of Rhode Island GreenShare Factsheets
Commonly grown onions are the mild types, such as White
and Yellow Sweet Spanish, and the more pungent globe types.
The pungent onions are better suited to long-term storage.
The common onion (Allium cepa), one of the most
widely grown onions, is grown from either seed, plants or sets
for use as both green onions and dry bulbs. The home gardener
will usually have more success with sets. Any standard onion
variety or hybrid can be used for green bunching onions if
harvested at the proper stage of maturity. Onions can be used
as green onions within 30 days if grown from plants or sets,
or 40 to 50 days if grown from seed. For dry onions from sets
or plants, 100 or more days are required from planting, depending
on the variety grown.
The potato, or multiplier, onion and
the Egyptian onion are grown from vegetative parts rather than
seed. The underground
portion of the potato onion (Allium cepa var. solanium) is
a compound bulb formed from the segregation of a large mother
bulb. Each bulb in the compound bulb produces 6 to 12 plants.
The principal use of these onions is the production of early
green bunching onions.
The Egyptian onion (Allium cepa var. viviparum) produces
clusters of small bulbs called bulbils at the top of the seed
stalk in late summer. The bulbils are used to produce very
early green onions. Both potato and Egyptian onions are planted
in the fall, overwintered with some mulch protection, and brought
into production in the early spring. These onions are referred
to as "winter onions."
The onion is adapted to a wide range of temperatures
and is frost-tolerant. Best production is obtained when cool
temperatures (55 to 75 degrees F) prevail over an extended
period of time, permitting considerable foliage and root development
before bulbing starts. After bulbing begins, high temperature
and low relative humidity extending into the harvest and curing
period are desirable. A constant supply of adequate moisture
is necessary for best results. For onions started from plants,
a light mulch will help conserve moisture for uniform growth.
An important aspect of onion development is the length
of day (photoperiod). Photoperiod, along with temperature,
controls when the onions form bulbs. Some onion varieties are
short-day and form bulbs when the days are 12 hours or less
in length. Other varieties are long-day plants, forming bulbs
when there are 15 or more hours of daylight. The specific photoperiod
of some onion varieties makes them unsuitable for northern
climates, as they will begin to bulb when the plants are too
small. The influence of day length requires that Sweet Spanish
and Bermuda onions be grown from plants rather than seed in
the Northeast. Southern types such as Vidalia do poorly in
Unfavorable growing conditions may result in onions bolting
or sending up flower stalks. If flower stalks should develop,
carefully cut them from the plant immediately or bulbing will
Onions grow best in a loose, well-drained soil with high
fertility and plenty of organic matter. Avoid heavier soils
such as clay and silt loams, unless they are modified with
organic matter to improve aeration and drainage. Onions are
sensitive to highly acid soils and grow best when the pH is
between 6.2 and 6.8.
As with most vegetables, lime and fertilizers are best
applied using the results of a soil test as a guide. Fertilizers
of a 1-2-2 ratio (5-10-10, for example) are generally good
for onion production. As the onion plant's root system is very
limited, high soil fertility is essential for good production.
Onions should be planted early in the spring as soon
as the soil can be worked. Onion seed is sown 1/2 inch deep,
while sets are planted one to two inches deep. A three-inch
plant spacing is desirable. Rows should be 12 to 18 inches
or more apart depending on the method of cultivation. For wide
row planting, plants or sets are placed on 3-inch centers.
Onions are ideal for wide row planting, but keep in mind that
weeding must then be done by hand.
After the plants are well-established, a mulch will conserve
soil moisture, prevent soil compaction and help suppress weed
growth. In windy areas, small plants must be protected with
a windbreak of some type to prevent serious damage or loss
of plants. Weeds, insects and diseases must be controlled.
Thrips, onion maggots, downy mildew, neck rot, pink root and
smut can all harm onion crops.
Harvest onions when the tops have fallen over and dried.
On sunny, breezy days, onions may be pulled and left in the
garden for a day or two to dry before they are taken to a curing
area. Curing must take place for the onions to be stored for
any length of time. Cure onions by placing them in a warm,
well-ventilated area until the necks are thoroughly dry. With
warm temperatures, good air circulation and low humidity, curing
should be completed within two weeks after harvest. Onions
are best stored in a cool, moderately dry area in ventilated
Adapted from Marianne Riofrio
and E. C. Wittmeyer, Ohio State University Extension, 2000
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
of Rhode Island Cooperative Extension provides equal program