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SESE LIBRARY - Multiplier Onion & Garlic Growing Guide
GARLIC AND PERENNIAL ONIONS
Copyright © 1993 by Jeff McCormack, Ph.D. All rights reserved.
Bulb-forming onions and garlic are members of the Allium species which share similar growth and storage requirements. Directions given below are written for multiplier onions (potato onions and shallots). Additional notes are included for topset onions and garlic. Multiplier onions and garlic are very productive crops when grown under the right conditions. Most gardeners will have yield increases of three to eight times per year (on a weight basis). Your success will largely depend on how closely you follow the directions in this planting guide.
SOIL PREPARATION
Multipliers will grow in a range of soil types, but yields will be greatly improved when you pay close attention to soil structure. Bulbs do best in a well-drained, sandy loam well-supplied with organic matter. Roots need air as much as they need water. Thus, it is best to avoid waterlogged soil which can cause onions to rot or grow poorly. Clay soil should be lightened with peat moss, compost, or well-rotted manure. Use at least one part organic matter to two parts soil. Sand can be used to increase drainage, but the sand grains need to be large (approx. 1/16 inch diameter) otherwise you'll make the drainage worse instead of better. Sandy soil should be fortified with well-rotted organic matter, using one part organic matter to two parts soil. Peat moss and well-rotted sawdust are excellent materials for loosening soil.
PRODUCTION IN RAISED BEDS
Raised beds are ideal for bulb production. The size of the beds does not matter as long as you can work the beds without walking in them. We use 4' x 25' beds with 18" aisles in between, but any size will do. The important point is that the soil be well prepared to a depth of at least 8", preferably 12". Beds can be framed with logs or rot-resistant lumber. Join the logs together with barn spikes. Plumbing strap is also useful for making the corners secure. Two inch boards, 6" to 8" wide, can be used for frames. Do not use wood treated with hazardous chemicals such as pentachlorophenol or creosote. The health effects of using pressure-treated wood are not known. If pressure-treated wood is used, coat it with a sealer or expose it to the weather for a number of months to remove leachable salts. Copper naphthenate treated wood may be acceptable, but rot-resistant wood such as locust or cedar is best.
SOIL FERTILITY REQUIREMENTS
For best results, keep pH at 6.5 to 7.0. Soil which is too acid or too alkaline will cause slow growth and late maturity. If necessary, raise the pH of your soil by adding dolomitic limestone, wood ashes (used sparingly), or other appropriate material. To lower pH, add sulfur dust or fertilize with ammonium sulfate. (Never use aluminum sulfate which is toxic to plants and humans.)
Onions are heavy feeders and respond well to fertile soil. It is important that phosphorous and potassium be in abundance. We use rock phosphate or colloidal phosphate to supply phosphorous, and greensand to supply potassium. These materials also supply many trace elements, and fertility is maintained in the soil for several years. Nitrogen is supplied in the form of compost, manure, or other suitable nitrogen- containing material. Work these materials into the soil well ahead of planting time. At the time of planting, bulbs may benefit from a teaspoon of bonemeal beneath each bulb. If the nitrogen level is low, add nitrogen when the leaves are 4" to 6" high, and again in late April. Fish emulsion (5-1-1) works well for this purpose. Nitrogen should not be applied during bulb formation. Too much nitrogen at this time could produce bulbs which are susceptible to rot.
If you use a synthetic fertilizer, use 5-10-10. Synthetic fertilizer is useful in bringing fertility levels up quickly, but don't overdo it. Too much synthetic fertilizer may burn your plants, and produce soft succulent bulbs which rot easily. The use of synthetic fertilizer can be addicting, resulting in poor soil health. Most synthetic fertilizers do not supply many needed plant nutrients, and when used over a long period of time, may have an adverse affect on soil structure and chemistry (unless you add organic matter to the soil).
