bulbs are called sets. they were grown last year as onions, pulled and stored, then you plant them in the spring. sets stink. the best is to start from seed but plants are ok too.
onions tolerate really cold temps and i suspect even snow would not effect them as it would just insulate them from really cold temps.
i start my seeds about 2/22 and plant out around 4/25. my last frost date is 5/31. i think 28 degrees is ok but there is a fair amount of water in the stalk so 20 may just be too much - too cold. the plant greens may die but i suspect the roots would regrow but then you lost a lot of growing time. that's why i wait to 4/20 4/28.
but the sooner you put them out the bigger they get by june 21. day long onions for the northern latitudes need to grow as much as possible before mid june. the longest days trigger bulb formation.
i use neptune's harvest fish and seaweed emulsion. onions do not need a lot of fertilizing. so i fertilize them just 3 times 30 days apart. give them 1" of water per week and keep them weeded, onions do not compete well with weeds.
tom
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