Saving Spinach Seeds: A Complete Guide for Organic Growers

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Spinach is dioecious, with separate male and female plants. In this photo, a female plant with immature seed already forming is on the left and a robust vegetative male at peak flowering is on the right.
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In "The Organic Seed Grower," John Navazio reviews the historical and botanical information for organic plant production as well as the basic, organic seed grower fundamentals for growing over 30 vegetable plants.

With a high nutritional quality in all varieties, spinach plants are well-worth the patience and precision. Each plant will add color to your garden and nutrition to your plate, as well as produce an abundance of spinach seeds for the next planting season. IThe Organic Seed Grower (Chelsea Green Publishing, 2012), John Navazio combines traditional, organic know-how and the latest scientific research with the intent to place the organic growing power back where it started, into the hands of local seed growers and farmers. Because the knowledge of growing and saving seed is too important to leave to the agribusiness “experts,” now is a great time to learn and begin growing. The following excerpt, from chapter five, “Amaranthaceae,” explains the process of growing spinach and saving spinach seeds.

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About Organic Spinach Plants

Common name: spinach
Crop species:Spinacia oleracea L.
Life cycle: annual
Mating system: cross-pollinated
Mode of pollination: wind
Favorable temperature range for pollination/seed formation: 65–75°F (18–24°C)
Seasonal reproductive cycle: early to mid-spring through late summer or early fall
Within-row spacing: seed-to-seed 16–24 in (41–61 cm); root-to-seed 18–30 in (46–76 cm)
Between-row spacing: seed-to-seed 30–36 in (76–91 cm); root-to-seed 30–48 in (76–122 cm)
Species that will readily cross with crop: All spinach seed growers should be conscious of nearby farm or garden plots of spinach grown for fresh market production as these crops can flower rapidly in late spring and summer before they are turned under.
Isolation distance between seed crops: 0.5–3 mi (0.8–4.8 km), depending on crop type and barriers present on the landscape

Spinach (Spinacia oleracea) is one of the most widely grown vegetables in temperate climates around the world. It is derived from a leafy, winter annual that evolved in and near the Fertile Crescent of the Middle East. Winter annuals are plant species that germinate in the cool of the fall and grow vegetatively until the short days and cold of winter slow their growth. In spring, winter annuals grow steadily until a combination of environmental factors prompts the reproductive phase of the life cycle. Spinach bolting is initiated primarily by daylength, and the ancestral forms of this crop bolt very early in spring with less than 14 hours of light per day. This allowed the predecessor of modern spinach to mature seed before the onset of the intense heat of summer in the Middle East. The seed then lay dormant, having evolved to only germinate with the onset of cool, wet weather in fall, and thus the cycle was repeated.

Modern forms of this crop have been selected to produce a lush and robust leafy vegetable that is widely adapted across environments and seasons. Various types of spinach still produce well as a fall-sown vegetable that can be harvested in fall, winter, or spring. It is easily cold-hardy to 15 to 18°F (–9 to –8°C) but can survive much lower winter temperatures if insulated by snow. Many contemporary varieties have been developed to be spring-planted and produce a bountiful crop before the summer daylength causes bolting. Much recent breeding work has been devoted to develop fast-growing, upright plants that thrive at high population densities and are harvested at a juvenile or baby-leaf stage for bagged salads. Spinach leaf surfaces are smooth, semi-savoyed, or fully savoyed, depending on the amount of leaf curl. The savoyed curl is due to varied rates of growth of ground tissue parenchyma between leaf veins. Leaf shape ranges from the putative older form of triangular blade that is referred to as having an arrowhead or Christmas tree shape, characteristic of Asian spinach varieties, to the rounder, less lobed leaf that has become the ideal in European and North American markets. Selection for color in variable spinach populations has created darker leaf pigmentation. Research investigating the nutritional quality of spinach has found that the dark green types have higher levels of a variety of phytochemicals with antioxidants such as the carotenoid lutein, which is important for the health of the macula in the human retina.