NUTRIENT DEFICIENCY SYMPTOMS
If your soil is nutrient deficient, your plants will show it in the following ways. Nitrogen deficiency is characterized by yellow-green plants with necks that do not collapse. Phosphorous deficiency results in light green plants which mature slowly. Potassium deficiency results in light green plants with brown tips and poor bulb formation. Note: Soils derived from red rock may be deficient in magnesium (correct with dolomitic limestone), and black soils may be deficient in iron (correct with compost or Sequestrene).
FALL AND SPRING PLANTING COMPARED
Multiplier onions (potato onions and shallots): Multiplier onions are planted in both spring and fall. In Virginia, most varieties of multiplier onions are planted in the fall, but in northern areas a portion of the bulbs should be held for spring planting. Fall planting can give significantly larger yields, often double or triple the yield of spring-planted bulbs. The trade-off is that fall-planted bulbs may be destroyed by freezing if you plant them improperly or fail to protect them. Spring planting reduces the risk of freezing (and bolting in some shallot varieties) but yields are significantly smaller. If you plant in both spring and fall, plant the largest bulbs in the fall and save the smallest bulbs for spring planting.
Topset onions, garlic and ophio garlic (rocambole): For best results these are planted only in the fall.
COLD HARDINESS RANKING
Cold hardiness varies according to allium type and variety. In general, members of the onion family may be ranked as follows from most cold hardy to least cold hardy: topset onions > ophio garlic (rocambole) > garlic > potato onions > shallots. When properly planted, all of our varieties have withstood winter lows of -30°F.
PLANTING DATES
Multiplier onions (potato onions and shallots) --- fall planting: In Virginia, plant potato onions and shallots from mid-October until about mid-December. Planting too early encourages disease and damage by rodents. Planting too late prevents roots from getting established before winter. Thanksgiving week is usually the best planting time. In northern areas, planting times should be adjusted at least 2 to 3 weeks earlier. In southern areas, planting times should be adjusted 2 to 3 weeks later. When planted in the fall, top growth will continue long enough to produce green onions for mid-December harvest. This top growth may be winter-killed, but growth will resume in the spring. Caution: Never plant all your bulbs in the fall, or you may lose the variety. Always hold some bulbs for spring planting.
Multiplier onions (all varieties) --- spring planting: Spring planting should occur as soon as the soil can be worked. Spring planted bulbs may mature a week or two later than fall-planted bulbs. Shallots have less tendency to bolt if spring-planted but yields will be smaller.
Garlic, ophio garlic (rocambole), and topset onions: All varieties are best planted in the fall. In Virginia, the best planting dates are from mid-September to mid-October. Areas north or south will need to adjust planting dates accordingly. Garlic bulbs should be separated into cloves just before planting time.
PLANTING DEPTH AND SPACING
Fall planting depth: Fall-planted bulbs need to be planted deeper than spring planted bulbs. Multiplier onions -- in Virginia, multiplier onions should be planted deep enough so that there is approximately 1/2" to 1" of soil over the top of the bulbs. This depth is adequate for most winters, but deeper planting is recommended for mountainous areas. In northern latitudes we recommend planting bulbs in rows with soil hilled 2" to 5" deep, but be sure to scrape away some of the soil in the spring, otherwise bulbs will be elongated and may not store well. (When scraping away the soil, be careful not to expose the roots which are quite shallow.) A straw mulch piled to a depth of 4" to 8" is recommended for extreme northern areas, but the mulch must be removed promptly in the spring with the arrival of the first spring thaws. Bulb plantings can be more shallow than the recommended depths provided that the mulch is deeper and a snow cover is present. Garlic and ophio garlic (rocambole) --- Most garlic varieties are hardier than multiplier onions. In Virginia, garlic and elephant garlic are planted with approximately 1-1/2" of soil over the top of each clove. In northern areas, approximately 3" to 4" of soil should cover the cloves. In extreme northern areas, add mulch once the tops of the bulbs die back after fall planting.
Spring planting depth: Plant multiplier onions approximately 1/2 to 2/3 the depth of the bulb. Small bulbs (less than 1" diameter) should be planted so that the top of the soil barely covers the bulb. Garlic, rocambole, and topset onions are planted with approximately 1" of soil covering the top of the cloves.
