The art of pollination and isolation
The two key principles for conserving plant varieties as seed are to prevent cross-pollination and to save seed from only the best examples.
Preparations for our summer growing started way back last winter when our horticultural lead Marcin Salnikow mapped out a planting plan to include the varieties we needed to grow. This involved careful consideration of crop rotation, flower types and population sizes.
In practice, only one variety of a cross-pollinating vegetable will occupy an isolated space, shared with around a dozen other crops per polytunnel. For example, our broad bean varieties are each housed separately. Bees love the flowers and will happily travel half a mile to visit, bringing significant risk of moving pollen between different varieties including any field beans growing on neighbouring farms.
Isolation is also crucial for purple seeded French bean ‘Violetta’, so it doesn’t cross-pollinate with the stocky, green seeded ‘Somerset’ and create a generation of hybridised seed that is not true to either.
By isolation, we mean physical barriers between varieties of the same species. By the time summer arrives, our HSL site will resemble an odd campsite of zipped-up polytunnels and mesh-covered fruit cages.
Methods of pollination 🔗
Isolating flowering plants in this way sets up a bit of a challenge. Some plants, such as peas, can happily get on with the business of producing seed without much attention. However, where pollen needs help to move, we now need to move it, and there are several methods involved.
The first way is for people to become pollinators. In the case of runner and broad beans, these crops require the greatest human resources per seed produced than anything else we do. We use small paintbrushes to ‘trip’ the flowers, exposing the stigma to pollen, which is moved from plant to plant, thousands of times.
Similarly, plants such as the ‘Slovenian’ dudi also need hand-pollinating. In nature, dudi are moth pollinated at night, so the team will get to the flowers as early in the day as possible. Each paint brush stays with the variety for the year, and brushes are firmly planted in the ground next to the plants to avoid any mix-ups.
Hand-pollinating is not practical for everything. Cabbage, onions, leeks and carrots, for example, have tiny but often self-incompatible flowers. Pollen must move to another flower to fertilise and set seed. This is when helpful companions such as blow flies are dispensed.
We make multiple, well-timed visits to the local angling shop and get through up to 20 pints of maggots per year. These are carefully placed in sawdust in sealed crates to pupate, and released as flies into cages and polytunnels to pollinate. It’s an interesting experience working alongside thousands of flies in a polytunnel (I recommend keeping your mouth shut!)
Tuning our blooms 🔗
We’ve also been using tuning forks to help pollinate tomato plants this summer. Tuning forks mimic the natural vibrations of bees – or ‘buzz pollination’ – which helps to release pollen from the flowerhead. The tuning fork is struck on a hard surface, and the vibrating prongs are held near the anthers of the flower. The shaking helps loosen the pollen, and the process is repeated with all the other flowers. Last year, a small trial proved promising with tuned blooms producing both larger fruit and more seeds per fruit.
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Assessing genetic wobbles 🔗
Saving seed from the best examples of plants means managing the visible differences in plants that are unrelated to cross-pollination risks. The strength of open-pollinated plants is that each is slightly genetically different from another even within the same variety. This is essential to adaptation and a mechanism that has underpinned selection and breeding for plants that grow well in certain conditions.
The team must assess the visible genetic wobbles that appear throughout the year, carefully ‘roguing out’ atypical examples that stray too far from what we want. This can include flower colour, habit or seed coat, to ensure plants represent the absolute best examples of ‘Uncle Bert’s Purple’ kale or ‘Mr Ying’s’ stem lettuce, for example. This is especially important at the flowering stage to ensure pollen moves only within the best plant population before seed is produced.
Other performance is carefully observed and recorded, along with photos at various growing stages. A huge advantage of growing several varieties side-by-side each year is seeing how each responds differently to the same growing season. Plants that show better resilience to a cold start, or a sudden hot spell, are a useful reminder of the importance of conserving this cultivated biodiversity.
Monoecious plants have separate male and female flowers on the same plant, e.g. cucurbits, and this year the spectacular dudi (bottle gourd) ‘Slovenian’ is jostling for position in a polytunnel.
Dioecious plants such as ‘Bloomsdale Longstanding’ spinach have separate male (pollen shedding) and female (seed producing) plants. Either way, plants need pollen to move from male to female flowers, typically by insect or wind, and must be isolated.
Some vegetables are much simpler to accommodate. Inbreeders such as peas, French beans and tomatoes typically have a lower risk of cross-pollination, and varieties happily share the same space without concern. While both inbreeders, and partial-outbreeders like broad beans, have ‘perfect’ flowers, containing all the components in a single flower to produce seed, consideration of other flower types is also now visible.