No. 3
Preserving varietal purity
Preserving varietal purity guideline also available here as a PDF document (275Kb)
Over the years plant breeders, farmers and amateur gardeners have produced a vast range of different vegetable varieties. These varieties have been bred for differing reasons. Anyone can preserve these and carry on growing them in the future. Preservation requires following two simple rules: remove undesirable individuals and prevent cross-pollination.
Roguing
You will need to save seed from open-pollinated varieties (not F1 hybrids). If you remember the two rules these will come true to type. Open-pollinated varieties are inherently variable, each plant being slightly different from every other plant. It may be useful to think of them as a 'group' rather than clones. The amount of variation may be slight, as is the case of tomatoes, or quite noticeable, as with some brassicas. This variability is one advantage of an open-pollinated variety because it allows variable resistance to diseases, pests and stress. It also allows future breeders to choose various characteristics from which to develop other varieties. However, if the variability is too wide the variety loses its distinctive character.
To prevent widening this variation when saving seed you will need to rogue out those plants that are too far removed from the original average for the variety. This involves some careful observation. Those plants that are markedly different need to be rogued out (removed) as soon as they are spotted so that they do not contribute to the next generation. When sowing seeds always assume that you will lose some to pests, disease and other factors, and consider sowing excess to allow for rouging out. In some of the guidelines we give a recommended minimum of plants to save from for seed, so bear this in mind and sow a few extras.
Pollination
Inbreeding varieties, for example peas and French beans, will not cross with each other or cross rarely, so most of the information referred to below will not be relevant. Bear in mind, though, that if saving seed precautions need to be taken even for those that cross only rarely.
For outbreeders, e.g. brassicas, runner beans, alliums and beetroots, keeping pollen from different varieties away from each other is more important than roguing as it will widen the genetic variability of the stock in the next generation. You may be able to rectify this by rouging out in the future, but it would take patience and several seasons to accomplish.
The easiest way to prevent cross-pollination in outbreeders is to grow just one variety as pollen of many varieties can be transported between plants by wind and insects. If a neighbour is growing a vegetable that could cross-pollinate with something you are growing, you will need to bear this in mind. One way of reducing the chance of cross-pollination occurring is by growing them far enough apart that the pollinating vector, be it wind or insect, cannot carry pollen to your seed saving plants — each species requires a different distance (referred to as the isolation distance).
Isolation distance need not just be in terms of physical distance but also in time. Growing in different years will work but you can isolate by ensuring flowering of more than one variety does not take place at the same time, e.g. by sowing one variety early and the other later. In Britain with our shorter season this isn't always possible.
Another way to isolate is by using barriers that prevent insects reaching the flowers. You can buy barriers such as isolation cages to prevent insects reaching your crop. Barriers need not cover the whole plant, just the flowers that you wish to save seed from. For instance, runner beans can have their flower trusses covered. Isolation cages restrict insects from transferring pollen between plants but they also restrict insects from pollinating the flowers of your precious seed saving plants. There are a few ways around this:
Alternate Day Caging
If you were growing two varieties of broad beans, for example, you could cage one the first day and then cage the other the next and repeat, alternating covers. This would allow available pollinators to access both varieties but on different days.Hand Pollination
This is very time consuming but effective. Be prepared for some flowers to abort if not done correctly. Use a fine brush. More details are given in crop-specific guidelines.Introduce pollinators
This can be done using captured bees, bee hives, mason bees or flies. Flies are the easiest and cheapest to buy and are easier to obtain and manage. Buy maggots from an angling shop (buy 'whites'), put them somewhere warm to turn into castors; then add them to your cage before they turn into flies. They will need to be protected from the rain but free to fly out when they hatch. An old lidded margarine tub with a hole cut in the side works well. The whole process can take between two and three weeks and needs to be done several times. Think ahead and get the maggots before the flowers open so they are ready to work when the flowers are out and need pollinating.
Isolation Barriers
Tent made from
old net curtains and willow
Here are a few notes on barriers from the experiences we have had at Ryton:
Placing a barrier over a plant or its flowers can lead to problems. Physical barriers reduce light and increase heat so be ready for a growth spurt, or flowers aborting because they get too hot. Another problem can be the development of an environment that suits fungal growth (wet and warm), and the proliferation of pests with no predator able to get inside (including you). The last problem is making sure the pollinators are ready to work at the time the flowers are open (including hand-pollinators).
Do not expect every flower to be pollinated. That doesn't happen outside and is the reason plants produce lots of flowers. Flies will pollinate around 40% of brassica flowers whereas bees will do more. Don't be daunted by this. Being forewarned will leave you better prepared.
There are numerous materials and methods of construction you could employ to make tents and bags. We have used various materials, from cheap (99p a metre) to expensive (£7 a metre). For the home grower the most available and cheap material would be close-weave net curtains — used as bags or cages and supported with wire, reinforcing rods for concrete, willow, bamboo or anything else you can think of. Contact the Heritage Seed Library if you would like a copy of an article illustrating a possible technique for constructing an isolation cage.
Another point to consider is rain cover. If, as in the case of leeks, the seed heads are ripening later in the year, rain can inhibit ripening and promote fungal infection. Many people who grow flowers for shows use plastic, conical covers to protect the blooms, a method that could be adapted for vegetables.
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