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1、Onion Seed ProductionBased on a presentation by Joel Reitenat the Organicology Conference on 10 Feb. 2011Photo credit: Joel Reiten.Onions (Allium cepa) are a cool-season biennial seed crop that can be grown either seed-to-seed or bulb-to-seed. The author describes them as one of the hardest seed cro
2、ps to grow, with highly variable yields from year to year. This tutorial will share some of the tips used by commercial onion seed producers to ensure good harvests of high quality seed, including the basic processes of organic onion seed production, fertility requirements, planting, irrigation, pol
3、lination, selection, harvest, cleaning, and storage.NEXT: Three Methods for Onion Seed ProductionOnion seed crops are generally produced in one of three ways: seed-to-seed, fall-replanted bulb-to-seed, and spring-replanted bulb-to-seed. The best method for you will depend on your climate and your go
4、als.In seed-to-seed production, onion seeds are sown in early to mid summer, which grow into small, bulbless onions. These onions are left undisturbed to overwinter in the same location. The following year, they flower and set seed. Seed-to-seed production can only be done in a location where the wi
5、nter temperatures do not drop below 1520 °F. Seed-to-seed production is common in large commercial seed production because it minimizes labor. However, leaving the onions in the ground makes weed control more challenging, increases the risk of disease buildup, and does not allow for selection d
6、uring seed production.In fall-replanted bulb-to-seed production, onion seeds are sown in late spring and grow to 12 inch bulbs in early fall. These bulbs are then pulled, selected, and replanted in a new field within a couple of weeks, where they overwinter and produce seeds the following year. Like
7、 seed-to-seed production, this method can only be done where winter temperatures are not cold enough to kill the overwintering onions. The advantages of fall-replanted bulb-to-seed production are that it allows some selection of obvious off-types, and moving the onions to a new field makes weed and
8、disease management easier.The third method of onion seed production is spring-replanted bulb-to-seed production. With this method, onions are sown in spring and allowed to grow to full-size, mature bulbs. The bulbs are pulled in the fall, cured, selected, and stored out of the ground for the winter.
9、 When soil temperatures are warm enough in the spring, the bulbs are replanted and subsequently flower and produce seed. This method is the only viable method for onion seed production in regions where winter temperatures are too low to allow onions to remain in the ground. Because the bulbs are gro
10、wn to full size and stored, they can be selected based on how they would appear as a vegetable crop and on how well they store. This method requires more labor than the other two methods and requires a suitable storage location. A fraction of the onions may also be lost during storage. NEXT: First Y
11、ear Production of Seed-to-Seed Onion Seed CropOn the West Coast, in climates where onions can be overwintered in the field, onions should be seeded in July. In the Willamette Valley, they are sown the week after the Fourth of July. They are planted this late because the object is to get a plant that
12、 is not large enough to bulb but is mature enough to vernalize (i.e., having about 3 or 4 true leaves before winter). Seeding density should be around 2030 plants per foot, with 5foot spacing between rows. Soil temperatures should not be too hot, as onion seed will germinate at 55 °F. Uniform i
13、rrigation is critical for uniform germination and good stands.Joel Reiten on planting onions for seed-to-seed production(Note: this video can take several minutes to load)NEXT: First Year Production of Fall-Planted Seed-to-Bulb-to-Seed Onion Seed CropFor the fall-planted bulb-to-seed method, onions
14、should be sown earlier than the seed-to-seed method. In the Willamette Valley, seeding should be done between the end of May and the end of June, once soil temperatures are above 55 °F. Onions should be seeded in rows 612 inches apart, with a population of 11.5 million seeds per acre. The goal
15、is to end up with a 12 inch bulb at harvest time.Field planted for fall-replanted onion seed production. Photo credit: Joel Reiten.Bulbs for fall replanting should be pulled up when they are 12 inches in diameter. They should still be immature, with green leaves. In the Willamette Valley, bulbs shou
16、ld be pulled up at the beginning of September. The tops should be trimmed either by mowing them before harvest, or by trimming them by hand after harvest. NEXT: First-Year Production of Spring-Planted Bulb-to-Seed Onion Seed CropFor spring replanting, onions should be grown similar to how they are g
