Potatoes
Solanum tuberosum
Potatoes were domesticated in central South America as long as 10,000 years ago, perhaps by the Incas.
Introduction
Potatoes were domesticated in central South America as long as 10,000 years ago, perhaps by the Incas. Little can those early farmers have imagined the importance of their breeding. Potatoes are now the world’s fourth most important food crop, after maize, wheat and rice.
The English word potato comes from the Spanish ‘batata’ and suggests a relation to the sweet potato, however they are quite different. Sweet potatoes belong to the bindweed or Convolvulaceae family and need more heat to grow than do potatoes, which are much easier to grow and are sometimes called white or Irish potatoes to differentiate them.
Potato plants are susceptible to frost so, for those of us in regions with cool winters, it’s perhaps surprising to know that potatoes are perennials. As are their close relations tomatoes, aubergines, peppers and chillies.
There is a false rumour that the English nickname for potatoes –spud – stands for Society for the Prevention of Unwholesome Diet. I thought it stood for Society for the Prevention of Unnecessary Digging!
- Contrary to common belief, potatoes flourish in firm soil, as long as there is loose material on the surface for tubers to form in.
Harvest period
- Days from potato seed (a potato!) to first harvest: 60, to end of growth: up to 160, for maincrop
Potatoes planted (seed potatoes)Potatoes ready to harvestFirst Earlies, early springEarly summerSecond Earlies, early to mid-springMid to late summerMaincrops, mid-springEarly autumn
- Best climate is any with a long enough growing time between last and first frost, which can be as little as ten weeks.
Check when buying your potato seed that you know which type it is. For example, a First Early type needs planting earlier then will stop growing earlier too, for a harvest that will not store as well as other types.
The total harvest period for all three types together is around four months, between early summer and first frost. The total eating period for potatoes, including storage, is most of the year, because they keep so well in sacks.
Why grow them
Homegrown potatoes often have a richer flavour than bought ones, especially when new but often when older as well. Flavour reflects the soil they grew in, as well as which variety they are.
- There is a wonderful choice of varietal flavours to tempt you.
Growth is fast, potatoes are easy to harvest, they store well and give a lot of food per area. This can double up too, because in many climates it’s possible to grow Second Earlies in the first half of a growing season, then transplant another vegetable for the second half. Meanwhile your potato harvest is stored for eating any time, at harvest or later.
- The average yield of potatoes in the UK and USA is around 18 tonnes per acre, which translates to over a tonne of potatoes from a full-size UK allotment.
- Or 4.5 kg/m², just under 1lb/ft².
Potatoes are around 80% water, and most of the rest is starch. In new potatoes, which are usually smaller and not fully grown, some of the starch is in the form of sugars, adding to their flavour. In old potatoes, there is less water.
Pattern of growth
What we call potato seeds are simply potatoes selected for growing quality. The plants do sometimes develop seeds, in the little green apple-like fruits which hang from stems by midsummer. However, their seeds may have cross-pollinated with other varieties and then need two growing seasons to achieve a harvest.
In the first year of growing from little seeds, the potatoes are cherry size by late summer. Similar to growing elephant garlic from its small bulbils – see Lesson 17.
Growth from potatoes we plant is rapid, because of the food stored in them. First they grow sprouts, then leaves and stolons. Next the stolons grow tubers, and pretty flowers may then appear as tubers swell. Finally, the leaves and stems go yellow and die, while tubers cure in the soil, with firmer skins developing.
- You can harvest and eat potatoes at any stage, for different flavours.
- After harvest, potatoes have a period of dormancy before growth starts again, seen as sprouts coming from potato tubers.
- Sprouting potatoes are still good to eat, simply rub off the sprouts and treat as normal. By late winter there can be long sprouts, and these will have taken some goodness and moisture from the shrunken potato, but you can still eat them!
To keep potatoes for seed, store them in light from midwinter, so that the sprouts are not long and fragile. Instead they grow short and green, and the potato also will turn green, which is fine for seed potatoes – they don’t need to be white.
The green colour is from solanine which is poisonous to eat, although for eating potatoes you can slice off any green bits and eat the remaining white part.
Suitable for containers/shade?
You can grow potatoes in shade, although they are better for sunshine and are then less likely to suffer from late blight. The spores need constant moisture on leaves in order to propagate when they first arrive, so disease is less likely when leaves stay dry.
Potato plants need a fair amount of space for their trailing stems. You can plant them at normal depth in a two-thirds full sack, pot or bucket. Garden buckets and 20–30 litre/5–8 gallon sacks are a good size for one potato plant, and they must have decent holes for water to drain.
Six weeks after planting, you should have long stems developing. Now fill to the top with new compost, in which more potatoes can develop.
- It’s fine to bury stems and the leaves on them.
At harvest time simply pull gently upwards, with your hands around all of the stems. Or you can feel in the compost before harvest to remove a few potatoes, for an early meal of sweet, new potatoes. After harvest, you could plant salads or leeks.
