When and How to Plant Potatoes: Depth, Spacing and Soil

Timing is set by soil temperature, not the calendar

Plant once the soil has settled at 45 to 50F (7 to 10C). Earlier than that the sprouts do not move and the seed sits waiting in cold, wet soil, where the risk of rot grows by the day.

Plant too late and the tubers are trying to swell in high summer. Potatoes bulk up best at around 68F (20C) in the soil and stop above 77F (25C), so late planting is one of the main causes of a crop of small potatoes.

Do not lime the potato bed

Most vegetables get lime before planting. Potatoes do not. They prefer a slightly acid soil, around pH 5.0 to 6.0, and pushing it towards neutral makes the organism that causes common scab far more active.

If you lime routinely, or the previous crop in that bed was limed, it is safer to move the potatoes elsewhere. If you use compost, use well-rotted compost at about four pounds per square yard, dug in two weeks before planting.

Fresh manure and poultry manure both encourage scab. For the potato bed specifically, using no manure at all and relying on a small dressing of fertilizer is a perfectly sound approach.

Ridge height and trench depth

A low ridge, two feet (60 to 70 cm) wide and four inches (10 cm) high, is all you need at planting. You will earth up twice later to bring it to ten inches (25 cm) or more, so what matters more is leaving soil in the path that you can draw up later.

1. Open the trench Draw a trench about six inches (15 cm) deep down the center of the ridge. Level the bottom so all the tubers sit at the same depth and come up together.
2. Set the tubers Cut side down, eyes uppermost. Some gardeners plant cut side up, but early growth is more reliable with the cut side down.
3. Cover them Two to three inches (5 to 7 cm) of soil. Too shallow and the new tubers reach the surface and go green. Too deep and emergence is delayed by nearly a week.

Spacing depends on the size you want

A foot (30 cm) between plants is standard. Closer together gives you more plants but less to each one, so the proportion of small potatoes rises. Wider apart gives you larger potatoes but less from the same area. Neither is right or wrong: decide by what you want to cook.

For a double row, allow two feet (60 cm) between the rows. The soil for earthing up comes from between the rows and from the path, and any narrower means you run out of soil at the second earthing, which is what causes greening.

Keep the base fertilizer off the tubers

Do not spread fertilizer along the bottom of the trench. Put it between the tubers instead. Fertilizer touching a seed potato damages the cut surface and starts it rotting. Slightly less than a small handful per plant is plenty.

Too much nitrogen and potatoes make stem and leaf while the tubers lag behind. On ground where the previous crop was heavily fed, halve the base dressing and top up at the first earthing up, once you can see the leaf color.

1. Where to put it Not as a band along the trench, but in spots between one tuber and the next. Keep it off the tuber and the cut face is safe.
2. How much About three to four ounces per square yard of a balanced fertilizer. Start at half that if the bed was fed for the previous crop.
3. Cover it lightly Draw an inch (2 cm) of soil over the fertilizer before you set the tubers. The roots then reach it after they have grown, rather than being burned early.

Plan for late frost from the start

A frost just after the shoots break the surface blackens and kills the foliage. The tubers underground survive and new shoots come, but the harvest is ten days to a fortnight later and the potatoes are smaller.

If the shoots emerge while frost is still possible, draw a little soil over them or lay row cover across the row. Watch for nights forecast below about 37F (3C), and remember that still, clear nights are the frosty ones.

1. Reading the forecast A still, clear night with a minimum below about 37F (3C) will produce frost. Assume your plot runs a degree or two colder than the forecast point.
2. Before it hits Draw soil lightly over the emerged shoots or lay row cover over them. Take the cover off once the morning has warmed.
3. Afterwards Blackened leaves do not recover. Do not dig, and do not feed. New shoots will come, and waiting is the right response.

Planting earlier does not mean a bigger crop. Find out when late frosts finish in your area and treat two weeks before that as the earliest sensible planting date.

Working out what went wrong

Nearly everything that goes wrong with potatoes comes down to soil temperature, water, fertilizer or frost. Recall how many days it has been since planting, then feel how wet the ridge is, then think back to how much fertilizer you used.

Replanting is only an option before the shoots appear. After emergence you are better off correcting things with feeding and earthing up than digging anything back up.

1. Count the days before you dig Slow emergence is far more often soil temperature than disease. Do not dig anything up until at least thirty days after planting.
2. Dig only one plant If you really cannot tell, dig the one at the end of the row. Digging the whole row dries out seed that is still alive.

A plant that lost its leaves to frost cannot be pushed back with fertilizer. The seed below ground is alive, so leave it until new shoots appear.