Pollination: All species of the Solanaceae family are self pollinating. Bees are not attracted to these flowers but many other insects are, and a lot of crossing can occur.
Isolation Distances: Tomatoes are usually self pollinating and are not usually visited by bees, so isolation distances do not usually matter. I have seen very little crossing (though others tell me they have). However, it is possible, and has been known to happen. If you are very concerned then isolate varieties by 10m with something growing in between them.
Peppers cross far more readily within species, and I would suggest you never plant hot peppers anywhere near sweet peppers if you are serious about saving seed. If I had a situation where blocks of 20 peppers of one variety could be 10m from another variety and there was a high block of corn (or similar) in between, then I think that would work between sweet peppers, but hot peppers need to be in another area, probably 30-50m away from any other peppers. Eggplants cross with the melongena species and need to be in blocks for good pollination rather than long rows, but keep varieties 20-30m apart.
Minimum Numbers: 1 plant is enough, although more is better, to ensure you are saving seed from most true to type fruit, and that you are maintaining a wide genetic base. With eggplants and peppers it is best to have minimum of 16 plants. 1 potato plant is enough to save seed (because we save the tubers).
When To Plant: Eggplant and pepper seed needs to be planted in late August and early September to crop well over Summer and achieve best seed results. Tomato seed should be planted in September or October for best seed, potatoes can be planted any time there is 4-5 months frost free available after planting.
Rogueing/Selection: Peppers, tomatoes and eggplants: if you weighed the fruit that comes from each vine you could select for weight of crop. Decide exactly what you want to select for, as this will affect your seed and fruit/plant characteristics over time. Do you choose early fruiting, or disease resistance over the whole season etc.? Always be on the lookout for your best plants and mark them carefully as your mother seed plants.
Potatoes need annual careful rogueing to maintain strong true to type seed lines. All plants that appear for any reason at all to be not quite normal must be removed as early in the season as possible. Disease is spread in potatoes by insects such as aphids that are sap feeders, so ensuring rogueing is done early in the season makes a huge difference to the potential spread of disease.
The first rogue is done during or before the seed is planted. When you take your seed from the onion sacks where it was kept hanging all Winter under the eaves your shed etc., discard any seed that is rotten or feels very soft, or has potato tuber moth faeces hanging from the eyes. Place seed in trays to grow shoots 4 weeks before planting, which now that we have the psyllid in New Zealand must be before mid September.
The second rogue can be done at planting time. Any seed potato that has weak shoots, or shoots of a different colour, can be discarded. Once the potato tops emerge, any leaves that show crinkling, yellow streaks, stunted growth etc. must be removed. Once flowering begins you can rogue out any that have different coloured flowers or are weak plants.
Harvest time is your next opportunity to maintain strong true and productive seed. Harvest your plants individually and keep the crop that grew under each plant in its own pile. Once you have finished digging you can go along your lines and select the plants that have the heaviest best looking tubers; these are your seed tubers. The largest tubers will grow your best potatoes next year but you may choose to keep the egg sized potatoes as your seed because of the suitability of that size for planting.
Harvest: Watch for birds eating the juicy seed part out of tomato plants before you get to them; they may need to be picked a little early or netted. Peppers must be fully ripe and coloured up before harvesting for seed. Eggplant fruits must be left to fully mature until they go hard with yellowish skin before harvesting for seed collection.
Processing: With tomatoes I cut the top (the end that was attached to the plant) off the fruit and squeeze out the seeds with the juice that surrounds them. Leave this juice to go mouldy on top for 3-5 days, then add water and stir vigorously. The good seeds will sink and the poor seeds and the rubbish will float. Pour the rubbish off the top, add water and repeat until you have clean seed in the bottom of the container. I then tip the seeds into a sieve and bang them into the drier or a piece of absorbent paper. Make sure that the temperature of the dehydrator is able to be controlled and is lower than 35°C if you are using one. Once the seeds are dry, you can rub them between your fingers to separate the seed again. You can just squeeze the seed straight from the tomato onto toilet paper or newspaper and dry the seeds in the sun, that will work fine, but they are hard to separate and use this way.
Pepper seeds are easier. Just scoop them out of the peppers (be careful if they are hot peppers as the heat is concentrated at the top of the seed stalks, wear gloves if necessary). Place the seeds in a container in the sun, greenhouse, drier etc., and when crunchy, separate seeds and extraneous material, and store.
To save the seed of eggplants, leave the fruit on the plant until they are mature (they change colour) and hard before picking. Then chop them up and whiz in a blunt old blender with water. If you then tip this mixture into a jar and add water, the good seeds will sink and the weak seeds and rubbish will float. It can then be tipped off and the good seeds dried.
Potato seed is easy to save, as most of the work will already be done by the harvest stage. Simply choose your best large or egg size potatoes and hang up in an onion bag in the light to stop them sprouting, or in a sack in the dark and cool to stop them sprouting. Those that begin actively sprouting first in Spring are the early potatoes, the ones that want to be planted first and that grow better in the cooler season.
Seed Life Expectancy: Peppers will retain 50% viability after 3 years if kept dry, dark and cool, tomatoes will remain viable for 4-10 years.