The flowers of the Allium giganteum are impressive by anyone's standards, and a personal favourite of my own. You can readily purchase them in the spring as bulbs - huge bulbs obviously - but as you can imagine, just one bulb can be pricey. However, growing Allium giganteUm from seeds is surprisingly easy and as a hardy plant you can choose to grow them inside under protection or outside in a prepared seed bed.
TIP. Your giant alliums should germinate straight out of the pack, but if you purchased them early and are not ready to sow them yet then I would keep them in the bottom of the fridge to help break any dormancy issues and improve germination rates.
Growing Allium giganteum indoors
Keep the soil damp but not wet, and place the seed tray either in a propagator or seal the seed tray inside a polythene bag until after germination - which usually takes about 3 months.
Make sure that the tray is placed in a bright position but out of direct sunlight.
Giant ornamental onion seed needs a cold period in order to help break any seed dormancy before they can germinate and so if germination does not occur by the end of 3 months, transfer the container to a fridge (not freezer) at 5C (40F) for a further 3 months.
Check regularly while in the fridge and remove once seeds start to germinate.
However, in my experience - and you can only truly know that you are using fresh seed when you collect it yourself - alliums have always germinated without incidence.
When seedlings are large enough to handle, transplant and grow them on into cooler conditions until they are large enough to move outdoors.
Move to a cold frame and plant out the during following spring, at a distance of 30cm (12") apart, in light sandy, well drained soil in full sun. When growing alliums, plant them where the leaves of other plants will cover the base of their stems. This will hide the old foliage which rather irritably dies back before flowering begins.
Growing Allium Giganteum from seed outside
When growing enormous Allium giganteum from seed, the secret is in the preparation. To begin with you need a sunny site with good drainage.
If you are planning on growing them outside then you can start by preparing a seedbed in the autumn by digging in plenty of well-rotted farm manure. This will give the ground a chance to settle over the winter period and allow frosts to break down the soil clods. If you soil is too acidic – below pH 5.5 – you will need to add lime to it according to manufactures recommendations. In general, giant ornamental onions prefer a pH of between 6 and 7.5.
It's possible to grow giant ornamental onions on heavy soil, but you must improve the drainage first before planting. Add plenty of horticultural grit and bulky organic matter to the soil and then create a ridges of soil 4 inches high to further reduce soil moisture.
You can sow Allium giganteum seeds as soon as your soil will allow which can be anytime from late February until the end of July.
Giant ornamental onions like a firm bed so tread over the area you have just raked. Try adding a general fertiliser like growmore for extra fertility.
Choose a dry day to sow Allium giganteum seed when the soil is moist but not too wet, then plant the seed very thinly into drills ½ inch deep. If you are planting more than one row then each row should be at least 4 inches apart. Carefully cover the Allium giganteum seed with soil and gently water in.
Germination should then take approximately 21 days to occur. Once the new seedlings have began to push through the soil they can be thinned out to between 1 and 2 inches apart. Remember to clear away all of your discarded thinning so as not to attract onion fly.
Giant ornamental onions are not very good at suppressing weed growth, and if regular weeding is neglected they will easily be out competed for nutrients resulting in your juvenile Allium gigantem plants becoming stunted. Try to leave enough space between the rows to get your hoe in for weeding, but always hand-weed any weeds close to your Allium gigantem as they can be easily damaged by garden tools.