Crop Characteristics

Reproductive Biology

Spinach is largely dioecious in its flowering habit. The number of male plants to female plants is roughly equal in large populations. Occasionally, there may be monoecious plants in a spinach population that express both male and female flowers on the same plant, but these are less common. Male spinach plants exhibit two basic plant forms. The first males to flower are quite short in stature (often only 4 to 6 in/10 to 15 cm tall at full development), with suppressed leaf development on all upper nodes, but prodigious in staminate flowers at all nodes. These extreme males, as they are often called, flower only for a short period of time and die after flowering but are capable of producing large amounts of pollen, ensuring that plenty of pollen is available for early-flowering females. The second type of male plant is known as a vegetative male and is a more robust, larger plant with both staminate flowers and leaves at all nodes. Vegetative males initiate flowering a week to 10 days after the first appearance of extreme males. They are longer-lived than extreme males and flower over a longer duration, ensuring ample pollen for the female plants throughout the period of pollen receptivity. Female plants are similar in stature to vegetative males and bear pistillate flowers and leaves at all nodes. They are long-lived and usually begin flowering within 3 to 7 days of the vegetative males.

Flowering is initiated primarily due to daylength. Heat can play a role by speeding the metabolic rate of the spinach plant, accelerating the flowering process once flowering has been initiated. Spinach is predominantly wind-pollinated and has extremely light pollen that is easily carried long distances by the wind. Both male and female flowers are very small and lack petals. Both types of flowers are borne in groups in the leaf axils of their respective plants. The calyx of the male or staminate flower has a sepal below each of the four stamens. In turn, two anthers are borne on each stamen. In the female, or pistillate flower, the calyx can have two or four sepals that persist after fertilization and combine with the pericarp to create a hard protective casing for the single-seeded fruit. The calyces of several fruits that are borne together can fuse and form tight clusters of seed that will require special seed-cleaning techniques when milling the seed.

The spinach seed coat may be either prickly or smooth. Historically, the prickly-seeded types were generally the large-leaved winter varieties (presumably, the more ancestral form), while the smooth-seeded types were the spring and summer varieties. The prickly-seeded trait is increasingly rare today, and there does not seem to be a genetic association between the prickly seed coat and either winter hardiness or large leaves.

Climatic and Geographic Suitability

Specific environmental conditions are required to produce high-quality yields of high-germinating, large-seeded spinach crops that are free of seedborne pathogens. Spinach is a cool-season crop that is very sensitive to temperatures above 75°F (24°C), both as a vegetable and as a seed crop. For this reason, there are few locations where spinach seed can be grown for commercial use. The two most important spinach seed production areas worldwide are the Skagit Valley of Washington and a large swath of central Denmark. Both areas have cool, wet springs followed by dry, cool summers (temperatures usually not exceeding 75°F/24°C) and relatively dry fall weather for harvest. The cool, wet spring weather of these ideal climates enables the spinach plant to establish a robust, large vegetative rosette of leaves before flower initiation under the longer days of late spring. Summer weather that exceeds 85°F (29°C), especially during pollination and early seed development, can dramatically lower germination rates, seed size, and seed yields. As is common to all dry-seeded vegetables, an extended dry period in late summer is favored for producing a seed crop that is relatively free of disease and disease-causing inoculum.

Spinach Seed Production Practices

Soil and Fertility Requirements

Spinach grown for seed can be planted on a variety of soils, but the soils must be well drained to avoid root rot problems. Soil pH should be maintained above 6.0, as spinach is sensitive to acidic soils. In seed crops, the amount of available nitrogen should not be too high in order to avoid excessive vegetative growth before bolting, as this promotes lodging of the plants during seed set. Well-maintained agricultural soils that are high in humus and microbial populations will supply adequate nutrients and water over the long seed production season. Spinach is somewhat tolerant of soil salinity and very tolerant of alkaline soils, although foliar fertilizer applications may be needed on alkaline soils to counteract the reduction in availability of micronutrients such as manganese under high soil pH.

Growing the Seed Crop

The optimum temperature range for germination of spinach seed is 45 to 75°F (7 to 24°C). Spring-sown spinach can be planted as early as the ground can be worked, though waiting another week or two till true spring weather prevails to plant will ensure vigorous seedling growth. Planting should not be delayed too long, however, as the key is to get as large a vegetative rosette as possible before flower initiation. This rosette of leaves (or frame, in the parlance of seed growers) will not grow appreciably after the flower stalk emerges and will be the photosynthetic factory responsible for producing the seed crop.

Sometimes spinach seed crops are planted in late summer or fall and overwintered for seed production the following summer. This eliminates ground prep and planting during inclement spring weather. It also enables the spinach to develop a larger vegetative frame to optimize the seed yield potential of the crop. Unfortunately, the overwintered crop may act as a green bridge, providing a live, vegetative host allowing a number of diseases to survive through the winter.

