View Seed saving of Potatoes
A seed potato is not a seed at all but a tuber. Potato plants do of course produce real seed in tomato like fruit that develop under their flowers. Growing seed from these fruit is the way to produce new varieties. The plants they produce will not resemble the parent plant. Seed potatoes are the tubers chosen for sowing the following year. The best tubers for seed are approximately egg sized. Before seed potatoes are planted they should stand, rose end up in chitting trays, which are light wooden trays which stack. The trays should stand in a light, frost-free shed, even a spare room, from about January onwards, growing stocky, dark green shoots from the eyes. If the seeds stay in the sack the shoots grow thin and white and long. If you look at a potato you will see that all the eyes are at one end known as the rose-end. If the potato is large and you want to make two seeds from it, you can cut your spud down from the rose end to the bottom, lengthways as it were, just before planting, ideally with the eyes upwards. Some gardeners dip the cut surface in slaked lime and put them back in the trays to sprout. If you want to grow large potatoes for baking or because you dislike peeling little ones, disbud your seed by rubbing out all the little shoots, leaving only the two best on each tuber. When you are raising your own seed you can pick out those with the most shoots for planting in your seed row because you want plenty of the smaller sizes,
You choose the best potatoes for seed because the ideal size and shape has the best chance of doing well. You discard any with the pustules of potato scab because you do not want to spread this non-serious but unsightly condition further in your garden. You grow your potatoes with good compost because you want your crop to have the finest flavour and your next year's seed to have the resistance to disease that is reputed to come from good feeding.
New gardeners should try a range of potatoes to see how they like them and how well they fit the local soil and then try seed saving, especially if they have managed to secure a stock of a kind that survives only by the grace of enthusiasts. Plant your potatoes in the ordinary way, putting plenty of compost in the trenches, because if your soil is short of magnesium from too much potash, you will get the leaves going yellow, with veins staying green and this can be mistaken for a virus. Ideally, reserve an end row for seed so you can watch them for symptoms, You are looking for leaf curl virus and mosaic, which resembles the markings on the leaves of the variegated laurel. It is unlikely that you will see any, but if you do, dig the plant up at once and use the tubers for eating. They are quite safe to eat.
Then by the middle of August for maincrop and the last week in July for earlies, cut down the haulm of your selected row with shears and compost the foliage. This means less than a full crop of course but you do not want large potatoes, you wand standard egg-size tubers that will keep. Spread your crop on the surface and leave it in the sun for 2-3 weeks. If the tubers go green with the light, then so much the better and, if you have time, turn your tubers so they have plenty of sun to ripen the skins all over. Store them in shallow boxes in the dark with not more than a three-inch layer in each so they have free air circulation, until they are ready to set out in the chitting trays. Potatoes, like apples, keep best at 34-35 F and it is more important to keep them cool than to worry too much about frost.
You can grow potatoes from the true seed which is held in tomato like berries that develop at the base of the flowers. The seedlings will produce a tiny potato or two the first year. They will produce larger tubers in subsequent years. Each seedling is a new variety and you can select for traits that you like.