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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Four species of domesticated squash are commonly grown in gardens—Cucurbita argyrosperma, Cucurbita maxima, Cucurbita moschata, and Cucurbita pepo. All four species have the same mating system and are essentially cultivated in the same manner when grown for seed. Generally, the four species are not interfertile, or cross-compatible, allowing a seed saver to grow one variety of each species for seed in the same season. And because squash plants produce large unisexual flowers that are easy to handle, hand-pollination is a simple way to produce true-to-type seeds of many varieties, regardless of species.
Each species has slightly different physical characteristics—most notable in the peduncle, or fruit stem, and the seed traits of the species. Seed catalogs and variety descriptions are the best resources to help gardeners determine the species—and the potential for cross-pollination between different squash varieties.
Cucurbita argyrosperma, which includes most of the cushaw squash and the silver-seeded gourds, is the species least commonly grown in gardens. This species often has particularly vigorous vines with large, rounded, shallowly lobed leaves and usually bears fruits that lack furrowing. Cucurbita argyrosperma can be differentiated from other domesticated squash species by its large pale-margined seeds that are covered in a cellophane-like membrane; many varieties, such as ‘Silver Edged’, are grown primarily for their edible seeds. Some have seed coats that are so thin that the seeds appear to be hull-less, or without a seed coat.
Cucurbita argyrosperma has been connected to humans since 3100 BCE. Genetic studies suggest that the species was domesticated in southern Mexico—possibly along with maize, in the central Balsas River valley.
Cucurbita maxima is comprised of a wide variety of edible winter squash types, including teardrop-shaped Hubbard squashes, the flat-topped buttercups, and the aptly named turban and banana squashes. Kuri and kabocha squashes are also Cucurbita maxima. The species varies widely in fruit size, rind texture, shape, and color, including the richly hued Australian blue varieties, such as ‘Triamble’, and the near-white tones of ‘Flat White Boer’. Most of its members are prized for their rich orange edible flesh, although the species also includes less palatable but nevertheless amazing mammoth pumpkins, such as ‘Atlantic Giant’. Most other pumpkins, such as pie pumpkins and jack-o’-lanterns, belong to the species Cucurbita pepo.
Cucurbita maxima generally produces plants with kidney-shaped leaves and distinctive round soft peduncles (fruit stems) that turn corky at maturity. Cucurbita maxima typically has thick seeds with narrow margins.
Cucurbita maxima was most likely domesticated from a wild squash in what is now Bolivia and Argentina. It was being cultivated along the coast of Peru by 2000 BCE. It is widely cultivated in Australia, where it arrived almost four thousand years later, sometime after 1788, along with British settlers.
Many varieties of Cucurbita moschata are recognized by their tan, buff, or pale-orange rind. The species includes neck pumpkins such as the historic variety ‘Canada Crookneck’; the more recently bred butternut cultivars, including ‘Waltham Butternut’, which has been loyally grown by gardeners since its introduction in 1970; and cheese pumpkins, so named because they appear to have been flattened like a wheel of cheese. Some varieties have piebald, or spotted, rinds, and others are green tinted or bicolor. Some of the orange cushaw squash also belong to the species Cucurbita moschata.
The traits common to Cucurbita moschata include the smooth, angular peduncles that flare where they attach to the fruit, and the large lobed leaves that often have white spots—called leaf silvering—at the intersections of the veins. Seeds have a wrinkled seed margin and a round seed scar.
Cucurbita moschata was most likely domesticated from a wild squash in the humid lowlands of northern South America. The earliest evidence of this species in human settlements places it in southern Mexico at around 4900 BCE. Over the centuries, the rich flesh of Cucurbita moschata has provided a flavorful, storable source of carbohydrates, vitamins, and beta-carotene. A quick glance at some cultivar names, such as ‘Musquee de Provence’, ‘Virginia Mammoth’, and ‘Futtsu’, point to its cultivation in France, the United States, and Japan.
Cucurbita pepo is comprised of summer squash, including zucchini, straightneck, crookneck, and patty pan (scallop) types, as well as certain winter types, including acorn, spaghetti, delicata, cocozelle, marrow, and dumpling squash. Jack-o’-lanterns, miniature pumpkins, and some pie pumpkins are also Cucurbita pepo, as are most warted or scalloped decorative gourds. The rind of this species ranges in color from almost white and pale yellow to orange and dark green; bicolor and striped varieties exist as well. Fruit shapes vary widely. Depending on the variety, the flesh of Cucurbita pepo can be consumed in either an immature or mature state.
