Growing sweet potatoes: care, varieties & harvesting

Growing sweet potatoes: care, varieties & harvesting

How to grow sweet potatoes at home? Here are the best varieties of the sweet potato and tips on planting, care, fertilising and harvesting.

Already in the 16th century, the sweet potato (Ipomea batata) conquered the European market, even before the omnipresent regular potato. However, while the potato became a hit right away, the sweet potato remained the exotic and hard to grow vegetable in the eyes of most. Within a very short time, the potato quickly became the most important source of food in many parts of Europe. After that, the cultivation of sweet potatoes was not pursued any further, which is a shame.

Although the sweet potato comes from the tropical and subtropical regions of South America, with the right cultivation tricks and adapted varieties cultivation is possible even in temperate climate zones. In recent years, sweet potatoes have been enjoying more popularity both in the kitchen and in the garden. A few years ago, the Weihenstephan-Triesdorf University of Applied Sciences in Germany undertook field trials to test the adaptability of sweet potatoes. The results show that sweet potatoes can thrive splendidly in moderate climatic conditions. This means that you too can grow and harvest your own sweet potatoes in the next gardening season in the comfort of your own garden. We have created an ultimate sweet potato cultivation guide exactly for this reason.

Growing, planting and propagating sweet potatoes

The basis for the success of sweet potato cultivation is the right planting material. Sweet potatoes are usually propagated using either tubers or cuttings. Seeds are rarely used because many varieties of sweet potatoes no longer produce seeds. So in theory, you can simply buy a sweet potato from the nearest supermarket and just put it in some soil. If the temperatures are warm, the tuber will form roots and shoots after a little while. There is only one problem: it can be challenging to estimate the exact variety of the sweet potato if you just purchase it from the supermarket. Therefore, it is possible that the purchased sweet potato could not be well adapted to the climate conditions of your garden. If you are not willing to take this risk and would like to prevent disappointment during the growing season, the safest option is to get rooted cuttings from specialist gardening shops. Specialized sellers offer a selection of varieties with clearly described characteristics, which makes the purchase easier and the future harvest a more likely triumph.

Once you have picked the suitable variety, place an order as soon as possible. To have a plentiful harvest in autumn, the plants should be slightly bigger when they are planted in the garden bed. Right from the initial stage the young plants should be given enough space. Therefore, plant them in a pot approximately half a litre in size and place them on a windowsill. In addition, the young plants need a lot of warmth and every ray of sunshine that spring offers. A windowsill facing south is therefore the best choice. Avoid temperatures below 10 °C and cool draughts caused by ventilation. Sweet potatoes are extremely sensitive to temperature changes: even single digit temperatures can easily damage the vulnerable plants. Sweet potatoes thrive best at temperatures around 25 °C. Water the plants regularly to keep the substrate moist (but not wet!). Fertilisation is not necessary if pre-fertilised soil has been used for repotting. The plants should then remain on the windowsill until the temperatures no longer fall below 10 °C at night. Young plants of sweet potatoes are relatively expensive. For this reason, you should continue to propagate the purchased cuttings yourself. This is the only way to make a bed of sweet potatoes inexpensive.

An overview of propagation methods

1. Using cuttings

A simple and quick method to obtain a number of sweet potato plants is by propagation of cuttings. A prerequisite to actually achieve this is, of course, an already existing sweet potato plant with longer shoots that can be cut off. The shoots at an approximate length of 10 cm can be removed from the plant and placed in a pot or a seed tray with some soil. Sometimes it is necessary to remove one or two of the lower leaves, otherwise the cuttings cannot be placed deep enough into the soil. To ensure that the sensitive cuttings do not dry out until they are rooted, keep them in a sufficiently humid place. Humidity is easily achieved with a mini greenhouse for the windowsill, which is available for just a few euros or pounds online. After this, the only thing to keep in mind is to let the temperature remain constantly at around 25°C. 20 °C is enough, but at higher temperatures the shoots will root much faster. Under optimal conditions, the shoots sometimes root after 1 to 2 weeks. A sure sign of successful rooting is the beginning of shoot growth. Next, it is time to slowly acclimatise the young plants: air them out more and more each day until they can do without the plastic cover.

