People often complain that their onions never get very big in Flagstaff. In general, larger onions are usually obtained when growing onions from seeds rather than sets. Here is an old “trick” that will help you get bigger onions from planting sets in a short season area.
When planting the bulbs, place them shallowly in the soil, just barely under the soil surface and space them at least three inches apart. As the plants grow, gradually pull the soil back from the bulbs so that by August, at least 2/3 of the bulb is above ground. It looks odd, but do it. In a long growing season the tops of the onions will eventually fall over and die back, and this is when the bulbs really start to enlarge. In a short season area such as Flagstaff there isn’t enough time for this to happen, so you must help them out.
Around September 1st ( I remember because it is Coconino County Fair time) go out into the garden and literally walk all over your onion plants and stomp them down. Do not cut the tops off, just leave the mashed mess as is. (Your shoes will smell of onions.) Now the bulbs will enlarge quickly and be ready to harvest around Halloween.
You will be amazed at how this little “trick” (who some think is an “old wives tale”) will help you grow beautiful large onions.