"Nicola"
I thought it might be a good idea to write a few words about how I grow my potatoes, so here we go...
Note: I'm assuming that most readers will know the basics of chitting, planting and earthing-up potatoes, but if not you'll find plenty of advice about those subjects elsewhere on my blog. Either use the Search facility in the sidebar, or follow the hyperlinks above.
1. Size matters! I have learned through experience that potatoes do better in big containers. I used to use florists' buckets and the containers in which pelleted chicken manure is sold. Yes, it is possible to grow potatoes in such containers, but bigger ones are better. Last year I bought some big plastic 35-litre pots, and I think these are ideal. Small containers dry out too rapidly, but larger ones do better.
35-litre containers, big enough for 2 seed tubers each
2. Use the correct growing-medium. Again, experience has shown me that plain soil, or plain multi-purpose compost is not going to be sufficient. I think it is important to have a mix that gives the potatoes enough nutrients (you could use proprietary "potato food", though I don't), but the texture of the medium is more important. Potatoes grown in very dry compost are prone to the disease called Scab, which produces unsightly lesions on the potatoes' skins. The best potatoes I have grown were grown in a mix that had a very high proportion of "organic matter" - in my case, composted stable manure. Grass clippings could achieve the same effect, as long as your grass has definitely not be treated with the dreaded Clopyralid weedkiller! I like to add a fair proportion of home-made compost too, because commercial compost is often very poor, and does not retain moisture well.
"Lady Christl"
3. Water copiously and often. Potatoes hate dry soil, and containers can dry out very rapidly, so you need to water very frequently - often daily if the weather is hot. And I mean proper watering, not just a light sprinkle. You need to ensure that the water gets right down to the plant's roots, not just moistening the top inch of soil. If you are using small pots, it is worth sitting them in pot-saucers which will maximise the benefit of the watering, by allowing the soil to absorb the moisture over a longer period.
"Charlotte"
4. Choose the right varieties. You will probably not manage to grow good Maincrop potatoes in containers. They just take too long, and often fall victim to Late Blight before their tubers have had a chance to mature. Better to go for First Early or Second Early varieties, which mature much more rapidly and therefore often avoid the blight - the ones that are usually called "new potatoes". My favourites at present are "Lady Christl" (First Early) and "Charlotte" (Second Early), but there are many different ones available.
"Kestrel"
5. Know when to harvest. A lot of people are too impatient with harvesting potatoes. They want to dig them up far too early, and therefore end up with a very meagre crop. The signs to look for are these: when the plants produce flowers, there are normally some useable tubers, though they may not be very big. If you want bigger tubers, wait until the plants' foliage has completely dried out. Here's another thing: if the plant's stems are still upright, they are probably still growing. When they flop over, the plant normally stops growing, and it's time to dig them up.
The moment you've been waiting for...
And the result? I normally hope for a yield of about 500g per plant, (i.e. one seed tuber), which will be very approximately 10 new tubers. Good luck!