Keep alliums growing hard

I've been asked a question as to why a guy's onions and leeks appear to be growing long and spindly and I think it's simply that he's growing them too soft. Now I'm no expert onion or leek man but I've gleaned enough to know that the more robust you can grow them the better. Mine are growing on in my totally amateurishly constructed growing area in one of my cold greenhouses. It's simply a few metal plates propped up against canes. If I had the inclination I would construct a proper wooden cabinet lined with reflective material, but this will do me for now. I do have a grolamp above them but no extra heat whatsoever. I find that the grolamp throws out quite a bit of heat and is more than enough to keep the plants ticking along even when temperatures outside are minus 3 or 4. In the morning the metal plates are quite warm to the touch so the restricted area does appear to hold the heat well, even though the lamp is a good 8-10" above the tallest plants being the leeks.

I throw a few old compost bags over the outer edges of the frame on the coldest nights as shown in the photo, but I take them off in the morning, turn off the lamp and remove the metal plates at the front so that the plants can get some natural daylight. When I get home from work about 6 it is usually dark and starting to get cold so I reassemble the metal framework and switch the lamp back on. I will do this until early March when there should be sufficient daylight hours to render the lamp unnecessary. Also, because I will have grown the plants in quite a 'hard' way it is unlikely the nights will get cold enough for them to require any extra heat. The key therefore is to give your plants a bit of cossetting but not to mollycoddle them,