What is 'Earthing Up' Potatoes?

The process of 'earthing up' involves adding more soil, compost or other organic material around the potatoes to avoid them pushing through the surface and being exposed to sunlight. Tubers left in full sun will become progressively more green due to the compound solanine - which also makes them mildly poisonous.

Earthing up can be as simple as dragging the surrounding soil or compost up around the plant or - if you would prefer to add extra material - using seaweed, compost, straw or grass clippings. Adding more garden compost (or Envirogrind) is probably the best option because it will feed the potatoes and improve the soil for the following crop.

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Potatoes push through the surface of the soil as they bulk up because the soil above them is less dense than the stuff below. They basically need more room and, as Yazz and the Plastic Population said in 1988, 'The only way is up'.

Early potatoes rarely need to be earthed up as the potatoes don't get large enough to breach the surface but you should always cover maincrop varieties. Early potatoes will most likely be out of the ground by the time late blight hits in July and August, but maincrop varieties will also benefit from the extra layer of soil or compost: it makes it less likely that blight spores washed off the leaves will reach the tubers below.