Cucurbita pepo can usually be distinguished from the other species by its characteristic five-sided peduncles, its leaves and stems covered with spiny hairs, and its deeply lobed leaves. However, some varieties have stems that are relatively spine-free or have little to no lobing. Seeds are smooth and oblong in shape, and the seed margin is usually the same color as the rest of the seed.
Cucurbita pepo is one of the first species that humans are known to have domesticated. Seeds from 10,000-year-old squash have been found in settlement sites in Oaxaca, Mexico. Cucurbita pepo spread throughout the Americas, and owing to the versatility of the species and its suitability to a range of climates, it became one of the most important agricultural crops in the New World.
All four domesticated species of Cucurbita are vining annual crops, although some have more compact vines, particularly summer squash varieties of Cucurbita pepo that are considered bush types, and some varieties of Cucurbita maxima. The process of growing winter squash and gourds to seed maturity is no different from growing these crops for food. Growing summer squash to seed maturity also requires the same cultivation methods used for growing them as a food crop, but the fruits—which are eaten when immature—need to remain on the plants in the garden long after they have reached market maturity in order for the seeds to fully develop.
Seeds germinate best when soil temperature is around 75°F (24°C) and may not germinate if soil temperature is below 60°F (16°C). Plant spacing is essentially the same for seed saving as for growing a food crop.
As with other species in the Cucurbitaceae, squash are monoecious, bearing separate male and female flowers on the same plant. At the onset of flowering, it is likely that only male flowers will be produced; fruit set becomes possible once flowers of both sexes are present in a population at the same time. The flowers—both male and female—are large and yellow. The female flowers are easily identified by an ovary, which resembles a miniature squash and is located just beneath the petals. Squash plants are pollinated by honey bees and native bees, including specialized ground-nesting squash bees (Peponapis spp. and Xenoglossa spp.) that tunnel in the ground at the base of a squash plant and emerge, often before sunrise, to visit flowers just as they open. Squash plants are self-compatible; insects carrying pollen from male flowers can pollinate female flowers on the same plant, or they can facilitate cross-pollination between plants.
After the female flowers are fertilized, the fruits will begin to develop. If fruits are harvested young—as with zucchini—squash plants will continue to produce new flowers and set new fruit. In both winter squash and summer squash crops, leaving fruits to mature will limit fruit production.
Within each species of squash, cross-pollination between varieties must be prevented. As with other insect-pollinated crops, a separation of 800 feet to half a mile (244 to 805 m) between varieties of the same species is recommended. Gardeners who exchange seeds with others or who garden in settings that do not provide many distractions for pollinators may choose to use the upper end of this range as a starting point when determining an isolation distance. Large-scale commercial seed growers isolate different varieties of the same species by 1 to 2 miles (1.6 to 3.2 km).
Because squash plants flower over the course of many months, different varieties of the same species must be isolated from one another, even if a gardener plans to save seeds from only one variety. With the exception of Cucurbita argyrosperma, squash is a very popular, very diverse garden crop, making isolation by distance difficult. Hand-pollination is recommended as an alternative method of controlling pollination.
It is also important to prevent the crossing of Cucurbita pepo varieties in the garden with wild Cucurbita pepo, or Texas gourd, which has naturalized in the United States throughout areas of the South and Southwest. In addition, although rare, fertile crosses between Cucurbita argyrosperma and Cucurbita moschata have been reported. If isolation between these two species is desired, it is recommended that gardeners separate them by 400 feet (122 m).
Although typically cross-pollinated, squash varieties do not need to be grown in particularly large populations for adequate maintenance—a population size of 5 to 10 plants is recommended to conserve the genetic diversity of a variety. And while viable seeds can be harvested from growing just one or two plants, it is best to save seeds from a larger population, especially if seeds will be shared with others. Because most squash are vigorous vining crops, population size will likely depend on garden space availability. A population of 25 plants or more should be maintained if the long-term preservation of a variety or stewardship of a rare variety is the goal.
