Growing Potatoes

This is the second part of a monthly series on potatoes. Last month I talked about Planting potatoes. Now they are in the ground, we turn our attention to growing healthy plants and doing all we can to maximize the yield.

Potato development stages and crop requirements

1. First the plant produces roots, stems and leaves. This vegetative state lasts 30–70 days. Bigger plants have more yield potential, so the goal for this stage is to produce robust large plants. Vegetative (leafy) growth of potatoes is favored by warm, 80°F (27°C) moist weather, but tuber growth is favored by cooler soil conditions of 60°F–70°F (15.5°C–21°C). This combination can be achieved either by planting in spring, when the soil temperature lags behind increasing air temperatures and is still cool enough for tuber formation, or by adding organic mulches to keep the soil cool if planting in early summer.
2. Tuber formation (a two-week process) and branching of the stems comes next. All the tubers (potatoes!) that will grow on that plant are formed in those two weeks. The number of tubers produced per plant depends on hours of daylight, temperature and available water in that short period of tuber initiation. Watering stimulates the production of more tubers. 5 gal/yd² (22.8 l/m²) is a good amount to supply when tuber formation begins. Short day length is optimal, with a night temperature of 54°F (12°C). If temperatures at night are 68°F (20°C), initiation will be reduced; and at 84°F (29°C), will be inhibited. High nitrogen also inhibits initiation. During this stage, leaf growth continues. Flowering can happen too, but it’s not essential, so don’t worry if you get few or no flowers. Hilling adds soil to the stems, encouraging stem growth and providing sites for tubers to form.
3. Third, the tubers grow larger, but don’t increase in number. When the leaves start to turn pale, the plant has finished its leaf-growing stage and will be putting energy into sizing up the tubers under the ground. Adequate water and nutrients are important during this critical stage which lasts until the plant reaches maturity for that variety, up to 90 days. Try to ensure at least one inch (2.5 cm) of water per week, up until two weeks before harvest. Avoid very uneven watering, or overwatering, as hollow heart could result. The size of the tubers depends on various growing conditions. Two or three weeks after flowers appear (if they do), the baby potatoes will be 1–1.6″ (2.5–4 cm) across. The best temperature is around 65°F (18°C), and I’ve read that potato size decreases by 4% for every Fahrenheit degree (7% per Celsius degree) above the optimal. Spacing is another factor — we got large potatoes one summer because we had poor emergence and therefore wide spacing! The heat of the summer didn’t stop them.

Potatoes dug during this tuber-sizing-up period will be “new” potatoes, and not have the thick skins necessary for storage. If you dig your potatoes during this stage (or “snitch” some, leaving the rest of the plant growing), you will be happy short-term, but your final total yield will be less than if you grew all the tubers to full size. If you planned for this, and grew plenty, you will be happy now, and happy later!

4. Finally, the tops naturally yellow and die. The skins of the tubers thicken, which makes them suitable for storage. No more growth is possible. Sometimes there are reasons to terminate potato growth early – see Early Harvest below for more about this.

Hilling potatoes

Start hilling (pulling soil up over the plants in a ridge) when the plants are 6” (15 cm) tall. Hill again two or three weeks later and two more weeks after that, if the plant canopy has not already closed over, making access impossible. Hilling also provides an opportunity for dealing with weeds, so if possible do this task in sunny breezy weather which won’t let the weeds re-root.

On a small scale, use a rake or standard hoe to pull soil up from the side of the row opposite to where you are standing. If you are sharing the job, one person can work each side of the row at the same time. If you are alone, turn round and work back when you get to the end of the row. Don’t be tempted to twist your arms around and move the soil up the side nearest you. You will damage your body by this distortion of your spine and shoulders!

At the next scale up, use a rototiller with a hilling attachment, or perhaps a wheel hoe with a hiller, if your soil and stamina allows. We have used a semi-manual planting method, making single furrows with our BCS walk-behind tiller, planting by hand in the furrows, then using the tiller again to cover the seed pieces and hill. Nowadays we use a tractor-mounted furrower that can make two furrows in each pass, and disks turned inwards in pairs to ridge the soil.

Alternatives to hilling potatoes

If you can’t hill, you can increase the effective depth of planting by covering the rows with thick straw or hay mulch. This is easiest to do immediately after planting, before the plants emerge. We don’t mulch our spring-planted potatoes because we want the soil to warm up some from its winter temperatures.

When we plant in June, we cover the seed pieces, then hill, then unroll round bales of spoiled hay immediately, like wall-to-wall carpeting. We choose this method to help keep the soil cooler through the summer. In warm conditions, deeper planting, hilling and thick organic mulches all help keep the plants cooler, as does irrigation.

Weed control for potatoes

Potatoes are sometimes said to be a “cleaning” crop, as if they did the weeding themselves. Not so! Any cleaning that takes place is a result of cultivation. As with many plants, the initial growth stage is the most critical for weed control. Hilling in sunny weather can deal with lots of weeds in a timely way, especially if the machine work is followed up by the crew passing through the field hoeing. Organic mulches also reduce weeds. Potatoes later in life produce a closed canopy that discourages more weeds from growing until the tops start to die. Mary Peet reports that potato yields were decreased 19% by a single red root pigweed per meter of row left in place for the entire season.

In wet weather it can be impossible to hill when you’d like to, and this is where flaming can save the day. Flaming is not an alternative to hilling, but it can be a way to buy time and deal with rampant weeds if the soil is too wet to hill. Potatoes may be flamed at 6″–12″ (15–30 cm) tall. Beyond that, flaming is not recommended. See ATTRA for more on flame weeding. Flaming when the potatoes are less than 8” (20 cm) tall is also an effective control measure for Colorado potato beetles (More on pests next month). Choose a warm sunny day when the pests are at the top of the plants. Flaming can kill 90% of the adults and 30% of the egg masses, according to ATTRA.

Early harvest of potatoes

Sometimes there are reasons to terminate potato growth early. If you need storable potatoes, cut, flame or mow the tops of the plants, and wait two weeks for the skins to thicken up. To test for storability, dig up two potatoes and rub them together, or rub them firmly with your thumb. If the skins rub off, wait a couple of days before trying again. If the skins are strong, go ahead and harvest. You might do this if you have a fast crop turnaround after spring-planted potatoes, such as we used to do when following our spring potatoes with our fall cabbage and broccoli. Another time I’ve brought potatoes to a rapid end was in England, when we got Late Blight. We cut and removed the diseased tops (so no spores went down into the soil), and were able to salvage the potatoes two or so weeks later. Back then, the recommendation was to burn the green tops. This would probably not be recommended these days. It made for a smoky fire we kept going for several days. Terrible air pollution!

For the earliest possible crop in a dry climate (but not the highest yield), plant “old” seed (ones with lots of hairy sprouts) in early spring, hold off on watering until the tubers are marble size, then give a single good watering at 5 gal/yd² (22.8 l/m²).

Next month I will write about potato pests and diseases.