As with all gardening advice, if you ask two gardeners you will get (at least) three answers and the information from the experts varies as well.
However, this gives some very basic information.
The general idea is to plant one potato – technically a tuber -
The seed potato could be one you have saved from last year’s crop or one left over from the kitchen but it is generally recommended that you obtain new ones from a reliable source to avoid diseases.
There are many varieties of potato. Early varieties produce a usable crop around June – these often need to be used quite quickly since they are said not to store that well. Main crop (or late) types are harvested in the autumn and can be stored for use through to spring. A third type bridges the gap between these – second earlies.
Opinions vary but at its simplest – you plant all types around the same time; the differences are in the time it takes them to produce a good crop.
Seed potatoes are available from December through to April (there are some in the Shops later in the year to give new potatoes at Christmas but they tend to be expensive and we do not stock them). It is recommended that you purchase the ones you want early in case they sell out.
You cannot plant them out and expect successful results until the soil is in a reasonable state – not too wet and not too cold – usually sometime in March at the earliest. But there are those who take a chance and plant very early. If you leave the seed potatoes in their bags while you wait for the weather to change the result can be a tangle of long root growth which will break off when you try to plant – a waste of the plant’s energy. To avoid this, you lay the potatoes in trays (with the eyes upwards) in a light but cool place – such as a spare bedroom – to chit – that is to form short strong shoots which will not break. There will be some shrivelling as they lose moisture but this isn’t a problem. Opinion varies as to whether chitting is actually a positive advantage to the crop but failing to do so could well be a disadvantage. Opinion also varies on whether or not to reduce the number of shoots before you plant – particularly on main crop varieties.
You have a choice about the way you plant your potatoes. The traditional approach is to lay the potatoes in a trench about 6 inches deep and cover them with soil. Alternatively, they can be planted individually using something like a bulb planter or just a trowel. As they grow you rake more soil up to the plant stems so that the plants are now appearing from a ridge. The theory is that this protects the plants from a late frost, encourages more roots (and thus a bigger crop) and also buries the new tubers deeper to keep them away from any light – light turns tubers green and they are not then edible. An alternative way to protect the crop from the light is to grow through holes in black plastic sheeting – this allows for very shallow planting and ease of harvesting but can encourage slugs.
Everyone has their own ideas about spacing. Tubers are normally planted about 12 to 18 inches apart in their rows – close for earlies, wider for lates. Potatoes will grow almost anywhere – they are often used as the first crop in newly cultivated or rough land. There are all sorts of options when planting. If your ground is particularly attractive to slugs then you might add a few slug pellets around the tubers. Some people add grass cuttings to counter the risk of scab. It’s probably worth adding a scattering of a general fertiliser – Growmore or the stronger specialist Potato Fertiliser. One thing to avoid is lime which can encourage scab.
There is usually little to do once the tubers have been planted. The foliage tends to suppress all but the most vigorous weeds so it’s just the thugs which need to be removed before they set seed.
The main concern is the arrival of blight – described in more detail elsewhere. Some people water their growing crop but if you do then note that wet leaves attract blight.
The general rule for harvesting is to start with earlies once the flowers appear, usually around June, and the idea is to dig the crop gradually as you need it. With lates, you wait until the foliage dies down in the autumn. For your own convenience or to release the land for other cultivation you may want to clear the complete crop and store the result. Alternatively you can leave the tubers in the ground until needed – whether this is sensible or not really depends on the variety (some keep better than others) and the state of the land. It’s not a good idea to leave a crop if the soil is likely to get very wet or if it is plagued by slugs.
I like a simple system with as little work as possible – so I ignore the rules. I plant tubers as deep as I can using a trowel. I divide the space I have in the beds I use by the number of tubers in the bag to get my planting distance, possibly planting two small ones together. Then I forget about them apart from removing any strong weeds. No earthing up and no watering. I leave all varieties in the ground until I need them – covering them with some plastic if the weather gets really cold or wet. It works for me.