While squash plants are young, seed savers should check leaf color, leaf shape, and growth habit (bush versus vine), removing any obviously off-type plants from the population.
After the first fruits have been set, summer squash should be checked for fruit shape and fruit color, and any plants bearing off-type fruits should be removed, so they cannot contribute pollen to the population of plants from which seeds will be collected. Seeds should not be collected from any fruits that were set before the planting was rogued. The plants will continue to produce new flowers and set new fruits from which seeds may be collected.
The most important traits of winter squash—fruit shape and size, rind color and pattern, flesh color, and taste—are not easily evaluated before maturity. Selection is most often approached through simply choosing true-to-type fruits, or maternal selection. Although the collected fruits are true to type, it is still possible—if there were any off-types in the population—that off-type traits may have been passed to the saved seeds. This will not be evident until the seeds are grown out in a subsequent season.
If flowers are being hand-pollinated, removal of off-types is not necessary as long as the off-type plants are easily identifiable and not used to pollinate any of the plants from which seeds will be collected.
At seed maturity, summer squash will be much larger than their market-mature size, and they typically undergo a color change. Fruits are ready to harvest when the rind is too hard to dent with a fingernail and the stem is dry. Winter squash are typically mature when fruits are normally harvested for eating: after they change color and fruit stems are dry. All types of squash benefit from a period of post-harvest ripening during which the seeds continue to mature. Fruits are typically held for at least 20 days beyond fruit maturity before their seeds are extracted. Although they can further ripen on the vine, squash are susceptible to diseases and sunscald, and it is generally recommended that fruits be harvested and moved to a shady location or indoors for post-harvest ripening. Commercial seed growers process winter squash after 20 days, but seeds can be extracted from squash later in the winter instead, when the fruits are used for cooking.
To remove squash seeds from the fruits, simply split the squash in half by shallowly cutting through the rind from top to bottom on both sides and separating the two halves. Cutting through the center of the fruit can damage seeds. Next, scoop out the seeds, massaging them free from the pulp as much as possible. Transfer them to a wide-mesh strainer—or any other container with openings large enough for pulp and strings to pass through—for rinsing. Rubbing the seeds under a strong stream of water will help dislodge the seeds from the pulp. When working with varieties whose seeds are hard to separate from the pulp, soaking the seeds for a few hours can facilitate cleaning. Large screens made from quarter-inch hardware cloth work well for cleaning and rinsing big batches of seeds. Immediately after cleaning, rinsed seeds should be spread out to dry in a thin layer on screens, if possible.
Decanting typically facilitates the separation of viable seeds from lightweight and underdeveloped seeds, but this method is only effective for some types of squash because even the viable seeds of many varieties will float, rather than sink. Alternatively, winnowing can help separate lightweight and underdeveloped seeds once the seeds are dry.
When fruits are processed individually—such as when winter squash seeds are extracted as a meal is prepared—the seeds should be cleaned and then mixed in and stored with the seeds of other fruits from the same planting, in order to maintain the genetic diversity of the seed crop.
When stored under cool, dry conditions, squash seeds can be expected to remain viable for six years.
LIFE CYCLE: Annual
SUGGESTED SPACING: Same as when grown for eating
OTHER REQUIREMENTS: None
FLOWER TYPE: Squash are monoecious (separate male and female flowers on the same plant) and self-compatible.
MATING SYSTEM: Mixture of self- and cross-pollination, depending on whether insects transfer pollen among flowers on the same plant or between plants
ADDITIONAL CROSS-POLLINATION CONCERNS: Wild Cucurbita pepo, commonly called Texas gourd, is naturalized throughout areas of the South and Southwest and can cross-pollinate with garden cultivars of Cucurbita pepo.
FRUIT TYPE: Fleshy fruit (pepo)
SEED MATURITY: Seed maturity occurs at market maturity for all winter squashes, pumpkins, and gourds. Seed maturity occurs after market maturity for all summer squashes and zucchini. Most fruits undergo a color change at seed maturity.
PROCESSING METHOD: Rinsing
EXPECTED SEED LIFE: 6 years
ISOLATION DISTANCE: 800 feet–1⁄2 mile (244–805 m)
For Viable Seeds: 1 plant
For Variety Maintenance: 5–10 plants
For Genetic Preservation: 25 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.
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