The planting density for spinach seed production requires much wider spacing than for producing spinach as a vegetable. For seed production, spinach plants are generally spaced 8 to 12 in (20 to 30 cm) apart within rows. Standard row centers are normally 22 to 26 in (56 to 66 cm) apart, but in raised beds spacing between rows can be dropped to 12 to 14 in (30 to 36 cm) apart. Wider plant spacing increases airflow through the crop, reducing disease pressure. Spinach seed crops are still often grown with overhead irrigation in the Pacific Northwest, although its use is limited to the early part of the season before bolting, flowering, and seed maturity. Avoid it during these reproductive phases of the life cycle, as the incidence of foliar and possible seedborne diseases will certainly increase as a result. Drip or furrow irrigation has the advantage of not wetting the foliage and can be used throughout the season.

Early-season weed control is critical for optimum establishment. If plants become leggy due to early competition they will be difficult to evaluate in the selection process and be more prone to lodging when full-sized. Spinach seedlings grow slowly and do not compete well with weeds. Starting with a stale seedbed is advisable, as well as avoiding fields with high weed pressure. Pre-emergence flame weeding can be effective. Thinning and blocking by hand are usually required in organic production, as is hand hoeing until plants are large enough to not be smothered by mechanical cultivation.

The fact that spinach is dioecious is an advantage in hybrid spinach seed production. The female plants can be used to develop all female flowering lines. This is possible because female spinach plants, if isolated from spinach pollen during flowering, will eventually “sex reverse,” subsequently producing male flowers in addition to their female flowers in order to achieve pollination. The subsequent generation will inherit the female genes. This sex reversal enables breeders to increase seed of a female line. During the development of female lines for hybrids, the breeder must perform meticulous selection on the female plants, ensuring that the lines uniformly produce only female flowers for at least 5 to 6 weeks before sex reversing. Then, when used as female parents in hybrid spinach seed production, these lines will have adequate time to receive all of their pollen from the male parental line of the hybrid in the field. Female plants can also be selected to have a monoecious flowering habit to serve as the male parent. This eliminates the variability of having both extreme males and vegetative males as your male pollen source and produces much better phenotypic uniformity in the resultant vegetable crop, especially as it just begins to bolt (when spinach is often still harvested). All of this can be done without resorting to chemical growth regulators and is therefore possible in organic production systems.

Seed Harvest

Spinach seed, as with all members of the Amaranthaceae, is formed and matures in an indeterminate growth pattern, beginning on older, lower branches and continuing up the flower stalk through the season. Due to this sequential maturation, only a portion of the seed continuously being set will reach maturity by the end of the season. In most cases only about 75% of the seed on any given plant will reach maturity by harvest. One of the most difficult steps in successfully growing a spinach seed crop is judging when this optimum amount of seed is mature on a majority of plants in the field. A standard method of gauging the maturity of a spinach seed crop is to make a visual assessment of the percentage of seed on most of the plants that has turned a tan-brown color, typical of mature spinach seed, and then harvesting the crop when 60 to 80% of the seed has become this color. This is an unreliable method of judging maturity, however, due to the considerable effects of genetic and environmental variation. Environmentally, the seed may prematurely lighten in color in the presence of the Stemphylium/Cladosporium leaf spot disease. Genetically, some spinach cultivars have much greener seed, even at full seed maturity.

The most important trait to monitor in gauging the maturation of spinach seed is the relative maturity of the seed endosperm. The endosperm and embryo, which mature concurrently, must be fully developed to produce vigorous, vital seed. The endosperm, which is primarily composed of starch, is easily monitored by cracking open the seed and squeezing it to visually inspect the stage of endosperm development. Appearance of the endosperm will transition from translucent or milky early in development to what is described as flinty, with a grayish, waxy appearance at the midpoint of development. At these stages, the endosperm can be squeezed out of the developing seed for inspection. When seed in the middle of the stalk is at the flinty stage, irrigation may be stopped to speed the maturation and drying process. This can be important if there is risk of maturation extending into the wet, disease-promoting conditions of fall.

