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Onions and Alliums

Growing Onions and other Edible Alliums in the Owens Valley

by Alison Collin



Allium cepa

Onions are biennial members of the Liliaceae family and Allium genus which also includes leeks, garlic and chives and shallots. They are an important food crop worldwide, and feature in the cuisine of many different cultures.

Numerous  varieties of onions are available and most are easily grown but not all varieties make good bulbs at this latitude so it pays to understand  the specific requirements in order to meet with success and not to waste time, money and effort planting varieties which are not likely to do well.

The most important criteria for growing onions is the length of daylight available.

Onions grow foliage during the cooler weather – each leaf representing one layer in the bulb - until a specific daylight length is reached. Once this point has occurred, foliage growth stops and bulb formation begins, so the larger and stronger the plant is at that point the bigger the mature onion will be.  If you plant the wrong type at the wrong time they will grow tops but not produce onions, or will simply bolt and flower.

 They are divided into the following groups:

  1. Short day onions bulb up when the daylight length reaches 10-12 hours. (Note: This is in spring, and NOT when the days are getting shorter)!  These are best started in the fall or early winter so that they can bulb up early the following year – they will be ready to use around late May -July. They are hardy, and overwinter well, although one has to remember to maintain irrigation and not let them dry out at any time. These are generally the sweeter flavored onions and do not store well.
  2. Intermediate day onions which are not so temperamental and will bulb when the light length is about 12-14 hours. In this area, if seed is started early in the year (January),  or transplants are planted out in February they will have several months in which to grow into good sized plants by the time bulb production begins so this will enable them to develop quite large bulbs which can then be dried and stored by the end of August/September.
  3. Long day onions that require 14-16 hours of daylight in order to bulb up. These grow well in the north of the USA but do not do well in this area. In general they are the more pungent varieties and tend to be the better keepers.  An exception to this are the Walla Walla onions which although technically  are long day onions they can be grown in intermediate day areas with good light since they mature early.  These are very sweet onions so do not store well.

Onions can also be further divided by color of their skins - yellow, white or red, or by their taste - sweet, pungent, or mild.  Each daylight length group contains a variety of these possibilities.  Some store well, while others only keep for a limited amount of time.

Mature bulbs vary in shape and size conical, round, flattened disc, and some, such as the Cippolini types only grow to 2”, while some aficionados attempt to grow individual onions weighing over 10lbs! The Egyptian Walking Onions (Allium × proliferum) make clumps of bulbs at the base which are rather pungent, and further clumps which are more generally eaten appear on the end of stalks in place of flowers. If left these flower stems bend over and the bulblets then root when they reach the ground.

Cultural Requirements.

Onions need full sun and prefer a loose, rich, well-drained soil.  They are very shallow rooted so need regular irrigation to avoid drying out, and benefit from careful mulching.  They do not perform well if they are crowded or have to compete with weeds.

Growing from Seed

This is an economical way to grow onions and gives a chance for several different varieties to be compared.

Onions may be sown directly into the ground early in the year for intermediate day varieties, or in fall for short day types.  Sow in rows ¼” deep and 15” apart.  They should be gradually thinned until they are about 4”-6” apart - the thinnings can be used as green onions or may be transplanted.

I have found that for intermediate day onions I have a much greater success sowing indoors in cell packs in mid-January.  I harden them off (outside in a protected place during the day, but in at night) ready to plant them out as tiny plants once they have two true leaves which is about  one month later. I put one seed per cell so that there is as little disturbance as possible to the plants when I plant them out.

I have also used cardboard fruit boxes such as tangerines come in, filled with seed compost and placed on a tray.  The box usually lasts for about one month by which time the cardboard is on the point of disintegrating which makes removing the little transplants easy.

Commercially grown transplants.

These are an easy way to grow onions if seed-starting is not possible.  Many catalogs now carry Intermediate day onions as small plants, and even sell them in mixed bunches of red, yellow and white varieties as a convenience.  They have their roots and tops trimmed, and depending on the company may have been lifted some time before they arrive so they may be rather dried out.  Also some companies don’t send them out very early in the season to zone 7 – a point worth checking.

They are comparatively expensive and come in large bundles of about 50-70 plants which is generally more than I need so it may help to find someone willing to share an order.

Onion Sets.

These are tiny onion bulbs which are grown the previous season in crowded conditions to keep them under developed and are then lifted and dried.  They appear in net bags and are usually just labelled “white” or “yellow” with no varietal name and are most often long-day varieties.  My experience with trying to grow these is that they bolt very readily or just grow foliage and some very small bulbs. In this area they are usually planted in the fall, but are often already woody in the middle before they are lifted.  They may be planted to provide a supply of green onions, and can be planted close together for this purpose.

My experience has been with using drip irrigation with ½ pipes and emitters every 1 foot and this I bury about just under the surface.   During  the recent drought years I have planted a row on either side of the pipe with those plants on one side about 2” to the left of an emitter, and the ones on the other side about 2” to the right so that they were checkerboarded and the plants were about 4”-5”away from each other.  If growing with soaker hoses or overhead irrigation  they can be planted about 4” apart depending on the cultivar.  Closer spacing will give smaller onions.

One month after planting I mulch the growing plants (with lawn thatch last year) and then just leave them to grow.

I have excellent soil so I fertilize only a couple of times using a general liquid fertilizer early in the growing season (March and May).  If growing sweet onions avoid using fertilizers containing sulfur.   It is important not to fertilize once bulbing has begun.  The areas between the plantings remained dry because of lack of water, and very few weeds appeared through the mulch but these were removed when small to avoid disturbing the onion roots.

