Growing Giant Onions in AutoPot Watering System

Some of the best reactions I get when visitors come to look around the greenhouse displays are when they turn a corner and their eyes suddenly fix on to the giant onions. “Wow!...Look at those onions!” they cry in astonishment and every time it brings a big smile to my face to see their amazement and wonder at these massive alliums. I have grown these impressive exhibition onions for a few consecutive seasons now, mostly for fun and the wow factor they give to the greenhouse displays, but also with one eye on beating the previous season’s top weight.

Writing this and thinking back to when I first had a go at growing giant onions in 1996, I realise that it has been 20 years. My first attempt back then was using an NFT channel system I had on display - I grew big onions but nothing like the ones I get now. Saying that, I think the current record holder for the heaviest onion (15lb 5oz I believe! That’s about 7kg), won’t be worried about losing his title to me….just yet.

If you are planning on taking that record for yourself, or you want to beat my top weight, or just to grow something to impress your friends and family with, then you need to start with the right seed varieties.  The two main varieties of choice for the exhibition grower in terms of size and weight are Mammoth, which holds the current record, and Kelsae.  You can get both varieties in seed form and can sometimes be found as ‘sets’ (a small bulb that grows in to a full sized single onion), and as young live plants ready to plant out.  Like many growers I like to start my onions from seed, I’ve heard that it's preferred by the pros for the best chance of a whopper – I like the satisfaction of starting with something so small, such as a seed, and seeing it end up as a huge onion.  The best seeds, if you are lucky enough, are directly from a giant onion grower who has produced seeds from his monster sized prize winner.

I managed to beat my personal best with an onion of 2.67kg this year with the Kelsae variety, I did grow a batch of the same onions using Biobizz organic medium and feeds as well as compost tea.  Not quite as big but that’s what I was expecting. You can see both results below.

CocoRocks  kg 1.65 1.74 1.77 1.85 1.89 1.95 1.99 2.14 2.15 2.18 2.2 2.67
 Biobizz kg  0.97 1.3 1.33 1.38 1.38 1.42 1.44 1.77 1.8 1.83 2.1 2.2

giant onions

If you are not man enough to eat a whole giant onion in one sitting (what’s wrong with you?) then chop it all up and freeze it.  You can then use just the amount you need straight from the freezer directly in to the pan – easy-peasy…er…oniony!?!
Have fun growing your own onions from seed and I would love to hear your experiences good and bad, funny or not. Or just drop us a message to brag about how good your onions are and we will give them the recognition they deserve.


  • giant onions propagationYou should sow your giant onion seeds between mid-December and mid-January for a harvest at the end of July to mid-August.  Fill a clean cell tray or small pots with a free draining compost medium. Choose a good quality seedling compost mix, 50/50 multi-purpose compost & coco fibre or perlite mix.  Alternatively you can choose to use a plug type product like Root Riot, Jiffey 7s or Grodan rockwool.  Fill the cell tray or pots and settle by banging onto the work surface a couple of times, then stand them in a tray of water to soak before planting the seeds.  Leave to soak for about an hour or until the medium is well saturated.  Remove from water and allow any excess water to drain briefly. With a small dibber or flat end of a pencil (the rubber is perfect) make a small indentation approx. 2-3mm deep in the center of each cell or pot.  Place one seed in to each indentation and cover with a thin layer of fine grade vermiculite – covering the seed and surface of the cell or pot thinly.  Now mist the vermiculite with water using a hand sprayer with a fine mist to wet it well.  Now place the tray or pots into a propagator with a lid on and vents closed and place somewhere warm.  Germination temperatures should be kept stable between 24-28 degrees C (75-82F).  No light is required until the first signs of germination, but at the first sign of the first shoot, light will need to be applied from thereon.  A T5 fluorescent propagation light or LED propagation unit is usually the preferred choice for seeds and cuttings.  In past seasons I have given the seedlings 24 hours of light from the first day of sprouting until moving into the greenhouse in April where artificial lighting is only used for a couple of hours at the end of the day.  Next year I am going to try lighting some of them for only 16 hours of light per day and compare them to the ones on 24 hours continuously.  I have read that someone was getting better end results by adopting this method at the start, so I’m going to be giving it a try. Germination usually takes 7-14 days with a high germination rate.  I regularly get 90-100% germination so you only need to sow a few extra than required, 10% is plenty.  After germination open the propagator vents, place under artificial lights and reduce the temperature to around 18 degrees C removing the propagator lid completely after 7-10 days.  If the cell tray or pots need watering at any stage, simply stand them in shallow water for 30mins then remove.

