Techniques
Earthing up
Drawing soil up around the stems of a crop — most often potatoes — to protect shoots from frost, stop tubers greening, and encourage more to form.
Earthing up means pulling loose soil up around the stems of a growing crop to form a mound or ridge. It is one of the simplest jobs in the vegetable garden and one of the most useful, and beginners usually meet it for the first time when growing potatoes.
Why earth up potatoes?
There are three good reasons to earth up potatoes.
Stopping greening. Potato tubers that are exposed to light turn green and develop a bitter compound called solanine, which is mildly toxic and makes them unpleasant to eat. A good covering of soil keeps the tubers in the dark so they stay creamy-white and safe.
Frost protection. In a UK spring, frosts can strike well into April and May. Tender young shoots that get caught are blackened overnight. Drawing soil over the shoots buries the most vulnerable growth and buys your crop a layer of insulation until the danger passes.
More tubers. Potatoes form on short stems above the seed potato, so the more buried stem you create, the more room there is for tubers to develop. A taller ridge generally means a heavier harvest.
When and how to do it
Start when the shoots are about 20cm tall. Using a draw hoe or a spade, pull loose soil up from between the rows and heap it around the stems, leaving just the top few centimetres of leaves poking out. Work along the row to build a continuous ridge.
This is not a one-off job. As the plants keep growing you repeat the process every couple of weeks, gradually building the ridge higher until it is roughly 25–30cm tall. Choose a day when the soil is moist but not waterlogged, as dry, dusty soil is harder to mound up neatly.
If you grow potatoes in bags or large containers, the same idea applies: start with the container part-filled and keep topping up with compost as the shoots grow. (This pairs neatly with chitting, the head-start you give seed potatoes on a windowsill before planting.)
Other crops you can earth up
Potatoes are the classic case, but the technique crops up elsewhere.
Leeks and celery are earthed up to exclude light from the lower stems, which keeps them pale, tender and sweet — a process known as blanching. Drawing soil up the leek shaft gives you a longer white section to cook with.
Brassicas such as sprouts and purple sprouting broccoli benefit from a little soil firmed up around the base for stability, helping these tall, top-heavy plants stand against autumn and winter winds.
In every case the principle is the same: a few minutes with a hoe, repeated as the plants grow, for a healthier and more productive crop.
In a UK garden
In the UK earthing up matters most in April and May, when tender potato shoots are still at risk from late frosts that can blacken new growth overnight.
Example
When your potato shoots reach about 20cm, you pull loose soil up around them with a draw hoe so only the top few centimetres of leaves show, building a long ridge over each row.