tonnes is not huge for the use of land for nearly 12 months. Of course, if you are producing organically then the crop must be grown on an organically certified site, which takes a few years to obtain, but clearly the potential rewards are considerable.

John Bostock is one of the few growers of organically certified onions in New Zealand, and in NZ Grower some years ago (Anon, 2019) it is stated that one of the major costs of organic onion production is weed control as no chemical herbicides are permitted, and there is a limit to how much mechanical cultivation is possible, so hand weeding becomes a necessity. This is because onions are very small seedlings initially and do not compete strongly with weeds. This is exacerbated due to their upright growth habit. When growing onions conventionally the practice is to spray with a contact weed killer just prior to seedling emergence, and to combine this spray with a residual herbicide which will kill weed seedlings germinating later. Thus the onions emerge into a weed free soil, which remains weed free for several weeks. Once the onions have become established there is a wide range of herbicides which can be safely applied to the onions as they grow larger. With organic onions the standard weed control method is to use a flame weeder (see photo) just prior to seedling emergence, and then to use a combination of between the row hoeing (using a steerage type hoe) supported by in the row hand weeding. Because direct seeded onions require a long growing season (June to January), much of the critical

Flame weeder

1 1 1 2

1 Onion sets being grown in a greenhouse, Onions (variety Early Long Keeper) late January, Palmerston North. 2 Sets planted mid-November

weed control is a winter to spring operation, when conditions for mechanical weeding methods are not good. The potential of using onion sets as a means of growing organically certified onions would appear to have considerable merit. In the summer of 1999–2000 Mike Nichols grew onion sets in seedling trays. The sets were harvested in February and stored at 2 degrees Celsius over the winter. He made two plantings of the sets, the first in early October and the second in mid-November, and was amazed how rapidly the sets grew and bulbed. Early Long Keeper, for example, matured in late December from the early October planting, and in late January from the mid-November planting. This means that the onions would only be in the ground for a mere three months, compared with the more normal six months plus for a direct seeded crop. Furthermore, the risk of diseases such as mildew are reasonably high when sowing in the winter due to the cool humid conditions, and are reduced during the dryer summer months. This becomes even more pertinent with the banning of the fungicide Mancozeb for use on onions in Europe recently. In New Zealand, a conventional onion crop may receive in excess of 10 sprays of Mancozeb.

Onion sets are essentially small bulb onions, which are grown by sowing seed at a high plant density in October or November and are harvested in summer (February) when the tops die down. They can then be stored (dry) over winter and planted in the spring when conditions for soil preparation and growth are good. The onion sets should weigh 3–6 grams, and can be stored in a cool dry shed, or preferably in a coolstore at 2 degrees Celsius. Depending on the variety it is possible that the larger sets will go to seed when planted out, but this can be overcome by giving a short burst of high temperature (20 degrees Celsius) for two weeks just prior to planting. The larger the set you plant the larger the onion plant at emergence, and the easier the mechanical weed control, but of course the greater the establishment costs, as essentially sets must be valued by weight. It is necessary to grade the sets by weight to minimise size variation of the final crop, and to plant sets of the same weight grade in each planting. Of course, the larger the sets, the larger the weight of the individual onion at maturity, but plant density will also play a role in this aspect of production. This summer (2020–21) Mike Nichols produced onion sets in a greenhouse, using large seed trays filled with coir. Sown at high density in mid-November, by mid-January many of the varieties had already started to bulb. (See photos.) There are huge advantages in producing sets under protected cultivation, including the fact that you are not hostage to the weather and can sow when you wish, harvest when you wish and have excellent control over plant density. For example, by using seeders developed for the nursery industry it is possible to accurately sow a seed tray to any desired seeding rate. The seeded trays can then be transferred to a temperature-controlled germination room until just prior to emergence, when they are moved into the greenhouse. Although the price of hybrid onion seed is higher than open pollinated onions, there can be advantages in using hybrids such as a greater diversity of maturity time and partial resistance to some diseases such as Downy Mildew and Pink Root. The uniformity of size is commonly better than with open pollinated onions. With the use of onions grown from sets the uniformity advantage and partial disease resistance can be enhanced.

Of course, another advantage of using sets is that by delaying the planting date until the spring, planting can be undertaken with more information on the optimum size of onions required for the market, and thus planting density can be adjusted accordingly. The size of the resulting bulbs will then depend on variety, plant density, planting date and of course the weight of the individual sets being planted.

ONION SETS ARE ESSENTIALLY SMALL BULB ONIONS, WHICH ARE GROWN BY SOWING SEED AT A HIGH PLANT DENSITY ... AND ARE HARVESTED IN SUMMER WHEN THE TOPS DIE DOWN

References

Anon (2019) “Organic onion exports a Kiwi first”. NZ Grower, 74 (7), 19. Nichols M A (2020) “The Final Word: Onion sets – 12 months on”. NZ Grower, 75 (6), 48–49.