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.
- 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.
In temperate climates, it’s unlikely that you need to water for the first six to eight weeks of growth. Watering is needed when you see flowers, but only if conditions are dry at that time.
Water, say, twice a week, and give a decent amount.
Stop watering when more than about 10% of leaves are yellowing, meaning that plants are maturing and making little new growth.
Potatoes are fast-growing plants, with a large surface area of leaves. Therefore they can transpire a lot of water, especially in the last third of their life. They are big plants, and the days are warm as they mature.
Check the soil after you finish watering, to see how moist it is. You may be surprised by a relative dryness. For the last two to three weeks of growth, give plenty of water in dry weather to bulk up the harvest.
Extra mulch for various reasons?
This is partly about clearing weeds, and links back to ground preparation you will already need to have done, according to whether there were weeds to smother. It also relates to earthing up, and whether there are developing potatoes to keep in the dark.
Suitable materials include black polythene, grass clippings, straw and sheep wool.
- Remember that undecomposed mulches offer habitat for slugs, which may damage potatoes and also make it more difficult to succeed with transplants for the following harvest.
- Nonetheless, potatoes often grow a worthwhile harvest under mulches like straw and polythene, which would cause slug and growth problems for other vegetables.
Earthing up, mostly Second Early and Maincrop
You need to keep light off developing tubers because they jostle upwards when short of space, through swelling close together. If new white tubers are exposed to light for more than about three days, they turn green with solanine, which is mildly poisonous and tastes bad.
For best results, drop a shovel of compost on top of each plant. It ‘rains down’ through leaves and stems, to land on the rising potatoes and keep them in darkness.
- Usually First Earlies do not need earthing up, because they finish growing before there are enough tubers to push each other up to light.
- Compared to the other mulches listed above, compost is easier to resow and replant into, once you finish harvesting potatoes.
How to judge readiness
Your choice of a harvest date reflects your food preference. Do you want an early harvest with the flavour of new potatoes? Or is your priority a maximum yield? A third consideration is when you want ground ready for succession planting
1. Earliness and flavour
- In early summer, while leaves are green on First Earlies, you can rummage near the surface and pull one or two tubers from each plant, leaving the rest to grow more. They are an early treat.
- If well grown but still with leaves all green, pull First Early plants from late May, for a few small and sweet tubers – quality not quantity.
- First Earlies in particular do not always flower, so don’t wait for that as a cue!
2. Biggest harvest
With Second Earlies and Maincrops, the largest harvests are once you see the leaves turning paler. Often this is two weeks after any flowers fade, with up to a quarter of the leaves actually yellowing.
Once growth is slowing, there is no advantage to leaving potatoes in the ground – pests may eat them and some surface ones turn green. Potatoes rot when frozen, so they must be harvested before first frost.
How to harvest
One option is firstly to remove top growth, by cutting through all stems just above ground level. You can put all of this material, and any other wastes from potato growing and cooking, onto your compost heap.
- There are false statements about composting potato waste material – it’s all good to compost, including blighted material.
- It does not spread disease, nor does it cause a problem of regrowth next spring, from pieces of potato that are supposedly living.
Find your potatoes by pulling the stems gently upwards, all at the same time, to reveal many tubers in the soft surface. Some varieties grow tubers on stolons further away from the main stems; in sandy soils, they may develop downwards and need a trowel.
Rub lumps of soil and compost from the potatoes. Then, if you will be eating them within a few days, clean them – place them all in a bucket of water and rub gently with a soft brush, or swill them around with your hand. They clean easily and very quickly at this point, when fresh.
Dirty potatoes keep better than washed ones. For long-term storage, after rubbing off excess soil, place potatoes where the skins can dry. This may even be in full sun for a day or two, which is not long enough for skins to go green.
The drying process is called curing, and allows a slight hardening of skins which results in better storage. Place the dry potatoes in paper sacks, of two or preferably three-ply thickness to exclude all light.
Sacks of dried potatoes do not need to be stored somewhere cool for the first weeks or months after harvest. How long you can easily store potatoes, before they sprout a lot, depends on the variety, because each has genetic information that determines its dormancy period, whether short or long, after which sprouting begins.
Here are examples from Homeacres:
- With potatoes we harvest in midsummer, then store in the brick shed where temperatures are 15–20 °C/60’s °F, I see no sprouting or deterioration at this level of relative warmth in late summer to mid-autumn.
- During winter in the same shed, the temperatures are below 10 °C/50 °F, cool enough to slow any growth of new sprouts.
- Each variety stores for a different length of time, and Maincrops mostly store for longest.
It is said that you should buy seed every year and not plant potatoes from your own harvest. As far as I can tell, the main concern is potato virus. If your plants had suffered this virus, you would know – see ‘Diseases’ below.
Despite this, I recommend you save potatoes to replant. These can even be green ones which you cannot eat.
- If, in late winter, you have some potatoes in sacks, for seed but with long sprouts on, it’s fine to knock off those fragile sprouts and then put your potatoes in full light. Then they make a second generation of short chits by the time of transplanting.
Potato leaves are damaged by the slightest frost. This matters in springtime, when fortunately it is rare to suffer any hard frost which might kill plants outright.
