In mid February, place the seeds in boxes (for small amounts, cardboard egg boxes are ideal) in a light airy position at a temperature of roughly 10°C / 50°F. We often use seed trays.
The potato seed should be positioned so the the sprouts are uppermost and the 'stalk' end (where they were severed from the parent plant) is at the bottom. Sometimes this is a bit difficult to judge, but if you get it wrong, and the potatoes sprout from the bottom end, simply rub off the distorted sprouts and turn the potato to the correct position.
As the potato seed produce sprouts, remove all but the topmost four to ensure that they receive all the goodness of the seed potato. The ideal sprout length at planting time is 2.5cm / 1in although this is not critical. What is critical, is that the sprouts are green and not white coloured. White sprouts are caused by not enough light.
If the sprouts appear too early for planting the potatoes outside, simply rub them off cleanly and they will re-sprout in a couple of weeks time. Research has shown that the sprouts can be removed five times without any ill effect on the sprouts which will replace them. Whatever you do, don't be tempted to plant them too early. Potatoes originated in tropical and semi-tropical South American conditions and cannot withstand frost. However, if you wish to grow many small tubers to eat like baby potatoes then leave all the eyes intact and plant the potatoes at half the distances shown below.
Planting seed potatoes
Mid March is the ideal time to plant the sprouted potatoes in the open ground. Plant 'earlies' about 30cm / 12in apart from each other, in rows which are 60cm / 2ft apart. Plant maincrop potatoes about 35cm / 15in apart, in rows which are 75cm / 2ft 6in apart. Where you are planting more than one row, the rows should (ideally) run from North to South to allow each plant its full share of sun.
Dig a trench about 20cm / 8in deep, scatter bonemeal or organic potato fertilizer in the bottom, add a few inches of soil and then place the potatoes in it with the sprouts pointing upwards. Hand fill the trench over the potatoes trying to avoid damaging any sprouts. It is often recommended that you then scatter bonemeal or similar long lasting fertiliser over the top soil and rake it in, but the roots grow downwards and it is better under the seed potato.
Caring for the crop
Frost damage is the first concern in the early stages. If shoots emerge above the soil level and frost threatens, draw a little soil from the bed edges over them.
After the potatoes plants have grown to about 20cm / 8in pull up the soil in between the rows around the plants leaving a few centimetres still showing. Repeat the exercise in two to three weeks time.
During the growing season, ensure that weeds are removed and apply some long lasting fertiliser again around mid-August. A month or so after planting, the dense foliage of the plant should then block out sufficient light to deter all but the most vigorous weeds.
As the potato plant grows, do not remove or 'stop' the foliage, it is supplying food to the tubers. However, remove any flower heads or buds which appear because these will produce potato seeds which will divert energy from the potato tubers.
Potatoes need to be well watered to produce the tubers, but water on the foliage produces damp, moist conditions just perfect for potato blight. So no spraying with hoses is the first rule, the heavens will produce enough water that way. The best way is to use porous hose resting in the trenches between rows, which will take the water down to the roots without wetting the foliage.
Harvesting the crop
Potatoes are ready for harvest when the foliage first starts to die and turn yellow. Early (new) potatoes can be lifted earlier (no earlier than June ) to get the very tastiest potatoes. In this case, harvest them about a week after the potato plant flowers first appear. New potatoes only produce a couple of handfuls of potatoes per plant, so dig up the whole plant.
Harvest times are subject to weather and climate. But as a rule of thumb, the first Earlies take 10-12 weeks, second Earlies take 13-15 weeks and maincrop take 15-22 weeks before they are ready for lifting. Remember, they don't all have to lifted in one go, the potatoes come to no harm being left in a pantry under the soil.
For all potatoes, dig them up from the side of the ridge (a fork or spade are both fine) to avoid damaging the potatoes. The foliage can be placed on the compost heap only if it completely free from disease - if not, burn it. If you don't need all the potatoes from a plant at one time or if you want a few early in the season, simply burrow around the roots with your hands and remove the potatoes you need. The remaining potatoes will continue to grow.
Remove any soil clinging to the potatoes and leave them on the soil for a few hours to dry out - if they are stored damp, they will rot. Leaving them on the soil surface in the sun will harden the skin slightly, doubling the storage time - this is especially important for maincrop potatoes which you may wish to keep eating into the next spring. Store the potatoes in boxes or sacks, checking them every few days, removing all but those in good condition. Damaged or blemished potatoes should be eaten immediately. Any potatoes showing green colouring should be disposed of immediately as they are poisonous and they are potatoes that have not been covered fully by the soil and the light has caused that effect to them.
One word of caution, some potato flowers may well turn to berries which are definitely poisonous. Eaten in sufficient amounts the berries can be fatal. You may wish to remove them if children are likely to see them. Children always enjoy lifting potatoes, and I well remember my eldest grandaughter saying to me, "It's just like buried treasure Grandad!"
Pests and Diseases
Potatoes grown from stock certified as free from disease and planted in soil not occupied by potatoes the previous season, are relatively free from pests and diseases. The most common ailments to affect potatoes are potato blight, scab, slugs, and wireworm. There is little protection available that can effectively solve these problems.
Potato blight has two phases - the first attacks the leaves, the second attacks the potatoes themselves. If your potatoes are attacked by the blight, there is no cure. However, cut the foliage back to 5cm (2in) from the ground at the first signs of infection. This will prevent the blight from getting to the potatoes themselves. With no foliage, the potatoes will not give of their best, but they will continue to grow for a month or so.