Scientific NameAllium cepa
General InformationPhoto: Lars Blankers, Unsplash
When to Plant
Plant starts November to January. Grow by seed for a much larger choice of onion varieties. Onion seeds do not store well so buy new seeds every year. To grow intermediate day and long day onions from seed, start seeds indoors by mid-October. Transplant into a compost potting mix in early December and put them in the ground in January while they are still less than a one-quarter inch wide. Once in the ground, the plants will develop their root system. Ideally, they will grow another ‘leaf’ every week or so. The longer they have to develop while the days are shorter, the larger the bulb will be. As soon as the day length hits the required number, the bulbs will begin to develop. Usually this happens by the end of May or the beginning of June.
Depending upon variety, start seeds indoors in mid-October.
Plant seedlings or starts outdoors in January.
Plant onions in well-amended, loose soil, in full sun, protected from wind.
Keep bed weed-free to eliminate competition for water and nutrients. Transplant — onions do not need to go in very deep. Plant them shallow so when the bulbs start forming they are partially above the soil.
Onions are sensitive to daylight hours. Know how many daylight hours you have, and what your onion variety requires. Don’t plant too early or the cold will cause the plants to flower. Don’t plant too late or the plant will not be big enough to support a good sized bulb. Plant your onion starts six to eight inches apart. Check seed packet for information about the specific variety you are growing.
Bulbing onions will be in the ground for approximately seven months.
pH Between 6.0 -7.5, optimal pH 6.8 Keep your bed weed-free to eliminate the competition for water and nutrients.
Frequent irrigation throughout the season. Root system is shallow so very little water is extracted from deep in the soil. The upper soil area must be kept moist to stimulate root growth. Water once or twice a week, depending upon the weather. Stick your finger in the soil to determine when it’s time to water. Regular irrigation is required anytime rainfall is not sufficient to provide one inch of water per week. Once stems show signs of dieback, cease irrigation and harvest when dry.
Add a low value nitrogen fertilizer, into the soil to provide nitrogen for green growth and if needed, a little phosphorous. (Most Marin soils have naturally occurring phosphorous as do many soil mixes, so test first.) Do not fertilize onions after the weather warms and they are nearing their bulb forming stage.
Onions are consumed prior to flowering so pollination applies to onions grown for seed. In fact, if you see a flower stem beginning to emerge on an onion cut it out and or harvest the onion as it is unlikely to form a good bulb. Flowers are self-fertilizing so pollen from one floret can pollinate another floret on the same umbel. The flowers are visited by a range of insects that collect pollen and nectar.
Harvest onions when most of the tops have fallen over and begun to dry. Carefully pull or dig the bulbs with the tops attached.
After harvesting, dry or cure the onions in a warm, dry, well-ventilated location.
Cure the onions for two to three weeks until the onion tops and necks are thoroughly dry and the outer bulb scales begin to rustle. After the onions are cured, cut off the tops about one inch above the bulbs. As the onions are topped, discard any that show signs of decay.
An alternate method is to leave the onion tops untrimmed and braid the dry foliage together. Do not remove the dried outer skins. Gently brush off any dirt and clip off the roots.
Store the onions in a cool, moderately dry location. Don’t store with apples or potatoes. Storage temperature should be 32 to 40F. Mature dry-skinned onions like it dry. The storage life of onions is determined by the variety and storage conditions.
When properly stored, good keepers, such as 'Copra' and 'Sweet Sandwich' can be stored for several months. Poor keepers such as 'Walla Walla' and 'Sweet Spanish' can only be stored for a few weeks. Allow onions to dry for several weeks before you store them in a root cellar or other storage area. Be sure to check your onions regularly and discard any that have gone soft or begun to rot.
Good Varieties for Marin
Intermediate day varieties: 'Walla Walla' 'Ruby Ring'
Long day varieties: 'Southport Red Globe' 'Copra'
Watch this helpful video showing how to plant, and harvest. Remember, in Marin we do not grow short day varieties.
Know how many daylight hours you have and what your onion variety requires. Don’t plant too early or the cold will cause the plants to flower. Don’t plant too late or the plant will not be big enough to support a good sized bulb.
Know how many daylight hours you have, and what your onion variety requires. Marin is between 37.8 to 38.1 degrees latitude.
Short day varieties require 11 to 13 hours of daylight. You need to be at 350 latitude. These cannot be grown in Marin.
Intermediate day varieties require 12 to 14 hours of daylight. You need to be at 35 to 38 degree latitude. Can be grown in Marin.
Long day varieties require 14 to 16 hours of daylight. You need to be at 37 to 42 degree latitude. Can be grown in Marin.
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