17、rown for fresh market. In the Willamette Valley, seeding is typically done from the end of March to the beginning of May. Spacing is 34 inches apart within rows, and 1524 inches between rows.NEXT: Bulb HarvestFor fall replanting:The primary goal with fall-replanted bulbs is to get the onion bulbs to
18、 reroot before winter. This is accomplished by harvesting bulbs before they go dormant and by planting into warm soil.For fall replanting, the onions are pulled in late summer or early fall while the soil is still warm and the bulbs are 12 inches in diameter. The onions should be immature, with gree
19、n tops. In the Willamette Valley, onion bulbs are pulled at the beginning September. The onion tops should be trimmed either by mowing the tops before pulling, or trimming them by hand after pulling. Onion bulbs sized for fall replanting. Photo credit: Joel Reiten.For spring replanting: If onions bu
20、lbs are produced for spring replanting, they should be grown to full maturity before pulling. In the Willamette Valley, onions planted in March or April will be mature in September or October. After pulling, the onions should be allowed to cure in a warm, shaded location with good airflow.NEXT: Bulb
21、 Storage and SelectionFor fall replanting:Fall-replanted onions should only be stored for a very short time20 days maximum. It is important to ensure good air circulation during storage to prevent overheating. Although fall-replanted onions will be immature and not fully representative of an onion a
22、s a vegetable crop, selection can and should still be done. Obvious off-types in color and shape should be discarded, as should any onions showing signs of disease. Generally, 8090% of bulbs will be kept, and 1020% removed.For spring replanting:Onions for spring replanting should be stored in a cool
23、, moderately dry location. Storage temperatures should be 3240 °F. The relative humidity should be 6570%. The onions should be stored in well-ventilated boxes or bags, and air should be circulated. The onions should be periodically inspected for signs of spoilage, and any spoiled onions removed
24、. Onions grown for spring replanting will be fully mature and appear like an onion grown for vegetable production. This method therefore provides the best opportunity for accurate selection for trueness-to-type. Spring replanting and selection on mature onions should be done for any stock seed produ
25、ction. Selection should be based on color, size, shape, disease-susceptibility, and neck thickness. For stock seed production, only 1020% of bulbs should be saved. Otherwise, for commercial seed production, 8090% should be saved, depending on overall quality and uniformity.NEXT: Bulb ReplantingFor f
26、all replanting:When replanting bulbs in the fall, the soil should still be warm and dry. In the Willamette Valley, bulbs should ideally be replanted before October 1 in single rows on beds that are 1.5 inches apart (7.58.5 tons/ac). Bulbs should be planted in a furrow 34 inches deep and covered with
27、 soil. Ideally, bulbs should be planted in a light sandy-type soil. If your soil is a heavy clay type, the bulbs should not be covered completely because the soil can trap excess moisture around the bulbs and cause rot. If your soils are colder than 55 °F by the time you can replant in the fall
28、, then they will need to be replanted in the spring. If circumstances require that you plant bulbs intended for fall-replanting in the spring, hot air curing (like what is done for onion sets) can decrease storage losses. For spring replanting:For spring replanting, it is important to wait until the
29、 soil temperature is at least 55 °F. Onion bulbs are highly susceptible to fusarium rot if planted in colder conditions. Planting depth and spacing should be the same as for fall-replanted onions. Some onions, especially storage onions, can be challenging to wake from dormancy. If you encounter
30、 problems with onions not coming out of dormancy in the spring, an alternate technique can be used. To break dormancy, cut off the top 2/3 of the bulb prior to planting. Do not re-cover the trenches until the bulbs have rerooted, as soil contact on the cut ends of the bulbs can increase rot. To furt
31、her minimize rot, a light coating of vegetable oil can be sprayed on the cut surfaces after planting.Next: Seed Stalk Growth and MaturationRegardless of whether the seed crop was grown as seed-to-seed, fall-replanted bulb-to-seed, or spring-replanted bulb-to-seed, onions will undergo a phase of stal
32、k growth and maturation in the spring of the second year. After soils warm in the spring, it will take about 3045 days for seed stalk extension. During this period, growth is very rapid and soil fertility is important. Second-year onion seed crops require around 100120 lb/ac of available N. For most
33、 organic systems, overfertilization is more of a problem than underfertilization. If there is too much available N in the soil, the stalks can become very elongated and the seed heads can become very heavy. These conditions can lead to seed heads falling over late in the season.NEXT: Flowering and P
