Building Digital Education of Heirloom Crops

for the Resilience of African Food Systems in the Climate Crisis

Lecture 5
Conservation of the Properties of a Variety

Selection of plants for propagation

Since late in the Stone Age, farmers and gardeners have worked to improve the quality of their crops. Perhaps they sought larger fruits, more robust plants, or an earlier harvest. This process is called selection, or if it is done using modern methods, plant breeding.

You should never take seeds from sick or weakened plants. These specimens should be removed from the bed before flowering. This way, they cannot participate in fertilization and cannot spread their pollen to strong and healthy plants.

You must consider the purpose of your seed saving activities to select the most suitable plants for seed saving.

Every seed saver may have different objectives tailored to their needs and farming practices. Each goal is valid, but it is essential to identify it clearly and choose the appropriate strategy.

Let's list some potential goals for seed saving:

Preserving the unique characteristics of heirloom varieties involves maintaining the traits of a specific, often older, plant variety.
Developing a customized selection from a mixed population. This involves carefully selecting plants from a diverse group based on specific desired traits to create a strain well-suited to your particular farming environment.
Improving an existing variety involves selecting and propagating plants that exhibit specific desirable traits to enhance those traits in future generations.

How to preserve crop characteristics

To ensure the preservation of heirloom varieties, we prioritize maintaining their distinctive traits. Our goal is to conserve these heirloom plant properties and prevent genetic variation. This involves selecting offspring that closely match the characteristics of the parent plants.

Conservation

If the purpose of your seed saving is the conservation of a specific variety, you have to consider the whole gene pool of the variety. This means you will have to take seeds from a broad spectrum of plants as long as they carry the characteristics of the variety.

To prevent the deterioration of old varieties, seed savers must know how to avoid losing parts of a variety's gene pool, which could lead to inbreeding and loss of vitality. At the same time, they must also know how to avoid contaminating these varieties with pollen from another variety of the same species.

Avoid inbreeding

Knowing how many plants you need to grow for seeds is essential to avoid inbreeding. The more plants you grow, the better the chance that all the original genes are represented. This does not mean you have to take seeds from every plant. If all the plants have participated in pollination, their genes will be represented in the seeds you get.

Some species, like cucumbers, can produce healthy seeds from a few plants over several years, but to ensure that the whole gene pool is intact, you'd better have 10 plants.

Cabbage is at the other end of the scale. In a few years, with too few plants, you will have lost or degraded the variety substantially. They are self-infertile / self-incompatible.

An excellent trick for increasing your available gene pool is to mix seeds from this year's harvest with seeds from last year's harvest and maybe even some from the previous year.

Therefore, never sow the last seed of a special variety unless it is the very last seed.

Avoid contamination

To avoid contamination from other varieties, you must know about your crop's "sexual preferences" and how far apart from other pollen sources the plants will have to grow. However, if your garden is too small for safe distances, you can use some of the isolation tricks mentioned in the fourth lecture to ensure the purity of your varieties.

Preventing unwanted cross-pollination in your garden

Let's take a closer look at how to save seeds from your favorite vegetables and flowers and avoid unwanted cross-pollination that can affect your harvest. In the following articles, we will equip you with the knowledge and techniques to ensure your seeds stay pure. We'll explore the differences between self-pollinating and cross-pollinating varieties, strategies to prevent unwanted crosses, and even delve into the world of vegetative propagation—a method for replicating plants without any pollination involved.

Keeping your self-pollinating plants pure: Strategies for maintaining the genetic integrity of self-pollinators. [1]

These varieties are the easiest to handle. Here, it would be best to have only a few meters between varieties and

5-10 plants of each variety.

Lettuce, peas, and most tomatoes are safe self-pollinators.

Garden beans, fava beans, runner beans, capsicums, and scorzonera self-pollinate, but they are also popular with insects and, therefore, prone to contamination with foreign pollen.

However, caution should be exercised, as sometimes a self-pollinated plant becomes accessible to pollinators, and unwanted pollen can get into the flower.

Photo by Frosamlerne [1]: Self-pollinating plants sometimes can cross-pollinate with other plants

Distance in time or space

As mentioned above, distance is one way to secure your plants from unwanted crossing. Grow only one variety each year, and keep an eye on what your neighbor is growing. Most seeds stay alive for at least a few years so that you can produce two varieties in alternating years.

Covering

Self-pollinators can be kept pure by covering. Pull a bag (paper or thin cloth – not plastic) over the whole plant, or if it is too big to cover, over a branch with a few flower buds. The bag must be tied closely around the stem so no insects can enter. Make sure you do this before the flowers open. To ensure good pollination, you can shake the bag now and then, and when the fruit is developing inside, you can take away the bag and attach a marker to the branch to remind you where to harvest your pure seeds.

Shielding

To reduce the risk of unwanted pollination or contamination, you can grow two varieties - one on either side of the house or a tall hedge.

Photo from https://blog.seedsavers.org/blog/tomatillopollination [2]

Use the link for more information

Challenges and solutions for cross-pollinating varieties: Techniques for isolating and hand-pollinating cross-pollinators. [1]

Cross-pollinating varieties are more complicated to deal with, and distances have to be much more significant. Foraging insects fly far and wide – bees can fly up to 3 km away from their hive. Moreover, plants depend on visiting insects and do whatever they can to attract them. They use scents and colors, and they offer pollen and nectar.

