How often

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.

How much

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.

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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.

Storing

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.

Saving seed

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.

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Frost

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.

Pests

Slugs

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.

Weedkiller

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.

Disease

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.

1. 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.
2. 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.

Potato virus  

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.

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