  • If you started your seeds in cell trays or plugs you will need to pot up into pots at the crook stage.  This is where the very early leaves still have the seed case attached and bend over.  Plugs are easier to judge as you can see when there are enough roots and before there are too many and being easy to handle they can be potted up at will.  In the case of cell trays, you want enough roots to hold the medium together so that when you lift it out of the tray it doesn’t crumble and snap all the roots.  I’ve read about some growers who trim roots at this stage before planting but I can’t see any advantage in doing that nor would I suggest it as best practice.  Pot on into 10cm/0.7L square pots or 11cm round pots using a well-drained potting soil mix or soilless media. Feeding is not required at this stage if using a soil mix, whereas a soilless mix should be watered in with a nutrient solution of 1.6 E.C (1120ppm). Regardless of your choice of potting mix I highly recommend using complimentary soil microbes and beneficial root fungi.  These naturally occurring bacteria and fungi perform lots of different functions within the growing medium all of which help your plants to grow bigger and better – I will explore this fascinating subject in more detail in separate articles. Choose a product with a broad bacteria profile and multiple fungi spores for best results.  Root Better by Better Organix is my preferred choice as I know the company well and know that it's the best of its kind I have come across. Great White by Plant Success is another good alternative and both products come as a dry powder that you sprinkle on to the root ball as you repot, plus a little in the planting hole to.  Use this at every potting stage.  I like to water in my plants with a mix of Nitrozyme (seaweed concentrate), Blackjack (Humic & Fulvic acid), Superdrive (vitamin and hormone supplement) and Aquaburst (wetting agent), all of these are made by Growth Technology Ltd.  They provide a great organic profile to the root zone containing all the good stuff your plants thrive on and that is missing from most plant nutrients.  They are really good for your beneficial soil microbes too, stimulating them into action and giving them an initial energy supply.  I use this mix regularly throughout the plants life, once a fortnight is sufficient for onions. Once your plants go in to an Autopot system, hand-water this mixture to each plant through the top of the pot rather than putting it into the tank.  Because these products are organic and contain microscopic solids they do not sit well in tanks and can build up in pipe work over time, with the exception of Aquaburst. The same four products make an excellent foliar spray for regular use.

    giant onions potting upI was advised by a wise, and now old man that for exhibition onions it is important to keep the leaves supported during all stages of development and not to let them get heavy and crook the neck of the onion.  Supporting the leaves allows new leaves to develop freely without restrictions and keeping the neck straight kept the onion growing for longer and therefore bigger and heavier. It also aids even development the bulb producing a uniform shape and more aesthetic appearance. True or not, I have always followed those guidelines because, to me, they all seemed to have a logical reasoning behind them. So at this stage I make a small wire support for the leaves. This support will last until potting into the final 15L AutoPot pots and will stop the leaves from bending the neck over.  For easy care of the plants until potting out into the Autopot system I use the Coco mat and Easy2Go kits from AutoPot.  The Coco mats come in sizes to fit most trays and propagators and provide a simple way to keep your small plants perfectly hydrated. Be sure to soak the mat and root control sheet before use and water everything through the top of the pots when first placed on to the mat.  Thereafter the AquaValve will wet the mat from below and keep your plants perfectly hydrated. For small propagators, simply fill by hand as required.

