Written by Dale Richardson - Updated: June 23, 2023
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If you are a fan of sweet potatoes, why not try growing the purple varieties? Similar to white sweet potatoes in texture but with a deep purple flesh, purple sweet potatoes are a great way to add diversity to your self-sufficient garden.
To grow purple sweet potatoes, add plenty of compost to a sunny garden bed and loosen the soil to a fine tilth. Procure purple sweet potato slips and plant them after all danger of frost has passed, and the soil temperature is over 10°C (50°F). Space slips 30cm (12inches) apart and water them well. Weed and water them throughout the summer. Harvest your tubers around 100 to 120 days or after the plants have yellowed and died back. In colder climates, harvest before the first frost.
Purple sweet potatoes are a heat-loving, long-season tuber. Even so, many people all over the world successfully grow this beautiful and delicious vegetable. Here's how:
Sweet potatoes are originally native to South and Central America. Many of the purple varieties we know today were developed and cultivated in Japan before spreading to the rest of the world. These tropical vines with edible root tubers require a long, warm growing season but they can be grown all over the world with a little help. No matter where you live, you can fulfil the needs of your purple sweet potatoes by:
Choose a sunny location and prepare the bed by incorporating in soil amendments and sufficiently loosening the soil.
One particularly important soil amendment is compost (including animal manure). Adding compost to your purple sweet potato bed adds humus which feeds the plant, improves drainage, and aerates the soil. The compost provides nutrients that are easy to absorb and creates an ideal soil tilth (or texture). Do not add too much nitrogen to your purple sweet potato bed. Excess nitrogen will produce plant growth at the expense of tuber development.
Read Next: How to grow the best creamer potatoes.
Image by Carmen Edenhofer
Kelp meal is also an excellent soil additive for purple sweet potatoes. It adds an abundance of nutrients and other essential growing elements, and it especially enhances rot development. It is also a natural fertilizer and can be grown and harvested sustainably.
Purple sweet potatoes prefer slightly acidic soil, so adjust to get the pH between 5.5 and 6.5.
Incorporate your compost and kelp meal while you are loosening the soil. Sweet potatoes need open, loose soil so their roots have plenty of space develop tubers. They will greatly benefit from tilling, but however you choose to loosen your soil is fine. Make sure you eliminate any large clumps and remove rocks and twigs as these can deform the growing tubers.
Loosen the soil to a depth of around 30cm (12inches). In another article, we go into more detail about the best soil depth for your sweet potatoes .
In cold climates, it is usually necessary to increase the soil temperature and extend the season to ensure your purple sweet potatoes grow and produce tubers. Laying black plastic over your garden bed once it is prepared will usually raise the soil temperature enough. Put the plastic down about two weeks before planting to give it time to warm up. Polytunnels or a greenhouse will also help you extend your season and keep the temperature high enough.
Purple sweet potatoes also grow well in planters or grow bags. This might give you the option of taking the plants indoors to avoid cool nights and fatal frosts. If you do grow in containers, prepare your soil in the same manner as discussed above.
Image by Walter Lim
Sweet potatoes are grown from slips, which are sprouts that grow on a mature sweet potato. You can buy slips from select garden centres or you can grow your own.
Purple sweet potato slips are hard to find, and you will probably have to order them online. You have the advantage of using certified seed stock so your chances of successful cultivation are usually "guaranteed." There are several different varieties to choose from but some are rarer than others. If you get slips from a garden centre, soak the root base in water for at least a few hours before planting.
You can produce your own slips by buying an organic purple sweet potato from a store. Organic is preferred since many conventional sweet potatoes are treated so they will not sprout slips. Place your sweet potatoes in a thin layer of soil or half-submerged in water and they will sprout and send up slips. Twist the slips off the tuber, and soak the base of the slip in water. The slip will be ready to plant when its has grown roots.
It is very important to plant your slips after all danger of frost has passed. Even a light frost will kill a purple sweet potato plant. Once the soil temperature stays above 10° C (50° F), plant the slips in the soil with the root side down. Keep the leaves above ground and pack the soil around the slip. Space the slips 30cm (12inches) apart. You can mulch around the new slips with straw or other organic matter to control weeds and help regulate the soil temperature. Keep the slips well watered.
Purple sweet potatoes require very little maintenance throughout the summer. Weeding and watering are all that is needed to keeping your plants growing and your tubers developing.
Despite being tropical, purple sweet potatoes don't require very much water to grow once they are established. Providing 2cm to 3cm (about 1inch) of water a week is sufficient.
Image by Carmen Edenhofer
What they really need is heat. Sweet potatoes generally require 100 to 120 days of warm weather. The more heat for longer, the more sweet potatoes you will harvest.
It is best to harvest your purple sweet potatoes about 4 months after planting. Realistically, however, most climates will force you to harvest just before the first frost in the fall. Even a light frost will kill the plant, and the tubers will quickly rot once the plant has been bitten by frost. If you live in a warm climate, you can wait until the vines have yellowed and started dying back. In colder areas, harvest before the temperature drop below 10° C (50° F).
Dig around the plant with a garden fork to loosen and turn the soil. Do not dig too close to the root base or you run the risk of spearing or damageing the tubers (damaged tubers will not store and should be eating right away). Brush off any extra dirt from the tubers but do not rinse or wash your purple sweet potatoes. Lay them in a single layer in a humid, well-ventilated area that is between 27° C to 30° C (80-86° F). Cure them like this for 8 to 10 days before storing them for the winter months.
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