2. Using tubers

Another way of multiplying sweet potatoes is with tubers in a similar way like the regular potato. To create several plants from a single tuber, it should be cut into several pieces. One sweet potato can be cut into 4 to 5 pieces. After that, the cut pieces should dry out for 2 to 3 days. This will reduce the risk of rot. After the waiting period, the tuber pieces can be placed into the soil and covered lightly. One of the most frequent questions about this topic is how deep should the potato be planted. Our answer: it doesn’t really matter! The new shoots will find their way. It is better to concentrate on keeping the temperature warm and the soil moist enough. At 20 to 25 °C and with moist soil, the first shoots should be visible after a few weeks.

Sweet potato varieties

Already 5000 years ago, the indigenous peoples of South America cultivated sweet potatoes. Because the indigenous people domesticated sweet potatoes, they have greatly contributed to today’s diversity of varieties. In today’s day and age, sweet potatoes are cultivated in almost all warm regions of the world. With approximately 7000 varieties, the sweet potato offers a huge selection of different plants. In most cases, the varieties differ in their tubers. There are tubers with red, yellow or white skin and the colouring of the flesh ranges from orange to white or purple. Similarly, the foliage is not always just green. Some cultivars have ornamental value because their leaves are strikingly colourful. Of course, other characteristics also play a significant role. The length of the cultivation period is particularly relevant to those who want to grow sweet potatoes in their own gardens. At the moment, the selection of available varieties for temperate climate zones is rather limited. However, this is likely to change as the interest in sweet potatoes continues to rise.

Before we move on to the topic of care, we would like to introduce you to four varieties that are easily available and can be grown in temperate climate zones. The following varieties have also been tested by the Weihenstephan University of Applied Sciences in Germany. It has been concluded that these can be cultivated in moderate climates without a problem.

  • Beauregard: a variety with a short cultivation period (90 – 95 days). The tuber has a red coloured skin; the flesh is orange. In field trials, this variety formed the largest tubers and yielded 3.5 kg per m². Varieties with orange flesh contain a lot of beta-carotene.
  • Bonita: This variety is sure to have a reliable yield but the tubers are somewhat smaller. The skin has a pink colour and the flesh is white to light yellow. Perfect for an all-year-round use in the kitchen.
  • Evangelina: The Evangelina variety is reported to be one of the best tasting varieties. Perhaps this is due to the relatively high sugar content. It is also suitable as an ornamental plant due to its wine-red shoot tips. The tuber colour is red-violet and the flesh is orange. Cultivation length: 100 – 110 days.
  • Murasaki: An eye-catching Japanese variety with white flesh and a pink skin. Has a sweet-nutty taste and is great to make home-made sweet potato chips. The cultivation period lasts relatively long (around 120 – 130 days). The yield is not as abundant as with some other varieties, but the unusual tuber colour compensates for this.

These sweet potato varieties are definitely worth a try but tend to be less available for purchase in some countries:

  • Oklamex Red: The root tubers have a dark red skin and a salmon-coloured flesh. The sugar content is said to be particularly high in this variety.
  • Sumor: This variety is said to have a very high vitamin C content. The Sumor sweet potato variety also has a lot to offer in terms of colour. The flesh is white to yellow.
  • Black Hearth: The Black Hearth variety is more of a decorative one. The leaves have an intense violet colour and should not be missing in any flower pot. In all well-assorted nurseries, you will often find sweet potatoes with special leaf colours.