Types and varieties
The variety determines time of harvest, tuber size, flavour and skin colour. Plants that grow white flowers generally grow pale tubers, while plants with coloured flowers grow pink-skinned tubers. There are now over 5,000 varieties, of which I mention only a few to highlight some of the differences.
The three listings of First, Second and Main refer to the speed of growth and maturity date. The boundary between the three is not precise, for example I would class Casablanca as a First/Second Early type.
Within the three types is a loose sub-division of salad potatoes, which are smaller and with firm, waxy flesh. Examples are Anya, Pink Fir Apple and Ratte. Sometimes Charlotte is called a salad potato, because of its waxy flesh.
1. First Earlies grow rapidly in spring and finish by early summer. They can be grown as a ‘catch crop’ before other vegetables, such as kale, carrots, beetroot and celery.
Harvest them slightly immature, as soon as leaves start to yellow – the flavour is ‘newest’ and sweetest at this moment. Do not try to store them through winter because they often start to sprout by autumn.
For sheer quantity in June, Casablanca is superb, although flavour is average. It continues growing for longer than some First Earlies, with a high yield by solstice time.
Swift is rapid as a First Early but a little watery in texture. Worth it for earliness!
Rocket is less rapid, smaller and waxier.
Blue Danube is an early Sarpo, a pretty colour but of average flavour I thought.
Yukon Gold, bred in 1960s Canada, has great flavour and a strong yellow colour after cooking – large tubers have a starchy texture.
Mayan Gold, by contrast, is a heritage variety for small and dark yellow potatoes, with nutty flavours and a floury texture. However, they turn mushy if you boil them for more than about five minutes!
2. Second Earlies mature in midsummer and can yield highly, they also store reasonably well. There is still time to grow many other vegetables, immediately after their harvest.
- For areas with damp summers, Second Earlies are worth growing as a ‘Maincrop’, because they do most growing before late blight arrives and store well.
Charlotte offers a large harvest with fine flavour and firm texture, not floury. It tastes almost as good in March from a sack, as in July from the ground.
Wilja and Estima, both from Holland, give high harvests of decent flavour – Estima is waxy.
Apache is a small, beautifully coloured and waxy Second Early.
Nicola excels in taste tests – it is a salad type, with pale skin.
Harlequin is a rose-skinned Second Early, with small long tubers, a creamy flesh and fine flavour.
3. Maincrops mature late summer to early autumn, with high yields thanks to their long growing period. However, there is more risk of late blight after midsummer, which can prevent them from growing right to maturity, reducing the harvest and possibly damaging potatoes with blight spores, such that they won’t store.
Lady Balfour has had publicity for its blight resistance, but I found the flavour unimpressive, and it suffers blight in any bad year.
King Edward is esteemed for its great flavour and is pretty, with rose-coloured eyes, but the yield may be average.
Desiree is another classic English Maincrop with an excellent flavour and dense texture, red-skinned, but this variety is susceptible to scab.
Pink Fir Apple is a red Maincrop of small tubers with excellent flavour, fiddly to clean because of their knobbly shape.
Ratte is a tasty and small Maincrop salad type.
Sarpo varieties resist late blight but occupy a lot of space with their long stems. At the end of August, those need cutting off when still green, to stop plants from growing any more. Otherwise the tubers become starchy and with less flavour; harvest in late summer before this happens.
Ground preparation
You often hear it said that potatoes are good at suppressing weeds. It's true that their vigorous growth and dense leaf cover results in fewer weeds germinating and growing. However, they don't magically clear any thick growth of weeds.
Ground preparation is about feeding soil organisms, providing a soft surface for developing potatoes and mulching weeds if currently present, or maintaining a weed-free surface. This can be easily achieved by adding a layer of compost on top, and/or a cover of plastic or cardboard under the compost, if perennial weeds are established.
- Potato roots do not need loose soil to grow in and develop well in undisturbed soil, which may even be clay.
- Potato tubers need loose soil or compost to swell in because they are soft and watery and cannot push against firm materials.
Potatoes are the only vegetable in my two-bed comparison to give higher harvests in dug soil.
- The increase is small in relation to the time and effort needed.
- Second harvests after the potatoes were lower on the dig bed.
The total harvests from these two beds, from 2013 to 2020, were 754.81 kg/1664 lb from the dig bed, and 854.53 kg/1884 lb from the no dig bed.
For potatoes, the compost used can be less perfect than, say, for carrots, because potato seeds are large and can push roots into places where carrots would struggle to grow. This validates the statement that potatoes are good at clearing ground. Not that they do, but growing them gives us the chance to do two things at once:
- Smother and clean ground of weeds, with mulches.
- Have an excellent harvest of food.
Choice of space
Thanks to no dig, you can grow potatoes on any soil from clay to sand. Then there is the question of rotation, and the common advice to wait three years between growing potatoes in the same ground.
