Regeneration Activities


Hundreds of vegetable varieties are grown at Heritage Farm each year. Varieties are selected for a number of reasons:



When seed stock of a variety is particularly low, the variety may be grown out to build seed quantity. While low quantity storage samples were the previous norm for the Collection, a new system is being implemented to keep larger quantities of each variety on hand. This reduces how often a variety must be grown out, minimizing both the risk of genetic change inherent with each grow-out and the operational cost of maintaining the Collection.


Seed viability decreases in storage. The rate of decrease varies with each variety depending on the initial quality of the seed going into storage. Although our Collection is stored in ideal conditions for seed longevity, each variety must be periodically regenerated to maintain a healthy, viable seed stock.


While evaluative data is taken from all accessions as they are grown in the field for the above reasons, some varieties are grown specifically for the purpose of observing varietal traits. Because the seed isn’t saved, evaluation varieties of the same species may be grown side by side without concern for cross-pollination. In 2010, selected varieties of lettuce, Chinese cabbage, squash, and peppers were grown in this way. See Plant Evaluation for a more detailed description of the process.


Some varieties are grown for the purpose of identifying new varieties suitable for the Seed Savers Exchange Catalog. These trials are similar to evaluation grow-outs.

Growing hundreds of vegetable varieties in close proximity creates potential for cross-pollination. Careful planning and coordination is necessary to maintain the genetic purity of each variety. Some plant types, like peas and lettuce, self-pollinate. Others, like broccoli and melons, are insect-pollinated, and still others, such as beets and corn, are wind-pollinated. The best method for preventing cross-pollination varies from species to species and depends on how pollination occurs. See Suzanne Ashworth’s Seed to Seed for more information on maintaining varietal purity.

Field Isolation Techniques

Isolation Gardens

Varieties with potential to cross-pollinate must be separated by distance, physical barrier, or time of flowering. Isolation by distance is the most common method used at SSE. Twenty-five isolation gardens are spread across Heritage Farm’s 890 acres. Most gardens are made up of rotating plots to maintain soil fertility and limit plant pathogens.


We often need to regenerate more varieties of any one species than we can isolate in a given summer. Isolation tents, or cages, are utilized to prevent insect cross-pollination by providing an insect-proof physical barrier. The tents used at Heritage Farm are constructed of fine mesh material supported by sturdy metal poles. A zippered opening allows access throughout the growing season. Tents are placed over plants before flowering begins, and pollinators are then released inside (see below).


Bees are some of nature’s most gifted pollinators. While pollinators are in abundant supply on our farm, they are intentionally excluded from our isolation tents. Instead, we work with solitary bees, incubating them at Heritage Farm and releasing them into our isolation tents throughout the growing season. The bees, foraging for food, visit the individual plants and transfer pollen from plant to plant. Horticultural technician Gabi Masek prefers two bee types for this purpose:

Mason bees (Osmia spp.) are used early in the season in the greenhouses, and then later as the first pollinators released into field isolation tents. Mason bees are solitary and sweet-natured, building nests of mud in anything hollow, such as dead trees and reeds. When the female lays her eggs, she lays the female-destined eggs at the back of the hole, and the male-destined eggs at the front. The males, which can be identified by a white spot on their forehead, emerge and leave first, ensuring that males are eaten instead of females if there are predators waiting. Mason bees are shipped to SSE as cocooned larvae, and bees generally emerge in less than three days of incubation.

Alfalfa leafcutter bees (Megachile rotundata) are small, very active, and non-aggressive. Because they are most active at warmer temperatures, they are used later in the spring and are especially adept at pollinating crops with small flowers, such as cucumbers and brassicas. The larvae are incubated for 3 weeks before they begin to emerge. Alfalfa leafcutters are solitary, making their nests in holes. They get their name from the female’s method of protecting her eggs. She lays a string of eggs, deposits a pollen ball into each compartment, and wraps the eggs with cut leaves, creating a partition between each egg.


The most time- and labor-intensive method of varietal isolation used at Heritage Farm is hand-pollination. We hand-pollinate when we have more varieties that need to be grown than we have isolation spaces. Most years, only two crops – squash and corn – are hand-pollinated by manually collecting pollen from male flowers and transferring it to female flowers. Timing is extremely important.

To pollinate corn, a pollination bag is placed over tassels (male flowers) each day to harvest shedding pollen. A small bag is also placed over shoots (female flowers), protecting the emerging silks (styles) from foreign pollen. Our method is to collect and combine pollen from the tassel bags and transfer it to the receptive silks of each corn plant, which are then immediately re-covered.

Squash is hand-pollinated by taping rather than bagging the flowers. Male and female flowers are taped shut the day before they first open. In the morning, the tape is removed and staff members move from male flower to male flower collecting viable pollen, which is then transferred to the stigma of receptive female flowers. The female flowers are taped shut again after they have received pollen to exclude any pollen-carrying insects from entering the flower before fertilization has occurred. Soon the ovary, visible on female cucurbit flowers, will start to enlarge—a sure sign that a new squash fruit is developing.

Greenhouses and Root Cellars

Our three greenhouses and two root cellars are integral parts of the regeneration process. The greenhouses fill with seedlings each spring, allowing us to extend Iowa’s growing season for warm-climate plants. Some varieties stay in the greenhouse for the entire season if they show poor germination or high disease susceptibility.

Each winter, our underground root cellars accommodate biennials and vegetatively-propagated plants. Biennials – like kale, leeks, and beets – require a vernalization period before they enter their reproductive phase. For most biennials, six to eight weeks at temperatures below 4 degrees Celsius are required for floral initiation. This means that it takes two years for biennial crops to produce seeds. Since the winters in many northern gardens are too harsh to over-winter plants in the field, we need to take extra steps to store them through the winter. The root cellars are climate-controlled and kept just above 0 degrees Celsius (32 degrees Fahrenheit) to create an ideal overwintering environment for the plants. Biennials are trimmed and dug out of the field in the late fall, stored in pots in the root cellars for the winter, and replanted outside in the spring. Some vegetatively-propagated plants, like onions and potatoes, are cured and overwintered in the root cellars as bulbs rather than as potted plants.

Seed Drying and Processing

A successful growing season is only the beginning of the seed-saving process. After seed is brought in from the field, it must be cleaned and dried for storage, a process that typically takes more than four weeks. “Dry” seeds, like those of beets and radishes, can be separated from other plant material with specialized machinery such as an indoor thresher. “Wet” seeds, like those of watermelons and tomatoes, are separated from the fruit flesh by hand. Special care is taken to ensure varieties are not mixed or damaged during processing.

Most seed goes through two or three steps before it is suitable for storage: separation from other plant material, removal of underdeveloped and non-viable seed, and forced-air blowing to eliminate light chaff. Fans and drying chambers are used throughout the process to ensure that seed moisture continually decreases. For home seed saving tips, see our online guide and Seed Saving Resources.

More Collections Maintenance Activities