The right location for sweet potatoes

Growing sweet potatoes in garden beds

The earliest possible time in the season to plant the young plants outside is after the Ice Saints (11th to 15th of May) have passed. If night temperatures remain stubbornly in the single digits, however, it is better to wait. A cold shock can cause the sweet potatoes to stop growing over a long period of time. Using gardening fleece or other type of crop cover can be of use to the impatient gardeners. This protects the sensitive plants from the wind and the temperatures under the fleece are always a few degrees warmer than outdoors. In addition, the fleece offers protection from the sun at the beginning. Even the light-greedy sweet potato has to get used to direct sunlight.

In addition to the planting date, the location itself is decisive for a lush harvest:

  • A full sunny location is preferred; partial shade is possible but not optimal
  • Light, sandy soil for good tuber formation and waterlogging prevention
  • Growing sweet potatoes in a greenhouse is, of course, also an option

When the days are warm enough, the young plants can move into their new home in the bed. Carefully remove the root ball from the pot. Now the root ball has to be loosened carefully. This step is important to prevent unattractive root tubers and adhesions later on. For optimum growth, the plants should also not simply be planted in the flat vegetable bed. When growing sweet potatoes, a certain method has proven to be more successful than others. For the lack of a better term, we will refer to this method as ‘the hill and row method’.  With this technique, sweet potatoes can also be successfully cultivated in heavy soils. To use the hill and row method, form 60 cm wide and 20 cm tall hills of mounded soil. Keep them at least one metre apart. The young plants are then planted on the hills at a distance of at least 30 cm from one another. This method of growing sweet potatoes has two major advantages. First, the soil in the hill can warm up more quickly, giving the sweet potato a growth advantage. Second, excess water drains off more quickly and the risk of waterlogging decreases.

In the initial stages of growth, sweet potatoes grow very slowly. This leaves a lot of garden bed space bare and unused. This is a shame but it can be easily avoided by planting mixed culture. In the spaces in-between the hills, plants that grow quickly and are harvested soon can be sowed. An example of such is lettuce, radishes or spinach. Mixed culture prevents soil erosion and the space in the garden is used efficiently.

Growing sweet potatoes in pots

Of course, sweet potatoes can be cultivated in a pot just as well as in a garden bed. It is important that the pot can hold at least 20 litres of soil (for a young plant). The flower pot can be placed anywhere where it is sunny. Opt for black plastic pots because they convert the sun’s rays into heat and thus improve the growth conditions for the tubers. By placing the potted plants close to a stone house wall, the heat stored during the day by the wall will ensure warmer temperatures for the sweet potatoes during the night. Similarly to the garden bed culture, a loose and permeable substrate should be used. We advise to pre-fertilise the substrate. In this way, there is no need to add extra nutrients in the initial growing phase after planting.

Watering sweet potatoes properly

If the plants are still small and have few leaves, it is usually not necessary to water them. If you are intending to grow your sweet potatoes in mounded hills in rows, keep in mind that, in this way, they will dry out faster than in the ordinary garden bed. For this reason, irrigate only if the top layer of the soil is dry. If the plants have developed a significant amount of leaves, irrigation should be increased too. If it does not rain and the temperatures remain high, it can be necessary to water the plants every day. Although sweet potatoes tolerate short periods of drought without any problems, tuber formation in the later stages can be negatively impacted by an inconsistent water supply.

As discussed above, sweet potatoes can also be grown in pots. Here, it is vital to pay attention to the water supply. If the sweet potatoes are larger, checking the soil daily is recommended. Compared to the cultivation in garden beds, growing potatoes in pots does require more work. A great rule to follow: the smaller the pot, the more watering is required. During especially hot summer days, excess water can remain in the tray under the pot, in which the plants grow. In the evening, any excess water should be removed. Once autumn comes, irrigation can be slowly reduced. The growing tubers do not need as much water as in summer. Additionally, by reducing water supply in autumn, the risk of rot is diminished too.

Fertilising sweet potatoes properly

If you prepare the bed correctly for the start of the season, no additional fertilisation is necessary.