34、ollinationOnions are insect-pollinated. They may be pollinated by a wide variety of insects, including honeybees, bumblebees, leaf cutter bees, and flies. Most commercial fields rent hives during pollination. According to the Pollinator Handbook: "Seed producers have generally adopted the pract
35、ice of renting five to 15 colonies of bees per acre and having them placed in or adjacent to their seed fields at flowering time. One suggestion has been to have about two colonies per acre delivered when flowering is well started, then an additional two per acre at 3- to 4-day intervals to take adv
36、antage of 'naive' bee behavior and maintain some level of nectar foraging activity throughout the blooming period." The reasoning behind bringing new bees in at intervals during the pollination period is, because onion flowers are not highly attractive to pollinators, bees that have bee
37、n in the field for a few days may have time to find more more attractive flowers further away.Providing ample habitat for native pollinators is also important. In some cases, native pollinators may be able to pollinate when honeybees will not. For example, leaf cutter bees can operate over a wider t
38、emperature range and are often found pollinating an onion seed crop earlier in the morning and later in the evening than honeybees.Flowering onion seed field. Photo credit: Joel Reiten.Onions do not reliably pollinate. The pollination window is short, lasting two weeks at most. During that time, if
39、temperatures are too high (above 100 °F), pollen may not germinate or flowers may abort. High temperatures and dry weather may also concentrate the nectar in the onion flowers. Depending on the variety, this concentrated nectar may become too sour and unpalatable to the pollinating insects. Far
40、mers experiencing hot and dry weather during pollination may use overhead irrigation to dilute the nectar and make it more palatable.NEXT: DiseasesOnion seed crops face the same diseases that affect onions grown for vegetables. These include downy mildew (Peronospora destructor) and purple blotch (S
41、temphylium vesicarium or Alternaria porri). The first approach to disease control should be good cultural practices. The following practices are recommended:· Use drip irrigation instead of overhead. One exception would be the use of overhead irrigation during pollination to dilute nectar if te
42、mperatures are above 100 °F (see Chapter 10). · Provide good airflow through the field by maintaining proper population density (see Chapter 3 and Chapter 8), and by orienting rows to allow prevailing winds to pass through. If spraying is required, two of the most common controls are coppe
43、r and peroxide. Copper interrupts the fungal reproductive cycle, while peroxide dissolves the fungal cells. Sprays should be applied using a drop tube to ensure that only the stems are treated and not the heads. Spraying should be done on a preventative basis according to expert advice and past expe
44、rience. Once disease symptoms are visible, it is too late.NEXT: Maturity and HarvestOnion seed is mature about 90 days after seed stalk formation. As maturity approaches, the field should be closely monitored. Onion seed is contained in capsules. When the first capsules begin to split to reveal blac
45、k seed inside, all irrigation should be cut off. When the onion seed heads have a cluster of split capsules that are approximately 11.25 inches in diameter, it is time to harvest.Onion seed heads ready for harvest. Photo credit: Joel Reiten. When harvesting, the seed heads are cupped with the palm o
46、f the hand while the stalk is cut about 68 inches below the seed head. The umbels can either be harvested into 55-gallon barrels and removed from the field, or cut and left in the field to dry. If the seed is harvested into barrels, it should then be placed on screens to dry for about half a day bef
47、ore threshing. If the seed is left in the field, it should be allowed to dry for up to two weeks before threshing.NEXT: Threshing and CleaningWhen threshing onion seed it is important to be very gentle. Onions have a very thin seed coat which is easily damaged during threshing. When the seed coat is
48、 damaged, important electrolytes can leak out from the seed and reduce seed viability over time. For commercial organic onion seed crops, threshing is often done with either an Allis-Chalmers All-Crop Combine or with a belt thresher. All-Crops are fairly gentle on onion because the cylinder bars are
49、 made from rubber. However, because All-Crops were last manufactured over 50 years ago, it can be difficult to find them in serviceable condition. Belt threshers work by rotating the capsules between two belts moving at different speeds. They are slower and gentler than the All-Crop, and more appropriate for smaller-scale onion seed crops.Once the seed is threshed, it is relatively simple to clean. Most of the cleaning can be done by winnowing the seed using fans. Seed companies also use gravity tables fo