Harvesting and Storing Onion and Garlic Narrative and Photo by Curtis E. Swift, Ph.D., Area Extension Agent (Horticulture)

Onion (Allium cepa) bulbs should be harvested when 80 percent of their tops have fallen over and yellowed and the necks have dried down. Garlic is ready for harvest when the tops become dry and bend to the ground or when the cloves begin to separate in the bulb. Bulbs intended for braiding or fresh market may be harvested at an earlier stage.

The bulb formation of the onion begins when the plant is exposed to a certain day length. Hence the classification of short-day (at least 12 - 13 hours of daylight/day), intermediate-day, and long-day (over 14.5 hours of daylight/day) varieties. The size of the mature bulb depends on the size of the plant at the beginning of bulbing. If bulb formation does not occur, or starts prior to the plant developing proper size, check to see that the day-length classification of the onion matches the day-length of your area. Once bulbing begins the rate of bulb growth accelerates with increasing temperatures.

Garlic (Allium sativum) develops cloves under conditions warmer and drier than those favoring bulb development of onions. As temperature and day length increase, bulbing begins. Bulbing is enhanced if the cloves used have been exposed to 32 ^o to 50 ^o Fahrenheit (0 ^o to 10 ^o C) with a relative humidity of 65 to 70 percent for 1 to 2 months prior to planting. Bulbing may not occur if the cloves were exposed to temperatures of 77 ^o F (25 ^o C) or higher prior to planting.

Elephant garlic (Allium ampeloprasum), a type of leek forming cloves resembling garlic, requires a longer growing season, and one somewhat cooler and wetter than required for onions. The bulb produced by this leek may consist of a few large cloves or a single, large, round clove.

Many gardeners believe that knocking the tops down hastens the development and maturity of the bulb allowing them to be harvested earlier than usual. Research has not shown this to be the case. Maturity of the bulb has to do with the number of days a plant is in the ground as well as soil and air temperature and day length. Tops will fall over when the plant is mature. Dean and Patil found that the optimum maturity harvest time of onions (80 % of the leaves have fallen over) will optimize flavor characters. Pungency and sugar levels (Walla Walla onion types) were highest at maturity. Plants should be harvested and bulbs properly cured before being stored.

Commercial onions that are slow to mature are undercut with a cutter bar. This breaks roots and lifts the bulbs. Such bulbs are usually ready to harvest within 7 to 10 days of undercutting.

After harvest the bulbs should be cured prior to storage. Curing is necessary to develop thick, tight outer scales, to maintain good skin color, and reduce storage disease problems such as Botrytis neck rot. Home gardeners usually cure onion and garlic by leaving bulbs in the field, or in shallow trays or mesh bags.

Field Curing:

When onion bulbs are cured in the field, they are placed in rows with their tops partially covering the bulbs. This helps prevent sunburn. Protection from the sun must be provided or bulb damage and greening may occur. This protection is especially critical in warm bright climates.

The bulbs are left until the tops wither. Tops are then removed leaving a one inch stub. If the tops are left on the bulbs, they can be braided together and the braids hung in the storage area. If rain occurs during the field curing process the bulbs may become discolored or rot. Onion bulbs should be covered during these rainy periods.

Field curing takes two to three weeks if temperatures are 75 ^o to 80 ^o F (24 ^o to 27 ^o C) and the relative humidity is less than 60%. Bulbs left in the field are susceptible to freeze injury. Garlic, if frozen, will rot. However, onion bulbs while freezing at 30 ^o F can withstand 25 ^o F without injury, as long as they are not moved while frozen and are thawed slowly. Frozen bulbs that thaw at higher temperatures will be more severely injured. Freezing injury may be limited to the individual scales or the complete bulb,. Freeze damaged, thawed bulbs are soft, grayish-yellow and water soaked.

Curing bulbs in trays or mesh bags:

When curing in trays or mesh bags, tops are cut about one inch from the bulb. The bulbs are left in the tray or bag until they rustle when handled. Trays can be made of lathes with half-inch spaces between lathes to allow for proper ventilation. Sacks need to be made of burlap or similar material that provides adequate ventilation. Bulbs cured by this method do not become discolored and seldom rot.

