The Seed Garden (Seed Savers Exchange, 2015) by Micaela Colley & Jared Zystro and edited by Lee Buttala & Shanyn Siegel brings together decades of research and hands-on experience to teach both novice gardeners and seasoned horticulturists how to save the seeds of their favorite vegetable varieties.
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Brassica oleracea has the largest diversity of crop types among the vegetable species; it includes heading and sprouting broccoli, Brussels sprouts, cabbage, cauliflower, collards, European kale, and kohlrabi, as well as less well-known forms such as Chinese broccoli. Although most of these crops are biennials, a few are annuals and can produce seeds in one growing season. The process of bringing these plants to flower varies between the different crop types, but the
methods used for harvesting and cleaning the seeds are essentially the same across the species. All forms of Brassica oleracea are cross-compatible, and isolation needs to be managed thoughtfully, but because most are biennials that will not flower until their second season, a gardener can grow multiple varieties for eating while simultaneously growing one variety for seed saving.
Over the millennia, farmers have created the incredible diversity within Brassica oleracea by selectively developing crops with edible stems, leaves, or flower buds.
Collard greens and European kale were probably the first cultivated forms of Brassica oleracea, and both were developed for their edible leaves. The leaves of collard greens appear most often in varying shades of green with white stems, but some varieties have purple leaves or stems. Kale varieties range in color from green—including dark greens that appear almost black—to reddish purple. They may have savoyed leaves, such as ‘Lacinato’, or frilly leaves, such as ‘Vates’ and other curly kales. Siberian kale (Russian kale) belongs to the species Brassica napus and is not cross-compatible with any of the Brassica oleracea crops.
Cabbages have a terminal bud surrounded by tightly cupped leaves that form a head. They vary in color from white and green to red and purple, and some types are known for their savoyed leaves. Seed catalogs often divide the listings of cabbage varieties into summer, fall, and winter cabbages based on a variety’s frost tolerance and when the crop is harvested. Chinese cabbage, which includes Napa cabbage, is a member of the species Brassica rapa and is not cross-compatible with any Brassica oleracea crops.
Brussels sprouts, which were most likely derived from cabbage, produce sprouts of tightly cupped leaves within the leaf axils of the central stem. This crop is thought to have originated in its namesake town in Belgium in the sixteenth century.
Over time, broccoli developed as farmers repeatedly selected for edible flower buds, and this crop is now divided into heading and sprouting types. Cultivars of heading broccoli—the most common type grown in the United States—produce a single large head for harvest, whereas sprouting broccoli types produce multiple small heads over a period of time. Heading broccoli is grown as an annual for both food and seed production; sprouting broccoli is a true biennial.
Cauliflower bears heads containing edible parts, commonly called curds, which are either undeveloped shoots (summer cauliflower) or undeveloped flowers (winter cauliflower). It is thought that cauliflower was originally derived from broccoli. The edible part of cauliflower can range in color from orange or purple to chartreuse or white, although the white forms are the result of blanching, which can occur on its own (in some cultivars) or by manually covering the developing curd with the plant’s leaves.
Chinese broccoli, also known as kailaan or Chinese kale, is an Asian vegetable that was selected over time to produce thick, glossy leaves, succulent stems, and small broccoli-like heads. The leaves, stems, and flower buds are all edible.
Kohlrabi, sometimes called German turnip or turnip cabbage, was developed by selection for an enlarged edible stem. The edible part of the plant, which can be either eaten raw or cooked, is either green or purple.
Brassica oleracea was most likely first domesticated somewhere in the Mediterranean region. By 300 BCE, Greeks were eating early versions of many of the cole crops grown today, including broccoli, cabbage, cauliflower, collards, and kale. Cabbage and kale spread with the Romans throughout northern Europe from 50 to 450 CE. However, scholars are still debating whether or not certain domesticated Brassica oleracea crops were grown in the British Isles before the Roman conquests. Northern Europeans developed Brussels sprouts and kohlrabi more recently, around 1500 CE.
