Earthing Up Potatoes and How Many Shoots to Keep

Thinning the shoots decides what each plant gives you

A single seed potato throws five to ten shoots. Grow them all and the plant spreads its resources across too many stems, and you get a lot of very small potatoes. Reduce it to two and the share going to each stem rises, which shifts the crop markedly towards medium and large tubers.

If you actually want a lot of small potatoes to cook whole in their skins, keep three. Bear in mind that the more stems you keep, the more crowded the base of the plant becomes, and the earlier blight arrives in a wet season.

When and how to thin

The right moment is when the shoots are four to six inches (10 to 15 cm) tall. Earlier and you cannot tell which shoots are the strong ones. Later and the roots have knitted together, so pulling one lifts the seed potato with it.

1. Choose which to keep Keep two thick shoots with short gaps between the joints. Pull the thin, drawn ones and any that came up late.
2. How to pull them Press the soil down firmly around the base with one hand and pull the shoot sideways with the other. Pulling straight up lifts the seed potato and damages the shoots you are keeping.
3. Afterwards Firm the soil back around the base with your hand. Do it on a dry morning so the wounds dry quickly and disease does not get in.

The shoots you pull will sometimes root if you push them into the soil between plants. They never yield anything like a proper plant, so treat it as an experiment in spare ground.

First earthing up, and the first feed

Earth up for the first time at the same visit as you thin the shoots. Draw two to two and a half inches (5 to 6 cm) of soil around the base so the thin stems are supported against wind. Scatter the feed around the plant first and then cover it, so the fertilizer is carried down to the roots by rain.

One to two pinches of a balanced fertilizer per plant is enough. Too much nitrogen gives you foliage while the tubers lag, so start light and top up at the second earthing if the leaves look pale.

1. Break the soil up first Loosen the soil in the path before you draw it up. Lumps laid on top do not let water through, and the base of the plant stays dry.
2. Place the feed Scatter it about four inches (10 cm) out from the stems. Fertilizer right at the base burns the developing stolons.
3. Cover the feed Draw soil over the fertilizer. Left on the surface it washes away in rain and ends up outside the root zone doing nothing.

The second earthing up decides quality

Two to three weeks after the first, when the plants are about a foot (30 cm) tall, earth up again. Add another two to four inches (5 to 10 cm), aiming to finish with at least eight inches (20 cm) of soil above the seed potato.

New tubers form above the seed potato, nearer the surface. If there is not enough soil they break through, and daylight turns them green and inedible. The second earthing up is, before anything about size or flavor, what prevents that.

Flower buds appearing is the sign that the tubers are starting to bulk up in earnest. Finish earthing up by then, because cutting roots after that stops the bulking.

If you did not earth up enough

If tubers start to show through during the season, add soil the day you notice. Even after flowering has finished it is not too late, as long as you only lay soil on top rather than digging around the plant.

If there is no more soil to take from the path, or you are growing in a container and cannot add any, lay a thick mulch of straw or grass clippings around the base to block the light. It is not a full substitute, but it prevents most of the greening.

1. When shoulders appear Add soil the day you see them. The green parts will not go back, but you can stop it spreading further.
2. After flowering Lay soil on top without digging around the plant. Cutting roots at this stage stops the tubers bulking.
3. When you cannot add soil Pile grass clippings or straw thickly around the base to block the light. Not as good as soil, but it cuts greening a long way.

In containers, top up instead of earthing up

Choose a container at least a foot (30 cm) deep and fill it only about two thirds at planting. Add compost twice as the plants grow and you get much the same effect as earthing up in the ground.

The usual container mistake is planting too many. Eight gallons (30 L) takes two plants and two and a half gallons (10 L) takes one. Crowded, the compost dries out fast, and you get drought stress and very uneven tubers at the same time.