Onion bulbs can be cured with forced hot air in 4 to 5 days if the temperature is between 85 ^o and 90 ^o F (29 ^o and 32 ^o C). Curing with forced heated air at (35^oC) for 48 hours has been found effective.

Spanish type onion bulbs keep well in storage. The mild types can be held in cold storage; but, because of their poor storage quality they are stored for much shorter periods of time. Only firm bulbs should be kept for storage. Those that are bruised, have a seed stalk or thick necks do not keep well. These bulbs should be used as soon as possible or disposed of. If poor quality bulbs are stored, rot may set in and spread to the rest of the crop. Ventilation must be provided to remove excess moisture.

If cured in mesh bags or trays, these can be taken directly to the storage area. Bulbs cured in the field can be hung from rafters if tops were braided together, or put in mesh bags or trays if tops were removed. Bulbs can be stored on shelves in the storage area up to six to eight inches deep. Commercial onion growers store onions several feet deep but move air through the onion piles on a constant basis.

Onions and garlic should not be stored with other vegetable crops as the flavor and odor will be absorbed by other vegetables.

Depending on how onions are cured, they can be stored from 1 to 8 months; garlic can be stored for 3 to 4 months or longer if maintained at 32^oF. (0^oC) and a Relative Humidity between 65 and 75% in a well ventilated facility.

The onion storage area should be able to maintain a fairly uniform temperature between 32 ^o and 36 ^o F (0^o to 3 ^o C) and dry air (relative humidity of 60% or less). A loss in bulb weight results when storage temperatures are above 32^oF. (0^oC) Onion bulbs stored at humidities above 70% (at any temperature) or between 41^o and 68^oF (5^o and 20^oC) will mold and often develop roots.

Some references indicate that onions raised from seeds do not store well. We have not found that to be the case. However, if the variety being stored has been a problem in the past, store them in a single layer with the necks pointing down. This can be done by suspending poultry netting from the ceiling and placing bulbs in the netting with the necks through the holes. These varieties should be used first.

Garlic can be stored satisfactorily at a wide range of temperatures up to 90 ^o Fahrenheit (32^o C.) but shrinkage increases above 70^o F. (21^o C.) The higher temperatures 80^o to 90^oF (26.7 to 32.2^oC) are satisfactory to store garlic for 1 month or less. Garlic cloves also sprout rapidly when stored at 40 ^o F (4^oC) or higher. Storage conditions of high humidity result in root and mold growth.

References Used:

Dean, B.B. and B. S. Patil. December 1996. Onion World. pp. 23 - 27.
Hardenburg, R.E., Watada, A.E., and Wang, C.Y. 1986. The commercial storage of Fruits, Vegetables, and florist and nursery stocks. USDA, ARS, Agriculture Handbook #66.
Hoffmann, M.P., C.H. Petzoldt, and A.C. Frodsham. 1996. Integrated Pest Management for Onions. Cornell Extension
Lorenz, O.A. and D.N. Maynard. 1980. Knott's Handbook for Vegetable Growers; Second Edition. John Wiley & Sons.
Pierce, L.C. 1987. Vegetables: Characteristics, Production, and Marketing. John Wiley and Sons.
Salunkhe, D.K., and Desai, B.B. 1984. Postharvest biotechnology of vegetables: Volume II. CRC Press, Inc., Boca Raton, Florida.
Schwartz, H.F., P. Westra and W. Cranshaw. 1990. Colorado Onion IPM Bulletin. Bulletin 547A. Colorado State University Extension.
Sims, W.L., T.M. Little and R.E. Voss. 1976. Growing Garlic in California. Leaflet 2948. University of California
Seelig, R.A. 1974. Fruit & Vegetable Facts & Pointers - Garlic. United Fresh Fruit & Vegetable Association, Washington, D.C.
Yamaguchi, M. 1983. World Vegetables: Principles Production and Nutritive Values. Van Nostrand Reinhold Company.

to the Vegetable Crops Homepage