Growing Annual Cole Crops for Seed
Most of the cole crops are biennials; however, a few can produce seeds in just one season—primarily heading broccoli, some cauliflowers, and Chinese broccoli. As a general rule, flowering in most annual types of Brassica oleracea is promoted by vernalization, but the cold requirement is minimal. These plants may require exposure to temperatures below 50°F (10°C) for one to four weeks before they will flower. In the cases of heading broccoli and Chinese broccoli, the production of the edible flower buds indicates successful vernalization. Because these crops are normally grown almost to the point of flowering, increasing spacing just slightly at planting provides additional space for the large flowering and fruiting branches that will develop when the crop is grown for seed.
Growing Biennial Cole Crops for Seed
Biennial cole crops—Brussels sprouts, cabbage, cauliflower, collards, European kale, kohlrabi, and sprouting broccoli—typically require 10 to 12 weeks of exposure to temperatures below 50°F (10°C) before they will flower and set seeds. If winter temperatures are cold enough to satisfy this requirement, without being so cold as to kill the crop, plants can simply overwinter in the garden. Straw mulch, leaves, or row cover can provide a bit of extra protection from the cold in borderline climates and also help mitigate widely fluctuating fall, winter, and spring temperatures. Appropriate sizes for overwintering differ based on crop type and climate, but plants need to have grown to a minimum size—typically eight true leaves or stems half an inch (13 cm) in diameter—before they are competent to flower.
Brassica oleracea plants grow quite large during their reproductive stage. After vernalization, they will require more space beyond that which is normally provided when these crops are grown for food. Typical spacing for second-year biennials is 18 to 24 inches (46 to 61 cm) between plants in rows that are at least 36 inches (91 cm) apart. However, when plants are young during vernalization, final spacing may be decreased because the plants will likely begin to flower when they are smaller in stature.
Cold-hardiness varies among the cole crops and even from variety to variety within a crop type. In addition, hardiness depends on other factors, such as the duration of cold temperatures and the age of the plant. In general most Brassica oleracea crops survive temperatures as low as 20°F (-7°C), and many varieties can survive temperatures down to 10°F (-12°C) or lower. Small plants are generally more cold tolerant than full-sized plants.
When plants cannot be successfully overwintered in the garden, they can be vernalized in storage. With the possible exception of kohlrabi, all of these crops are prepared for winter storage in the same fashion as other fibrous-rooted biennials. Leaves should be trimmed off, with the growing point left intact, and trimmed plants should be replanted into containers filled with slightly moist potting mix or sand for storage. The optimal storage conditions for Brassica oleracea crops range from 34 to 39°F (1 to 4°C) and 80 to 95 percent relative humidity. A more practical goal—and one used by seed-saving farmers and gardeners for ages—is to meet basic vernalization requirements and prevent stored plants and roots from freezing. A traditional root cellar is ideal for this, but garages, sheds, and other unheated structures can be equally useful in some climates.
Brussels sprouts are one of the most cold-hardy crops in this species, often surviving the winter in areas with temperatures that briefly dip to 10°F (-12°C). In mild climates, Brussels sprouts are typically grown to near market maturity in their first season in order to evaluate the quality of the sprouts, but in other areas plants are vernalized any time after their juvenile stage, since smaller plants are more cold hardy. If plants are dug for storage, leaves should be trimmed off, but any developing sprouts should be left intact. If sprouts are of a harvestable size, however, a portion may be harvested for eating. While this may affect seed yield, it does not prevent seed production or reduce seed quality.
Cabbages vary greatly in their days to maturity, winterhardiness, and length of time they can be stored. Winter cabbages are generally hardy to 10°F (-12°C), fall cabbages are typically hardy to 20°F (-7°C), and some varieties of summer cabbages are hardy only to 30°F (-1°C). The hardiest winter cabbages can overwinter outdoors with fully developed heads in some climates. Another approach is to time the sowing of seeds so that plants are just starting to form loose heads at the end of the growing season; these less mature heads can withstand colder temperatures. It is best to overwinter cabbages in storage as full, tight heads, because they store better than loose heads and resist rotting. Only the loose leaves that are not part of the cabbage head should be trimmed before storage. Cabbage plants have dense heads that can prevent the flower stalks from emerging. To facilitate flowering, a gardener can cut a 1-inch deep “X” in the top of the heads in the early spring. This process is useful even if the plants have only formed loose heads before vernalization.