  • Work 3 litres of mature compost and 100 g horn shavings per m² into the soil
  • If you are planting a mixed culture, the amount of nutrients should be increased by applying more fertiliser
  • Alternatively, other full-nutrient fertilisers can also be used; use the manufacturer’s recommended application rate as a guide

If the sweet potatoes are grown in a pot, fertilising just once is not nearly enough. The best way to go is to lightly fertilise every week with a high-quality liquid fertiliser. Once the sweet potato sprouts enough leaves, begin to fertilise regularly.

Care tips for sweet potatoes

Generally speaking, the sweet potato does not have any special care requirements. If the garden bed or a pot is well prepared and a good location is chosen, the plant grows quickly and independently. Only in the beginning, when the plants are still small and do not cover the whole bed with their creeping shoots, weeds have to be removed from time to time. Later in the season, annoying weeds stand no chance against the omnipresent arms of the sweet potato.

Sweet potato pests

Here, too, we have good news: the sweet potato does not seem to be particularly popular with insect pests native to temperate climate zones. In the large-scale field trial of the Weihenstephan-Triesdorf University of Applied Sciences in Germany, no damage was observed either. Some gardeners have reported, however, that mice seem to take interest in the tubers every now and then. With timely harvest, this can be prevented, though. Other than that, the gardening community can only hope that with time no new pests will develop a liking to sweet potatoes.

Harvesting, storing and curing sweet potatoes

Finally, in September, the sweet potato plant begins to form tubers. Before that, all the energy is put into the formation of the leaves. Once the leaves turn yellow (around October), the tubers can be harvested. At this stage, the sweet potatoes contain the highest amount of carbohydrates. Depending on the variety or the weather, the harvest can be earlier. What is important to keep in mind, is that sweet potatoes do not handle frosts well and must therefore be removed from the garden bed before it gets too frosty outside. For this reason, we recommend staying on the safe side and rather harvesting the sweet potatoes a little earlier.

home-grown sweet potatoes
Some might be surprised to find out that home-grown sweet potatoes are less sweet than those bought in a supermarket – the sugar content can be increased with a process called curing, though [ QueSeraSera]

Harvesting and the subsequent storing of sweet potatoes requires some patience and care. Sweet potato tubers have very thin skin. If it is damaged, the shelf life of sweet potatoes shortens dramatically as they can rot easily. For this reason, when digging for the sweet potatoes, be gentle and careful. Most of the time, the tubers are right in the upper layer of the earth anyway, so digging too deep is not necessary. The excavated tubers should be thoroughly washed and dried in the sun before storage. Although the sweet potato can be eaten right away, initially, the tubers contain a lot more starch and less sugar. You might be surprised if you eat a home-grown sweet potato and discover that it is less sweet than those bought in stores. However, with a treatment called curing, the sugar content of the sweet potato can be increased and storage life extended. Here is a brief explanation of sweet potato curing:

Curing sweet potatoes in summary:

  • Curing is a standard procedure for sweet potatoes available in supermarkets (4 – 10 days at 30 °C and 80 – 95 % humidity) and can be used to increase the sugar content in home-grown potatoes too
  • The warm and humid climate closes small wounds in the skin and starch turns into sugar
  • At home, curing can be imitated by wrapping the sweet potatoes in foil
  • The foil-wrapped sweet potatoes are placed in the sun for 5 days during the day and in the warmest room at night

Normally, sweet potatoes keep for 3 to 4 weeks after harvesting at 12 to 16 °C. While the temperatures below 10 °C can cause damage by cold, temperatures above 16 °C shorten the shelf life of the vegetables. Curing increases the storability of the sweet potatoes by a few weeks. Professionally done curing allows the vegetables to last even months longer. High humidity also extends the shelf life. In any case, only healthy and undamaged tubers should be stored. Potatoes with more substantial injuries add to the risk of rotting.

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