Spacing: When planting in intensive raised beds, large bulbs (3" to 4" diameter) should be planted a minimum of 6" to 8" apart, depending on bulb size. Smaller bulbs (1/2" to 2" diameter) should be planted 4" to 6" apart, depending on bulb size. When planting in rows, large bulbs should be spaced a minimum of 4" apart, and small bulbs a minimum of 2" to 3" apart.
CULTIVATION
It is essential that the soil be kept moist during the entire period of active growth. If the soil is allowed to dry out just once, growth will be arrested and you'll have smaller bulbs and a reduced yield. One or two inches of mulch between rows will conserve moisture and control weeds. Keep well-weeded: onions and weeds don't mix.
When planted at the proper depth, bulb clusters of multiplier onions form near the soil surface, not beneath the soil. This allows the bulbs to become larger, reduces susceptibility to rot, and improves keeping quality. Bulbs of garlic, rocambole, and topset onions develop from cloves beneath the soil surface.
HARVESTING AND CURING BULBS
Most problems with multiplier onions and garlic can be traced to improper care during harvest, curing, or storage. Therefore, it is important that you carefully follow the procedures below.
Multiplier onions: Prior to harvest, the leaves of multipliers will begin to weaken in the neck region and the green tops will begin to fall down. Do not apply additional water during this period. When approximately 75% of the tops have fallen, the crop has sufficiently ripened, and is ready for harvesting. There is no advantage to breaking over the tops of onions still standing (in fact this may shorten the storage life of certain varieties). Those with tops still standing should be given another 7 to 10 days to ripen. Those not ripened after this period should be kept separate from the rest, and should be eaten first because they will not keep as well. To harvest multipliers, pull the clusters, or gently dig them out. Try to do this during dry weather. Remove loose soil from the bulbs, handle gently and do not attempt to clean bulbs thoroughly. Leave the bulbs on the surface of the soil for several days. In southern areas, the bulbs should have some shade to prevent sunburn. This can easily be done in the field by laying the tops over the bulbs as the bulbs are pulled from the rows. As soon as the bulbs are dry they are ready to be cured.
Topset onions: Topset onions are usually grown as a source of onion greens. Thus, the purpose of harvesting is usually to transplant or to increase the crop. If the bulbs are to be harvested for eating or transplanting, wait until the stalks are mostly brown. Do not delay harvesting, because bulbs of some varieties may resprout within a week. After harvest, handle the bulbs in the same manner as multiplier onions. The bulblets at the tops of the stalks may be used to flavor pickles, or they may be used for increasing the crop. Select only the largest, well-formed bulblets for increasing your stock.
Garlic and ophio garlic (rocambole): as soon as the lower leaves turn yellow, the bulbs are ready to harvest. Do not wait until the tops fall over and turn brown, because by then the outer bulb scales will have decayed and the crop will lose its attractive appearance and the bulbs will be more difficult to clean. The bulblets which form at the top of the stalks of ophio garlic can be replanted in the fall; however we recommend cutting off flower and bulb stalks before they form, because they reduce the yield of the ground bulbs. The best time to remove the stalks is as soon as they uncurl and become "woody." Elephant garlic produces small bulblets or corms along side the main bulb. These may be planted in the fall, but may take 2 to 3 years to reach full size. Bulbs of garlic and rocambole may be of two types: segmented (cloves) and unsegmented. Both types may be eaten, but the unsegmented bulbs are usually saved for replanting.
CURING THE BULBS
Curing is a very important step which improves the flavor and hardiness of the bulbs. Move the onions and garlic to a shaded, warm, dry, and well-ventilated area to complete the drying and curing process. Spread them out in a thin layer on wire screens or wooden shelves. Allow the bulbs to remain undisturbed for 1 to 2 months. It is important to check for spoiled bulbs during this period of time, but handle them gently and as little as possible. Because the bulbs have a high water content after harvest they are easily bruised and susceptible to rot. Once curing is complete, separate the bulbs from the clusters, and cut the dried tops 1" above the bulb. Your multiplier onions and garlic are now ready for eating, storage, or planting.