Cauliflower can generally withstand temperatures as low as 20°F (-7°C), but some varieties can survive exposure to more intense cold. Most cauliflower varieties are biennial and require vernalization, but there are some varieties, developed in the tropics, that will produce seeds in one season. Cauliflower is difficult to overwinter in storage because it succumbs to rot if stored for prolonged periods. Cauliflower is also more difficult to overwinter in the field than cabbage and Brussels sprouts because it is less cold hardy and is prone to rot. Cold tolerance in the field is improved by tying the outer leaves together over the head to protect the curds.
Collards and European kale are both grown for seed in the same way. Varieties differ in cold-hardiness, but most withstand temperatures of 15°F (-9°C), and some can tolerate brief exposure to even colder temperatures. Although collards and kale can be grown throughout the growing season, plants for seed saving are typically planted in the summer, as a fall crop, so that they will be younger, approximately 8 to 12 inches (20 to 30 cm) tall, and better able to withstand cold temperatures than mature plants. Smaller plants are also more easily dug up and transplanted for vernalization in storage in areas where winter temperatures prohibit overwintering outdoors.
Kohlrabi is hardy to 15°F (-9°C). Seed savers commonly grow kohlrabi almost to full size before vernalizing it in storage. Although kohlrabi has a fibrous root system, its swollen stems can be repotted, or they can be stored similarly to root crops such as beets, buried in containers separated by layers of sand or wood shavings. When the goal is to vernalize kohlrabi plants in the garden, sowing should be timed so the crop will be slightly smaller than market maturity at winter’s onset.
Sprouting broccoli is a true biennial; it requires at least six to eight weeks of vernalization. Some varieties will only withstand low temperatures of around 20°F (-7°C), while others may survive exposure to temperatures that drop as low as 14°F (-10°C). Because the flowering shoots are the edible portion of the plant, sprouting broccoli needs to be vernalized whether it is being grown for food or for seeds. Plants are transplanted into the field in mid to late summer so that they can grow to approximately 12 to 24 inches (30 to 61 cm) before vernalization. Plants at this stage can be overwintered in the garden in appropriate climates or can be dug up and stored. Plants that will be vernalized in storage must have their leaves trimmed back; gardeners can follow practices similar to those used for kale and collards. After vernalization, when temperatures are once again favorable for growth, plants will produce many shoots with small broccoli-like heads that are normally harvested for eating. As with heading broccoli, these heads are the immature buds that will eventually produce flowers and seeds. Although seed yields may be reduced, it is feasible to harvest some shoots for eating and leave others to produce seeds.
Flowering, Pollination, and Seed Set
Similar to many crops in the Brassicaceae, the cole crops bear four-petaled, cruciform flowers arranged in indeterminate racemes. Although individual flowers are perfect, containing both male and female parts, most Brassica oleracea crops are self-incompatible and obligate outcrossers. Pollination is performed by a variety of insects, including flies, wasps, and bees. Flowers are most commonly yellow, but some types, including Chinese broccoli, bear white flowers. The flowering stalks of most Brassica oleracea crops usually grow 4 to 5 feet (1.2 to 1.5 m) tall; staking is recommended to keep the fruiting branches off the ground.
Brassica oleracea is insect-pollinated, so the recommended isolation distance between varieties—including those of different crop types—is 800 feet to one-half mile (244 to 805 m). Gardeners who exchange seeds with others or who garden in settings that do not provide many landscape barriers may choose to use the upper end of this range as a starting point when determining an isolation distance. Commercial seed growers separate different varieties by 1 to 2 miles (1.6 to 3.2 km). Managing isolation may be difficult given the diversity and popularity of Brassica oleracea crops, but most cole crops are true biennials that are seldom seen flowering in gardens. Only plants that are flowering need to be isolated from each other.
Options for controlled pollination are limited because the cole crops are outcrossing; without introduced pollinators, caging plants prevents their pollination altogether. Additionally, their indeterminate racemes bloom continuously, possibly for a month or more, making isolation by flowering time impractical.
