Growing Allium giganteum indoors
Use a seedling tray, sow Allium seeds from January to mid-March at 15-20C (60-68F), on the surface of a good quality seed, then gently firm down.
Keep the soil damp but not wet, and place the seedling 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 when seeds start to germinate.
You may need to repeat this cycle if germination does not occur as germination, can often be erratic taking from 30-365 days to emerge!
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 Alliums are growing, 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 the 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 have to improve the drainage first before planting. Add plenty of horticultural grit and bulky organic matter to the soil and then create a ridge 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 fertilizer like grow more 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.
Then germination should take 21 days(approximately)to occur.
You will need to keep a particular eye on the newly sprouting shoots as these can attract the attention of inquisitive birds – particularly pigeons and blackbirds - who will lift them straight out of the seedbeds for nothing more than a little mischievous fun. If you don't have some kind of protection in place you can end up losing almost an entire batch!
Giant ornamental onions are not very good at suppressing weed growth, and if regular weeding is neglected they will easily out-compete 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.