  • giant onions autopot1In April I move the onion plants out in to the frost-free section of the greenhouse and on to the heated benches for a couple of weeks to acclimatise.  These are not hot benches but warmed to maintain a temp of 15 degrees and prevent the pots from chilling at night.  By mid-end of April the plants are ready for potting up in to their final pots.  I use the 15L square pots and combine them with the Easy2Grow 2 pot trays from AutoPot.  You can use the 1Pot trays instead of the Easy2Grow trays if you prefer, but I find that the Easy2Grow trays are just right for giant onions and you can push them up to each other so that they are touching which saves space. The pot size and volume are great for giant onions, they have the perfect height creating the ideal moisture balance where the medium just starts to dry on top and gets progressively moist the further down the pot you go. I like to put the root control disc into the pot first with the copper-side down.  Again, choose a well aerated free draining potting mix with a low-medium E.C.  As before, add some more beneficial microbes and water in with the Nitrozyme, Blackjack, Superdrive & Aquaburst mix. At this point I remove the small but well-crafted wire supports and replace them with a Circular Trellis per pot.  giant onions autopot2This will last until the onions are ready for harvest and provide all the leaf support you will need – they are so quick and easy.  If you choose to go all out hydroponics then look at using a 50/50 perlite and vermiculite mix or a 60/40 coco and clay pebble mix.  This year I used CocoRocks mix, a 40/60 mix of small grade clay pebbles and coco fibre made by Growth Technology Ltd, with Ionic Coco Grow formulation nutrients in the tank.  If you are using a soil mix then use Ionic Soil Grow nutrient or other similar mineral nutrient solution in the tank.  It's generally considered that onions are heavy feeders so I suggest maintaining a constant E.C all the way through.  1.8 E.C / 1260ppm in soilless mix and 1.6 E.C / 1120ppm in a soil mix for the AutoPot system.  If you want to grow organically and use the AutoPot Watering system together then you will need to take a slightly different approach.  Either select a complete in-pot feeding solution such as the Biotabs organic feeding system, or add some organic slow release feeding granules to the potting medium and regular watering through the top of the pots with a liquid organic feed.  Remember to turn the tanks off a couple of days before watering through the top of the pots to avoid over saturation.  Remember to turn the tanks back on the following day.

  • Try using potting medium additives to give your plants a boost such as worm castings, bat guano and Charge (beetle frass), all should be premixed into the potting medium. Brew yourself some compost tea and water your plants with it weekly.  It's a great microbial boost and a boost to your beneficial microbes you added at the potting up stage.
    It's best to regulate the water supply by hand at the beginning by turning the tank on and off as required rather than leaving it on constantly.  This avoids early oversaturation while the root mass is increasing and it allows this to happen faster and healthier. I usually wait until June to leave the tank on constantly, up until then I just switch it on for a couple of hours a day in the morning only.
    Onions don’t tend to suffer with many pests but spider mite, and to a lesser extent thrips can sometimes be a problem.  Prevention is better than cure and that’s particularly the case with spider mite, so adopt a regular spray program using more than one natural pest control product and alternate their use.  If you are using natural pest control products then you have to apply these on a consistently regular basis for them to work at all.  You can also consider getting in some predatory mites that will eat all the bad ones and not your plants – Bug Fight!  Pick off any old leaves once they have died back and begin to dry.
    The onions are ready for harvest soon after they have stopped growing.  You can tell this because the plant stops producing new leaves.  If you leave them too long in the pots after the leaves have stopped growing then the bulbs will start to deform and will start to divide in to segments which is undesirable.  Once lifted trim off the excess foliage leaving a palms length of neck, this will allow for shrinkage as the outer layers dry out ready for storage or display.
  • Propagation:

    • Propagator (Heated for cool or cold spaces, Unheated for warm spaces)
    • T5 or LED propagation light.
    • Cell tray or 5cm pots.
    • Fine Vermiculite
    • 10cm square or 11cm round pots
    • Free draining compost mix or soilless mix
    • Nitrozyme
    • Blackjack
    • Superdrive
    • Aquaburst
    • Root Better.
    • Propagator Tent (if required): DP60 or DP90
    • Thin galvanised or plastic coated wire for supports

    Growing On:

    • 15 litre Square AutoPot Pots
    • AutoPot Easy2Grow or 1Pot trays, Root Control Discs, AquaValves, pipe and connectors.
    • Reservoir Tank, outlet tap and inline filter.
    • Circular Trellis
    • Free draining compost mix or soilless mix.
    • Root Better
    • Nitrozyme
    • Blackjack
    • Superdrive
    • Aquaburst
    • Your choice of nutrients: mineral or organic
    • Growing medium additive
    • Plant protection products

Posted in All You Need To Grow


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