Brassica oleracea is typically self-incompatible, so growing a large population will help ensure a successful seed set. Viable seeds can likely be collected from 5 plants, though 20 to 50 plants is the recommended population size for seed saving. The upper end of this range helps capture a variety’s genetic diversity and prevent inbreeding depression, and it is most appropriate if a gardener’s intent is to save seeds for multiple generations or to share seeds with others. For those saving seeds for genetic preservation, a population size of 80 plants or more is recommended. Some loss can be expected from overwintering, so gardeners may want to grow more plants than the desired population size.
Each of the cole crops can be rogued or selected based on various crop-specific traits. Important broccoli traits include shape, color, and size of head, and uniformity of maturity. Brussels sprouts are selected based on sprout size and color. Selection in cabbages and cauliflower typically focuses on head size and
color. Important traits of collards and European kale include foliage and leaf-stem color, as well as other leaf qualities such as size and texture. Kohlrabi is typically selected based on size and color of its edible stem.
Seed Maturity and Harvest
At maturity, siliques dry and turn light brown, and seeds turn brown. As with many of the Brassica crops, the window of time for an optimal harvest may be short because seeds can be lost to shattering or bird predation.
Depending on the size of the plants at flowering and on the quantity of seeds desired, seeds can be gathered by cutting branches or by harvesting whole plants. Because of this species’ tendency to shatter, the harvested material should be placed on drop cloths or in containers to prevent seed loss. If weather conditions allow, harvested plants can be spread out on breathable fabric and left in the garden to dry. Otherwise, they should be moved to a protected area for further drying. Depending on the moisture level of the plants at harvest, plants should continue drying for one to five days or until they are too hard to be dented with a fingernail.
Cleaning and Storage
Brassica oleracea can be threshed using one of several methods. Small lots and cut branches can be threshed by hand by rubbing the siliques between one’s hands or against any surface that will cause them to break open. If the pods are dry, they will release their seeds easily when threshed. Larger lots and whole plants can be placed in large tubs or on tarps and treaded upon, or they can be beaten against the inside of a garbage can. Stalks can be discarded once seeds have been dislodged and the remaining material should be screened and winnowed. If a seed lot requires further cleaning, a seed saver can drape a piece of felt over a wide board or other hard surface, raise the surface to an incline (20 to 35 degrees), and pour the seed lot down the ramp. The chaff will stick to the felt, and the round seeds will roll to the bottom of the board to be collected and stored.
When stored under cool, dry conditions, seeds of Brassica oleracea can be expected to remain viable for six years.
LIFE CYCLE: Annual or biennial
SUGGESTED SPACING: When growing Brassica oleracea crops for seed, increase spacing to 18–24 inches (46–61 cm) apart in rows that are at least 36 inches (91 cm) apart. Staking is recommended.
OTHER REQUIREMENTS: Biennial types require a vernalization period of temperatures below 50°F (10°C) for 10–12 weeks in order to flower and set seeds. Annual types may have a short vernalization requirement, but this is typically met in the early part of the growing season when temperatures are cool.
FLOWER TYPE: Perfect, self-incompatible flowers are produced in racemes.
MATING SYSTEM: Cross-pollination is the primary mating system due to self-incompatibility.
ADDITIONAL CROSS-POLLINATION CONCERNS: None
FRUIT TYPE: Dry, dehiscent fruit (silique)
SEED MATURITY: Seed maturity occurs after market maturity (in the second season for biennial crops), when siliques dry to a light brown color. Shattering is common.
SCREEN SIZE: 3⁄64–8⁄64 inch (1–3 mm)
EXPECTED SEED LIFE: 6 years
ISOLATION DISTANCE: 800 feet–1⁄2 mile (244–805 m)
For Viable Seeds: 5 plants
For Variety Maintenance: 20–50 plants
For Genetic Preservation: 80 plants
For more on seed saving, see our Seed Saving Guide.
Reprinted with permission from The Seed Garden, by Micaela Colley & Jared Zystro, edited by Lee Buttala & Shanyn Siegel and published by Seed Savers Exchange, 2015. Buy this book from our store: The Seed Garden.