If you live and garden in Maine, you probably already know that onions should be planted in the early spring as soon as the soil can be worked. What you may not know is that planting your onions too late in the season may result in large tops with tiny bulbs. There is a very good reason for that. Onions produce the leafy tops first and then the develop the bulbs, but it's not that simple. Onions rely on the number of hours of sunlight to determine when the tops cease growing and the bulbs begin to form.
Long Day Onions
Long day onions - the kind you should be growing in Maine and other northern climates (above the 36th parallel) - stop producing leaves and begin forming bulbs when the day length reaches 12 to 16 hours, depending on the variety. That means you need to plant long day onions early enough in the spring for ample foliage to form before sometime in the middle or end of May. Interestingly the number of 'leaves' on an onion plant tells you the number of rings the onion bulb will contain. The bigger the leaf, the larger the onion ring. If your onions have only produced a few scraggly tops by the middle of May, your onion bulbs will be small. Conversely, if you plant them too late they may develop bushy tops with little or no bulb formation.
Short Day Onions
Short day onions perform best in southern climates and begin to develop bulbs when the day length reaches 10 to 12 hours, depending on the variety. The same rule applies to planting them early enough for the tops to form before the daylight hours reaches 10 to 12 hours. Because the soil is too cool - or covered with snow - in northern area at the time when short day onions need to be planted, they are not suited to Maine gardens.
Neutral or Intermediate Day Onions
New varieties of onions labeled as neutral or intermediate day onion typically form bulbs when daylight hours reach 12 to 14 hours. These onions can be grown in either norther or southern gardens and may perform well in either.
Before you choose onions for your garden, determine whether you should grow long day or short day onions in your location and choose an onion variety according to day length.