'Fertilizing onions is essential for bigger, stronger crops' – an onion growing expert reveals how to do it

Onions are a hugely popular vegetable to grow and have a multitude of uses. They can be a simple crop for a grower of any experience, but if you are wanting to cultivate the best bulbs then correctly fertilizing onions is key. 

Onions can be planted in fall or spring and the planting date dictates when to start fertilizing them. If you want big and strong onions, it means adding fertilizer when you plant them and consistently through their growing period. There does come a point where you need to stop feeding them.

A correct routine of fertilizing is an important aspect of how to grow onions. Understanding what types of fertilizer to apply at the right time can make all the difference, especially if you are seeking to grow the biggest onions possible.

Drew Swainston

Drew is a former professional gardener who specialized in growing vegetables and fruit. He worked in kitchen gardens for the National Trust in the UK for several years and then ran a productive walled garden growing vegetables for a high-caliber restaurant in England. He always grew onions in the kitchen gardens and he has grown and harvested many different types over the years.

When to fertilize onions

Growers need to be adding fertilizer when planting onions, whether from sets or transplanting onion seedlings, and also carrying on a fertilizing regime during the plant’s growing season. 

Fertilizers contain three key elements in nitrogen (N), phosphorus (P), and potassium (K) and the packaging will show three plant fertilizer numbers that correspond to the percentage make-up of NPK. 

Bruce L Frasier, President of Dixondale Farms in Texas, a family farm that has been growing onions for over 100 years, advises a regular feeding regime and recommends the best nutrients the crops need at each stage of fertilization.

He suggests: ‘A balanced fertilizer (10-20-10) is needed at planting for the onion to establish a healthy root system. 

‘After 3 weeks, the onion needs an application of straight nitrogen (21-0-0) every 2-3 weeks. Nitrogen promotes foliage so continue applying it periodically until bulbing begins.’

Onions can be grown either from onion sets or seeds. If you plant onion sets in fall, then an application of a high-nitrogen fertilizer at the start of winter both provides nutrients to grow strong and also helps prevent bolting – where the onion prematurely flowers due to stress.

How to fertilize onions

Fertilizing onions is a fairly simple process and you should only apply the fertilizer at the recommended rate that will be listed on the packaging. 

Getting the timing right is important, as you should only fertilize onions when their tops are dry. Another important fact, as recommended by Bruce Frasier from Dixondale Farms, is to only use a granular fertilizer.

He says: ‘Onions prefer granular since they don’t get fed all that well with liquid fertilizer. Sprinkle along the side of the plants and water in. The roots will take up the nutrients. That is the only reason they are there.’

Make a small furrow around 2-3 inches deep between your rows of onions and then work the fertilizer into the soil. Water the furrows well after each application of the fertilizer, taking care not to wet the growing foliage when watering the plants.

If you are growing onions in containers, make the furrow through the center of your plants or around the outer edge of the pot or planter.

When to stop fertilizing onions

All watering and fertilizing of onions should stop once the bulbs have swollen, which will be around mid-summer. Signs that the bulbs have stopped growing include the leaves starting to turn yellow and the neck starting to feel soft. This should be around four weeks before they are ready for harvest.

It is vital to stop watering and feeding the plants by mid-summer as continuing to do so will impair their storage ability. 

If you are thinking of storing onions, then getting the longest lifespan for your crops is important. Stopping fertilizing onions and watering the crops is a vital cog in ensuring your harvest will keep for as long as possible.

Best fertilizer for onions

Vegetable growers can now get specific onion fertilizers from retailers that are slow-release and designed to give you tasty and bigger onions. Balanced granular fertilizers will come in ratios such as 10-10-10 or 12-12-12, as well as the 10-20-10 recommended by Bruce L Frasier, and will work perfectly for onions. 

Nitrogen-rich fertilizers are commonly available and can include the likes of ammonium sulfate, calcium nitrate, or feather meal. A good example is the Hi-yield Ammonium Sulfate fertilizer from Walmart  or Calcium Nitrate from Amazon.

If you are growing your onions to organic principles, then digging in plenty of well-rotted manure or compost before planting is vital. You can also use organic fertilizers, such as blood meal and bone meal granular fertilizers, which you can both get from Burpee.

While the onion is growing, consider mulching around the plants with compost, grass clippings, well-rotted manure, or straw to add extra nutrients as it decomposes. Mulch after planting the crop, however if you transplant onions seedlings wait for them to get larger before mulching to make sure you do not smother the small plants.

Tips for getting bigger onions

If you want the biggest and best onions then it is imperative to make early preparations, as they are heavy feeders. 

The key to growing the largest onions is to get your soil loose and fertile prior to planting. Work in a layer of well-rotted manure or compost into the soil. This will add vital nutrients as compost contains lots of NPK as well as helping to improve the soil structure and the moisture-retention capability of the soil.

By preparing the ground potentially months before planting you will be giving your onions the best possible base. That groundwork, coupled with a consistent routine of fertilizing the crop when they are planted and as they grow, should mean bigger bulbs when it is time to harvest onions.

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