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
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
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
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
Greenhouses and Root Cellars
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
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.