For the characteristic long white stalk – the blanched portion is where the subtle sweet allium flavor is – Negi must be "hilled", sort of like you would when growing potatoes. Soil is pushed to cover the plants so only the green tip is exposed, making the plants elongate further to reach for sunlight.
Here you can see what Negi looks like when it is transplanted:
In Japan, Negi transplants are planted into 4-6" deep furrows with ridges on either side. As they grow, they are hilled at least 3 times during the season, and the ridges are used to cover the plants so the white shank is never exposed to sunlight.
The stalks are staked and tied (like you might for tomatoes) to prevent the greens from falling over in hard wind and rains.
And that is how you get ridges around the plant that are over 18" high, and Negi that is over 30" long. We'd love to hear if you've tried other methods and what has worked or not worked – leave a comment and let us know!