STORAGE
Multiplier onions and garlic store very well over the winter provided that they are well cured, kept cool, dry and well-ventilated, and are not packed over 4" deep. Ideal conditions are a temperature of 32°-40°F (0-4°C), and 60-70% humidity. Maintain good air circulation. Most varieties store reasonably well in a cool room if hung from the ceiling in mesh bags, or spread on shelves in a layer less than 4" deep. Bulbs may be stored in the refrigerator in open paper bags. Do not store in closed containers. Inspect onions and garlic once a month or more often, and remove bulbs which have sprouted or spoiled. If spoiled bulbs aren't removed, the whole batch may spoil. Large multiplier onion bulbs should be eaten or planted in the fall because they have a tendency to sprout easily. Large bulbs may also be grown in pots or flats in your home during the winter, and used as a source of greens all winter long. Although garlic, rocambole, and topset onions have the same basic storage requirements as multiplier onions, the majority of the sets should be planted in the fall in order to obtain good yields.
FLAVOR NOTES
Flavor of multipliers and garlic will be affected somewhat by your growing conditions. Hot dry weather and high soil sulfur levels may produce stronger tasting bulbs. Bulbs develop their best flavor after they are fully cured.
FACTORS AFFECTING BULB DIVISION
When multiplier onions and garlic are grown from small sets or cloves they may not divide the first year, but they will divide and produce clusters the following year. Cultural conditions greatly affect bulb size, and may also have some effect on bulb division.
NOTES ON PEST CONTROL
Onions and garlic are relatively pest free but there are three pests which could give you some problems, especially if you grow large quantities of bulbs. Basic pest management information is presented in the following:
Onion fly (onion maggot): The onion fly prefers seed onions to multiplier onions. It will attack multiplier onions when grown in large quantity. Prevention is the best method of control. Never grow onions in the same location repeatedly: rotate with other crops. Use a minimum three year rotation and quickly compost onion debris, otherwise you run the risk of substantial crop loss. The onion fly is most likely to cause problems when 2 or 3 wet growing seasons occur in a row. Relatively dry growing seasons tend to inhibit the fly. Garlic appears to be totally resistant. If your onions develop a severe infestation, grow your onions in an isolated plot, or spring plant all your onions. There is a predatory nematode (Neoplactana) available which is reported to give good biological control. If chemical control is used spray when leaves are 4 to 6" tall and later in the season if necessary.
Onion thrips: Thrips are most likely to cause damage during hot, dry weather. Humid, wet weather and frequent spraying with soapy water (2-3 tbsps/gallon) will reduce their numbers below damage level. Insecticidal soap is suitable for this purpose, but since thrips can be difficult to control it may be necessary to use a stronger insecticide such as an organic combination of rotenone, sabadilla, and pyrethrin.
Voles and field mice: voles and field mice travel in the tunnels created by moles. As a preventative measure, moles should be discouraged, even though they do little direct damage. Since damage is most likely to occur in fall plantings, set out mouse traps (baited with peanut butter) in the fall. Continue trapping until you have trapped out most of the local rodent population. You may find it necessary to trap two to three times in the fall. This is one reason why it is best not to plant too early in the fall. Raised beds with wood frames may be protected from rodents by stretching hardware cloth or 1" chicken wire netting over the bottom of the frame before the bed is prepared. Rodents may also be controlled by vitamin D organic rodenticide or by Juicy Fruit gum rolled into balls and dropped into tunnels. In both cases the bait must be as free of human scent as possible.
NOTES ON ONION DISEASES
Proper attention to crop rotation, growth conditions, harvesting, and curing techniques will greatly reduce the incidence of disease. The most likely and serious problem is onion neck-rot, which is encouraged by the following conditions: (1) late applications of fertilizer, especially nitrogen; (2) extended wet weather during harvest and initial stages of curing; (3) reduced air circulation around bulbs due to weeds or mulch applied too close to the necks of the bulbs; (4) presence of downy mildew due to extended rainy weather; (5) cutting off tops before the necks have completely dried. In general, strong-flavored varieties are more resistant than mild-flavored varieties of onion.
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