As the foliage grows take care not to break or bend it at the neck as this will inhibit nutrition  from the foliage reaching the bulb.  (Dogs, soccer balls etc.).

Harvesting and Curing.

When the bulbs have grown to full size, usually around early August for intermediate day varieties, the foliage begins to turn brown and bend over at the necks.  At that point I remove the mulch, reduce the irrigation, and as the onions dry I disconnect the irrigation pipe completely.  When the foliage is dry I lift the onions very carefully so as not to damage them or tear off the drying skins. They have usually grown so that the bulb is almost sitting on top of the soil.  Then the onions are traditionally laid out on the soil surface to dry, with the foliage of one plant laid over the next to prevent sunburn.  However, I have found that since this is often at the hottest time of year in our area, they tend to get sun damage anyway, so I have found that  using an old mesh fly screen supported on a couple of saw horses and placed away from strong, direct sunlight often gives better results.

Once the necks are completely dry, and the outer skins papery,  the onions may be placed in mesh bags, or if suitable braided and stored in a cool, airy place. (Unfortunately, almost an impossibility here).  I use a spare bedroom for the hotter months before transferring to the garage once the outside temperature cools off.  Inspect for softening or rotting and remove those that are affected immediately;  use any with thick or soft necks first.

Pests and Diseases.

I grew onions without any noticeable problems for a few years, but two years ago I began to get thrip infestations which affect onions grown in dry conditions, and appeared in July.  The foliage became streaky and slightly grayish and sticky.  Insecticidal soap has proved effective in controlling this pest, which is not so likely to occur where onions have sprinkler irrigation.  Damage to the crop was not severe since by that time the bulbs were well formed. 

Onion pink root is a fungal disease and can be devastating.  So far I have never seen any evidence of it in my crops.

For other pests and disease symptoms check UC IPM.


Bolting, or sending up a premature flower spike during the first year of growth, is often the result of  a sudden change in temperature, irregular irrigation or other stress.  Some varieties are more prone to bolting than others, so check for this when choosing varieties.  If a flower spike emerges, snip off the top of the stem, and use that onion quickly because it will not store.  The center of the bulb may be woody, but the rest is still usable.

Yields vary but in 2016 I grew  one  17’ row of Candy from seed started early indoors  and planted out as tiny plants in mid-February  - a total of 35 plants which yielded just under 31lbs of onions, the largest being 1lb 6¾oz, and the smallest 9 oz.  They stored through mid-December.

However another Master Gardener in a nearby neighborhood grew Candy from commercial transplants planted out on March 29 as part of a comprehensive trial of onions in 2014.  She grew 30 plants of Candy yet the yield of those was only 11lbs 9oz, and they grew to only 2.5” in diameter.

http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=16873 (Bishop trial of 9 different varieties)



http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=19933  Fall planted onions in Bishop




Allium ampeloprasum, Porrum Group (Zones 3-11)

These are biennials grown for their long cylindrical blanched stalk, and are harvested at the end of their first growing season before they develop a flower spike.  They may be started from seed or planted as transplants and in this area grow quite happily in afternoon shade.  They have a distinctive, milder flavor than onions and have the great advantage of being able to be left in the ground for most of the winter so do not need lifting and storing, but can be used directly from the garden.  I have tried several different varieties here – American Flag, Zermatt, Blue Solaise, Lancelot – and they have all performed well with no failures!

Their cultural requirements are very similar to onions with the exception that the lower stalk is blanched during the growing season and consequently the planting out technique is a little different:

Planting out:  A wide trench about 1ft deep is dug, and the leeks planted at the bottom of it about 18” apart, then as growth develops soil can be drawn up gently against the plants to blanch them, great care being taken not to let the soil fall between the leaf layers since this gets embedded and makes for a gritty meal.

My preferred method is to dib a hole and drop the plant into it, then water it gently so that some soil from the edge of the hole falls in against the roots.  Gradually with further irrigation more soil falls into the hole and fills it in.  When the plants are about 1ft high they are ready for my magic ingredient:  Toilet paper tubes!  I gently gather up the leaves and slip these over so that they cover the base of the plant which will then become blanched with no soil between the leaves. This method is best suited to drip irrigation.



Allium var. aggregatum

These are smaller and produce clumps of cloves which are larger than garlic and  which have a mild onion flavor.  They are often eaten raw in salads, and when cooked become sweeter.  They are often useful if only a small quantity of onion is needed in a recipe. They are generally expensive to buy in grocery stores.

Like onions they come in different colors and shapes.

They have much the same cultural conditions as onions and can be grown from seed or sets which are planted in the Fall, after the first frost but before consistently cold weather arrives.  Plant so that the top is just under the soil surface and mulch.  New growth will occur in spring.  Leaves can be used as green onions, and the bulbs will mature like onions, ready for drying in early summer.  They are good keepers.



A. fistulosum

These are perennial bunching onions which can do well in part shade.  They are fast-growing, do not form a large bulb but make clumps of individual plants which can be harvested year round.  The mother clump can be kept for several years and new plants can easily be grown by removing plantlets from the outside of the clump and transplanting them.  The blanched bases and green leaves are usually eaten raw in salads. 



Allium schoenoprasum

Chives form perennial clumps of tiny bulbs.  They have hollow leaves which grow to about 1ft tall and which can be harvested through most of the year although they die down during the coldest months.  They prefer regular water but can withstand some dry spells, and they will grow in part shade.  The easiest way to propagate is by division, dividing overcrowded clumps into groups of about 6 bulbs before replanting.