Maybe you hoped you could just drop your seed potato pieces in the ground, cover them over and wait for potatoes. More likely, you’d heard that you would need to hill your plants. Here I’ll explain why we do this, when to do it, how to hill, plus a couple of alternatives to hilling.
Initial Potato Development Stages and The Main Reason to Hill: More Potatoes!
The first things the seed potato does after planting is to produce roots, stems and leaves. This vegetative growth stage lasts 30 to 70 days. Bigger plants can yield more potatoes, so the goal for this stage is to produce large sturdy 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 to 70°F (15.5°C to 21°C). This combination can be achieved in spring, when the soil is cooler than the air temperatures most of the time, or if you are planting in early summer, add organic mulches to keep the soil cool.
Tuber (potato!) formation and branching of the stems comes after the vegetative growth stage. All the potatoes that will grow on that plant are formed in this important two-week period. Flowering can happen too, but it’s not essential, so don’t worry if you get few or no flowers. The number of tubers produced per plant depends on the hours of daylight, temperature and available water in that short period of tuber initiation. Hilling adds soil to the stems, encouraging stem growth and providing more sites for potatoes to form.
Watering also stimulates the production of more tubers. When tuber formation begins supply 5 gallons per cubic yard (22.8 liters per square meter) of water. Water at this critical time, even if you can’t water at any other time. Short day length is optimal, with a night temperature of 54°F (12°C). High nitrogen also inhibits initiation. During this stage, leaf growth continues (the plant gets bigger).
When and How to Hill Potatoes
Start hilling (pulling soil up over the potato 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.
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 when you get to the end of the row and work back up the same 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 our BCS walk-behind tiller, with a hiller/furrower attachment. Nowadays we use a tractor-mounted hiller that has disks turned inwards in pairs to ridge the soil.
Hill Potatoes for Weed Control
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 time for weed control of potatoes.
As well as providing more stem length underground for potatoes to grow from, hilling in sunny weather can deal with lots of weeds in a timely way, especially if machine work is followed up by the crew passing through the field hoeing, as we do. Sun and wind kill the weeds quicker, giving them little chance to re-root. Organic mulches also reduce weeds. Potatoes later in life produce a closed leaf canopy that discourages more weeds from growing until the tops start to die. Mary Peet in Sustainable Practices for Vegetable Production in the South reports that potato yields were decreased 19% by a single red root pigweed per meter of row left in place for the entire season.
Hill Potatoes for Frost Protection
A potato plant after two late frosts of 30F and 29F.
Frost will kill potato leaves, but the plant underground is not killed and can quickly recover and grow more leaves. If you are expecting a heavy frost after your potato plants are 4 inches (10 cm) tall, try to hill them before the frost. We had a heavy frost May 9 this year which just about killed all the above-ground growth. But we knew it was coming and hilled to give our plants the most protection we could from the soil. I was amazed at how quickly the plants recovered. On the third day after the frost, the plants weren’t looking good. By day 6 they had new green growth and on Day 8, they looked almost as good as before the frost.
Two frosted potato plants on the third day after frosts, already recovering.
A potato plant on the eighth day after two late frosts, showing lots of new growth.
Alternatives to Hilling Potatoes: Thick Mulch
If you can’t hill, or really don’t want to, you can increase the effective depth of planting by covering the rows with thick straw or hay mulch. It does need to be very thick, if you are not hilling at all. So-called “lazy beds” of potatoes are made by planting the seed potato pieces under only 2 inches (5 cm) of soil, and then piling 12 to 18 inches (30 to 45 cm) of mulch on top. If you are buying the mulch, or spreading it by hand, the cost and time may leave you feeling far from lazy!
Mulching is easiest to do immediately after planting, before the plants emerge. It’s difficult to spread the mulch around the plants after they emerge. We don’t mulch our spring-planted potatoes because we want the soil to warm up from its winter temperatures – I don’t recommend mulching potatoes if the soil is cold.
When we plant in June, we cover the seed pieces with soil, 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. A couple of weeks after planting and mulching, we walk the rows, investigating the spots where there should be a potato plant but none is visible. Sometimes the shoots get trapped under the mulch and need to be freed up.
Alternatives to Hilling: Flaming
In wet weather it can be impossible to hill when you’d like to, and this is where flaming can save the day, as far as dealing with weeds. Although not an alternative to hilling in terms of providing more stem length underground, flaming can deal with rampant weeds if the soil is too wet to hill and it can buy you some time. Potatoes may be flamed at 6 to 12 inches (15 to 30 cm) tall. Flaming is not recommended for potato plants taller than that. See ATTRA’sFlame Weeding for Vegetable Crops
Flaming when the potatoes are less than 8 inches (20 cm) tall is also an effective organic control measure for Colorado potato beetles. 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.
Third Stage of Potato Development
After the two-week tuber initiation period, the potatoes 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 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.
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 to 1.6 inches (2.5 to 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. Finally, the tops naturally yellow and die. The skins of the tubers thicken, which makes them suitable for storage. No more growth is possible.
Pam Dawling works in the vegetable gardens at Twin Oaks Community in central Virginia. She often presents workshops at MEN Fairs, as well as sustainable agriculture conferences. Pam also writes for Growing for Market and other magazines. Her books, Sustainable Market Farming, and The Year-Round Hoophouse are available at www.sustainablemarketfarming.com. Her blog is on her website and also on Facebook.
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