Frost is a potential problem, mainly for early potatoes – any damage to leaves and stems results in a smaller and later harvest, depending on the level of damage. Protect plants with any of fleece, cardboard and paper, draped on leaves in the afternoon or evening before frost is forecast.
- Common dates for late frosts in Northern Europe are 12th–14th May, days known as the Ice Saints days.
If slugs are numerous, they make holes in tubers at all stages of growth. Should slugs be a common issue in your garden, I would not grow potatoes through mulches of straw, cardboard or polythene. Use compost only, plus:
- Harvest as soon as the leaves start to fade, rather than leaving them in the soil to harvest later.
- Reduce habitat close to your vegetables, including any decomposing wooden bed sides.
Potato cyst nematodes (Globodera rostochiensis and pallida)
These cause damage in overworked soils – too much cultivation and potatoes often grown in the same piece of ground, as one might find in many allotments. They feed on potato roots, not the tubers; the result is weak growth and yellowing leaves.
- The best remedy is to rotate! Allow three to four years before growing potatoes again in the affected soil.
- No dig, with a 4 in/10 cm mulch of organic matter, will start a healing process in damaged soils that are often full of pests.
Wireworms (Agriotes spp.)
The slender white and wiry grubs, up to 3 cm/1.2 in long, can be a serious pest. However, they are not common and mostly occur where there was recently grass, or close to grass edges.
They are the larvae of click beetles, and have an unfortunately long life in soil – up to five years. They mostly eat root vegetables, including potatoes, carrots, onions and beetroot, but also tomatoes. They eventually pupate one summer, before hatching into beetles.
Cultivation is supposed to reduce their numbers, by disrupting habitat such as weed roots, and exposing the wireworm to bird predation. However, I know a grower who cultivated fields for six years and was still experiencing significant damage at the end of that time. He will have caused a lot of soil damage too, at no little expense.
- Trapping in spring is the best remedy. For example, place potato halves face down on the soil, check the following morning to see if wireworms have gathered there, and then collect them up.
Pyralid weedkillers are growth hormone disruptors, which interfere with plant processes to confuse growth, resulting in deformity and sometimes plant death. They are a problem resulting from corporate power and ineffective legislation.
Most damage is to the growing tips of plants. If you suspect this, check for deformity there.
Avoidance is difficult because of the weedkiller’s unseen presence in many composts and compost products, and also in hay. There is more to say about this – do check my video. I am just mentioning enough to warn you, as sadly it is very damaging to potato plants. You would notice curling inwards of the newest leaves – see the photos below.
Clopyralids are common in lawn herbicides, from where they accumulate in commercial composting facilities where lawn clippings have been added to the mix. They are also in some potting composts.
Late blight (Phytopthora infestans)
This is the main disease threat for tomatoes and potatoes. It arrives during summer, when night temperatures stay above 10 °C/50 °F for 48 hours, and air humidity is above 90% for all of that time.
This level of warmth and moisture enables growth of newly arriving blight spores. They are widespread in such conditions. Potato leaves quickly turn translucent brown, followed by dark spots appearing on stems and potato tubers.
In cooler and especially in dry summers, blight is unlikely to be a problem. Blight warnings are sometimes issued by manufacturers of fungicide when there is no need to worry. Often there is more worry than blight.
However, in warm and damp conditions, watch the health of your potato leaves and get busy when you see any sudden browning or limp leaves.
- Cut off all infected foliage and stems. If weather is wet all the time, best cut the plant to ground level. You can put this material on the compost heap because blight spores do not survive in soil or compost, only on living material.
- Check that the cut stems are fresh green and not brown; if the latter, potatoes may be rotting already. If cut stems are green, leave for two to three days so that any blight spores on the soil die. Then pull your potatoes and put them somewhere to dry – perhaps in a box in the sun for a day or two – before placing them in paper sacks.
- Check the potatoes a week later, to remove any rotten ones. Eat them quickly if some are rotting because that suggests the blight managed to enter many tubers, and they will not keep for long.
This is seed-borne and rare, because most seed grows healthy potato plants. The main symptoms are bright yellow or curling leaves, with yellow veins.
I keep my own seed and this works fine. Just be vigilant, because potato virus is a main reason quoted for not replanting your own potatoes. If you suffer it, best dispose of plants and buy new potato seed next winter.
Scab (Spongospora subterranea – cercozoa, or Streptomyces scabies – bacteria)
These organisms live in many soils. You see corky lesions which may become dry, brown pustules on potato skin. Fortunately, the damage is skin deep and can be peeled off before cooking.
Scab is more common in alkaline soils and in dry summers. It is relatively rare on tubers developing in compost, while parent plants are rooting into soil. Therefore ‘earthing up’ with compost, and no dig in general, are good precautions against it.
Blackleg (Dickeya solani)
This arrives as blackleg bacteria in seed potatoes and fortunately is rare – stems turn black and slimy near ground level. As soon as you see this, it’s best to remove all of the plant and some of the infected soil, and bin it.
If blackleg bacteria are left in the soil after removing infected plants, they may cause trouble next summer. However the bacteria mostly develop in very warm weather, so are rare in areas of cooler summers.
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.
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.