At a mature stage of development, the endosperm turns a starchy white color, which can be observed by cracking open the seed. Once the endosperm turns to this solid, true white color and is firm, the seed is mature and ready to harvest. When the majority of the plants in the field have about 75% of the seed at this advanced, starchy white stage, it is time to cut the plants near the base of the stems to stack into windrows. This should be done preferably during a warm, dry period. The windrowed stalks will be ready to thresh in 4 to 10 days, depending on the weather. Rotating stalks in the windrows facilitates uniform drying of the seed. Cutting or windrowing the crop in wet weather should be avoided. If a crop is ready to cut and an extended wet weather pattern is forecast, cut the crop and bring it into an airy, dry shelter to cure. Once dried, seed stalks are usually threshed and then cleaned by screening and fanning.

Threshing is best accomplished with a belt thresher to break up the clusters of seed that form when the calyces of several spinach fruits fuse while maturing. Many seed companies then clean away any remnant of this fused material, as well as prominent burrs in prickly-seeded types, with a rotary deburrer in a process that approximates decortication in beets and chard.

Genetic Maintenance

When planting spinach with the intent of performing selection on a population, the initial planting density that I use is anywhere from six to eight plants per foot (30 cm) within the row.

Planting six to eight spinach plants per foot (30 cm) for the initial stand before practicing selection establishes a population that is large enough to perform multiple selection events from the time of emergence to flower initiation and ensures that you will still have an adequate population to maximize your seed yield at the end.

As with all crops, seedling vigor and early robust growth are always important traits to select for in organic production systems. Spinach prospers from increased vigor as the spring crop is often planted into cold, wet soils and must make rapid growth before longer daylength and hot weather induce bolting. Selecting for vigor may be accomplished by eliminating any slower, poorly formed seedlings after all (or most) of the seedlings have emerged. This can be repeated in another 7 to 10 days, eliminating any slowpokes or malformed plants.

There are various stages at which leaf size can be selected depending on the leaf shape desired by the vegetable market you serve. As an example, much of the modern baby-leaf and teenager-leaf spinach is selected for rounded leaves. Alternately, some prefer the triangular or arrowhead-shaped (also called Christmas tree) spinach leaf for shape variation in prepared salad mixes. Leaf textures are characterized as savoyed (extreme leaf curl), semi-savoyed, or flat, and are very important in determining the market class of spinach varieties. Open-pollinated spinach varieties can have considerable plant-to-plant variation in their degree of savoy curl, and it is important to select for uniformity of this trait. Anyone selecting for this trait should also realize that the degree of curl varies with the stage of growth and seasonal temperatures. Likewise, variation in leaf color is common in open-pollinated spinach populations, and gain from selection is possible. Darker leaf color is usually preferred and is correlated to higher levels of nutritionally significant carotenoids. Selection for color can be done across growth stages.

Plant stature is another important trait for spinach growers, especially for harvest of leaves cut for salad mix. The ability of the plant to hold its leaves in an upright position can reduce the amount of soil that may get trapped in the underside of leaves. Upright foliage also lessens the amount of fungal and bacterial pathogens that can be splashed onto leaves due to rain or irrigation.

Incidence of disease should be monitored and identified accurately for proper field management and for selection purposes. Routinely selecting for plants free of disease or with relatively low incidence of disease symptoms can help develop partial or horizontal resistance to a particular pathogen. This requires familiarity with the symptoms of spinach diseases endemic to your region.

Isolation Distances

Spinach, like all of the chenopod crops, has pollen that is known for its ability to travel great distances. It is almost exclusively wind-pollinated, and its pollen is relatively small, light, and easily carried by the wind. The male plants of this dioecious crop will usually produce so much pollen that it is possible to see yellow puffs of it moving across a spinach field with a gust of wind at the peak of pollination. Depending on the direction and force of prevailing winds and the relative humidity, spinach pollen can pollinate other flowering spinach crops within 2 mi (3.2 km) if grown in open terrain. While this may represent a rather small amount of outcrossing to another spinach crop at this distance, it is especially important when the two adjacent crops are distinctly different types of spinach or are foundation seed or stockseed, where purity is very important.

If a substantial physical barrier is present, then the minimum isolation distance needed between two different spinach crops can be lowered to 1 mi (1.6 km). However, it should always be remembered that these isolation distances are not absolute, and there may still be a small amount of crossing even at these higher isolation distances, especially with wind-pollinated chenopods.

Don’t stop at spinach! Prepare for next season by Saving Tomato Seeds: A Complete Guide for Organic Growers

Reprinted with permission from The Organic Seed Grower by John Navazio, published by Chelsea Green Publishing, 2012. Buy this book from our store:The Organic Seed Grower.