Pumpkin and squash (Cucurbita species), cucumber (Cucumis sativus), and melon (Cucumis melo) are two different genera (plural of genus) and several different species, which do not cross with each other. Still, they cross energetically with other varieties of their species. The Cucurbita family contains several species that do not cross with each other (e.g., C. maxima, C pepo, C. mixta, and C. ficifolia). There are only minor visible differences between them. Therefore, you must know the Latin name to ensure which ones cross and which do not.

In this group, the male and female flowers are separate but so big that hand-pollination is possible.

"Cross-pollination Circle" - indicates the risks of unwanted cross-pollination between plants.

Photo by Egils Murnieks, Cross-polinated flower

Circle of the risks of unwanted cross-pollination between plants [1]

Hand-pollination

To be successful, you have to keep an eye on your plants. The day before the flowers open, you cover them with a bag tied around the stem. You need at least one female flower and 4-5 male flowers – all bagged. The next morning, when the flowers have opened, you pick the male flowers and strip them of the petals. Then, you remove the bag from the female flower and smear the pollen from all the male flowers on the stigma. Finally, you cover the female flower again. When the fruit is developing, you can remove the bag and put a marker on the fruit so you do not forget where the pure seeds are.

Cabbage (Brassica oleracea) is a troublesome insect-pollinated species. Everybody crosses with everybody, and you need at least 20 – preferably 40 – plants to avoid inbreeding. Therefore, to grow seeds of a certain variety of kale, you must watch all kales, cabbages, cauliflowers, and Brussels sprouts in the neighborhood. If they flower at the same time, they will cross. Danish seed savers show their examples of selection, overwintering, and seed saving for Amager White Cabbage. https://www.youtube.com/watch?v=b4XEGL2pdvc

Carrots (Daucus carota) are also insect-pollinated and even more troublesome. They cross not only with other carrot species but also with wild carrot. To avoid inbreeding, you need at least 40 plants, and being biannual, they must survive a winter before producing any seeds. Carrots do not like frost, so you have to protect them either with a thick layer of straw or by transplanting them into a greenhouse for the winter. If you want to play with carrots, it is recommended that you have an experienced seed saver handy.

As mentioned above, shielding is not enough to protect a cross-pollinating variety, but it can reduce the necessary distance between two varieties.

Covering is a possibility with cross-pollinating varieties, but you need to cover the whole row and add some insects to secure pollination.

Vegetative Propagation: Asexual reproduction for maintaining desired plant traits

Many plants are commonly propagated vegetatively, ensuring "children" inherit the parent plant's traits exactly. This is because no cross-pollination or gene exchange occurs when creating a new plant in this manner.

Vegetative reproduction definition:

Asexual reproduction occurs in plants where a new plant grows from a fragment or cutting of the parent plant or specialized reproductive structures, sometimes called vegetative propagules. [3]

Many plants naturally reproduce this way but can also be induced artificially. Horticulturists have developed asexual propagation techniques that use vegetative propagules to replicate plants. Success rates and difficulty of propagation vary greatly. Monocotyledons typically lack a vascular cambium, making them more challenging to propagate. [3]

Photo by Ilze Mezniece: Potatoes grown under hay

Types of Vegetative Propagation:

Tubers, corms, rhizomes (e.g., potatoes, dahlias)
Division of clumps (many herbs)
Cuttings, layering (many shrubs, berry bushes - currants are very easy to propagate)
Bulbs, bulbils
Grafting (fruit trees)
Tissue culture (meristems - excellent for virus-free propagation and mass production of plants from a few cells, but requires a laboratory)

It is important to select healthy plants for vegetative propagation.

How to use the "Guide To Seed Saving" table

The "Guide To Seed Saving" table offers recommendations for avoiding both deterioration and contamination in seeds intended for two types of seed-saving goals – hobby and conservation/sale. [1]

Hobby

Hobby means that you save seeds for personal use only or share them with other amateurs. Furthermore, you are saving seeds of varieties that can be easily replaced.

Conservation/sale

Conservation/sale means that your seed saving is about conserving the whole gene pool of a specific variety, or your seeds are for small-scale marketing.

Distances

In the column for distances, you will sometimes find two numbers. This does not mean you have a free choice. It means that you have to consider the topography of your garden. Do you have buildings or tall hedges to shield different crops from each other? Do you have very few or many insects in your garden and many flowers everywhere to distract the insects?

Pollination notes in the table:

SS: Almost exclusively self-pollinating.

S: Primarily self-pollinating, men will sometimes be pollinated by insects or wind.

S/O: Can self-pollinate but will also frequently be pollinated by other plants of the same species.

O: Outcrosser, but can also self-pollinate for a few generations before being weakened by inbreeding.

OO: Outcrosser, strongly self-infertile/self-incompatible. Few or no viable seeds from a single plant suffer from inbreeding quickly if too few plants are grown.

Example of Table 2

Source "Guide to Seed Saving" from Growing Seed Savers [1]

References:

[1] - Growing Seed Savers. (2019). A Guide To Seed Saving [Brochure].

https://growingseedsavers.org/content/pdfs/GSS_Guide_to_Seed_Saving_ENG_Spreads.pdf

[2] - Seed Savers Exchange (2015). Tomatillo Isolation Requirements - Set The Records Straight , https://blog.seedsavers.org/blog/tomatillopollination

[3] - "vegetative reproduction | horticulture". Encyclopedia Britannica. Retrieved 11 September 2024

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The content of this page was created as part of the project 'Building Digital Education of Indigenous Inherited Crops for the Resilience of African Food Systems in the Climate Crisis Development.' The project was funded by the Ministry of Foreign Affairs in 2024 from the development cooperation budget. This content reflects only the views of the project partners.