I run a trial where we have reached the sixth consecutive year of growing potatoes in the same bed. The results have surprised me so far, with harvests similar now to when we began. There is no increase in disease.
The trial has three strips as described in the table. Each strip has six beds, so there are six beds that run across the three strips.
- Each bed grows different vegetables.
- In some beds I grow the same vegetables every year, to learn about the necessity or otherwise of rotation.
- In the potato beds, we transplant leeks every year as soon as the potato harvest is finished – see Lesson 19.
The only downside of continuous potato cropping, that I have noticed, is the regrowth of volunteer potatoes in early spring. They are tubers left in the ground from a previous harvest. In a way they are free seed; however, if several grow close together your harvest will be lots of tiny potatoes. Hence we remove them.
To find volunteer potatoes, watch for new shoots in early spring, then use a trowel to lever them out. They are often the deepest potatoes that were undiscovered at harvest.
Planting
Old-fashioned methods include digging a trench to place seed potatoes in. It's easier and quicker to use a trowel – see below.
- First shoots appear in 15–25 days, depending on the type of potato and soil warmth.
Chit, or not
Chits are new shoots or sprouts, whose growth is powered by only the potato. It's often said that seed potatoes with chits will grow more quickly than potatoes planted without any. I find that any difference in growth is small, but potatoes grow best when sprouts/chits are short and strong.
Keep seed potatoes in light after the new year begins, so that their sprouts stay compact and sturdy. In contrast, seed potatoes in the darkness of a bag grow long and thin sprouts, often damaged when planting.
Potato size
Seed potatoes can be any size, but ideally should be about the size of a large egg, not smaller, as harvest would then be less, from growth starting more slowly. If you are short of seed at planting time, you can cut larger ones in half lengthwise as long as there are at least two sprouts on each half.
First Earlies with fewer chits grow medium-sized rather than small potatoes, in their short time of growth.
For Second Earlies and Maincrop, don't remove any sprouts and use medium-sized potatoes.
Planting time
You can plant potatoes from early spring, as the soil starts to warm. However, the key factor in deciding when to plant is your last frost date. You need to know this average date for your area.
For example, at Homeacres in 2020, potatoes planted even as late as mid-April grew large quickly in unusually warm weather. Then suddenly, on 11th May, a cold NE wind dropped the day temperature by 7 °C/13 °F, compared to the day before. On the following night, my potato plants lost many leaves to frost. The amount of damage depended on how much fleece I had managed to cover them with. We had three nights with temperatures as low as -2 °C/28 °F.
- Homeacres' last frost date is 16th May, so I was not surprised. We were still in gain, because we managed to protect most growth with fleece covers.
- Plant First Earlies up to seven weeks before the last frost date, as long as you are prepared for spring frosts with covers (which can be any material, including large pieces of cardboard).
- Plant Second Earlies and Maincrops two to four weeks before the last frost date.
Late planting
You can plant potatoes in late summer for harvests of new potatoes in late autumn, though this would not work with any potato harvested during that summer – you need to set a few seeds by from the spring planting, preferably Second Earlies. Keep them on a windowsill through spring and summer, where they will grow sprouts slowly.
However, leaves are at a high risk of blight, then perhaps of frost, and harvests are small.
How to plant
Plant them the right way up! You can usually see a top, with the most chits, and the bottom with few. Also, the bottom has an indent where last year’s stolon was feeding into it.
Insert a trowel vertically into the ground, to its full length, then pull the surface compost and soil towards you. This makes a slit into which your seed potato can drop. The top is about 5 cm/2 in below surface level.
- For First Earlies, this planting depth means no subsequent earthing up is needed. For Second Earlies and Maincrops, you will need to do some, because the larger number of developing potatoes push upwards.
- If planting through a polythene mulch, first cut two 10 cm/4 in slits at right angles, to make a cross at each plant hole. Use a trowel as above, and slide the seed potato down. After a few weeks, check that the new shoots have found light as they grow upwards, through the slitted hole.
Spacing
Plants can be more or less equidistant, rather than in a line or ridge. Space First Earlies at 30 cm/12 in, Second Earlies at 40–45cm/16–18 in, and Maincrops at up to 60 cm/24 in.
Clear
After the harvest your soil should be clear of weeds, just remove any that are still there. Be sure to remove every last little potato, which would otherwise regrow in autumn or the following spring.
Use a foot to re-firm the surface, then rake level. You are now ready to replant.
Follow with
First Early harvests allow time for almost any other vegetable to be transplanted or sown. I sometimes look at First Early potatoes as a catch crop, a starter before the main dish.
After Second Early harvests there is time to transplant leeks, kale, celery, Savoy cabbage, broccoli and beetroot, as well as many salad crops and Florence fennel.
Main crops which finish in early autumn offer less scope for second plantings. Mainly you could be transplanting salad vegetables, plus spring cabbage and spring onion at the end of September.
