brown onion lot

Preparing For Onion Season

Onions are one of New Zealand’s most lucrative vegetable exports. Yielding over 200,000 tonnes per year, about 90% is sold overseas. The next growing season is about to begin, so let’s look at how growers can ensure a healthy crop.  

New Zealand onion growing season

With winter fast approaching, it’s almost time to begin onion seeding. The correct time to plant onion crops varies depending on the region, which can begin as early as May. Crops should all be in the ground no later than September. 

Onions are ‘photothermoperiodic’, meaning temperature and daylight play a significant role in the quality of yields. Onion seeds ideally germinate at temperatures in the low to mid twenties celsius. Dramatic fluctuations in temperature can result in onions bolting to seed, producing flowers and seeds rather than a bulb. 

The number of daylight hours that onions are exposed to is also a determining factor in when bulbs develop, which varies between onion varieties.

Preparing your soil

The soil needs to be well conditioned and nutriated, as well as have good drainage. Our humic acid product, Humax, is an excellent option to condition your soil. It can provide stability, which improves drainage, and allows for better uptake and retention of nutrients and water.

Onions have a notoriously difficult time taking up nutrients due to their shallow roots. Our Rootella line of Mycorrhizal fungi form a network of thin fungal strands that attach to roots and increase their effective range. This increases a crop’s ability to uptake nutrients and also contributes to soil stability. One of the major benefits of Mycorrhizal fungi inoculation is the ability of the organism to scavenge and uptake phosphorus in particular, and also nitrogen. This alleviates mineral loss to waterways which can affect our environment and importantly reduces the need to apply expensive Phosphorus fertiliser, which has dramatically increased in price.

green plant on brown soil

Fertilising your onions

Before applying any fertiliser to the ground, it’s important to know where your soil’s nutrient levels stand. Conducting an initial soil test will help you understand what nutrients will be needed. Overapplying nutrients can be as detrimental as mineral nutrient deficiencies. Excesses of elements can retard quality, cause environmental issues in our waterways and also cause lockup of other minerals. The utilisation of Mobilizer at key periods such as planting or before bulbing can unlock locked up soil minerals making them available to the crop.


Onions need all macronutrients. Early applications of phosphorus help stimulate root growth and establish the crop. Nitrogen is also important for high yields, though over-applying it can affect the maturity of the bulbs. Potassium assists in maintaining good yields and the accumulation of sugars in the bulbs. Try our Synergizer product for balanced NPK needs. 

Sulphur is also very important in onion growing. S has been found not only to increase the bulb yield and dry matter but also improve onion quality especially pungency and flavors.


Micronutrients, while needed in smaller quantities, also play a critical part in producing a healthy quality yield. Studies into micronutrient removal rates by onions show that boron and iron are particularly important. Added to this, Magnesium and Zinc have also been found to positively affect the size and quality of onion crops. 

Our Biomin Booster V trace mineral fertiliser provides an ideal solution to help your crops achieve a good balance of all micronutrients. 

If you have questions, send me an email –