Sweet potato

3 Dec 2021

Sweet potato / kumera
Ipomoea batatas

This is a meandering self sufficient tropical plant who also grows really happily in subtropical-like locations like ours. They are easy to grow, though a bit of attention is needed to grow them well to be able to feast, and both the ‘tubers’ and leaves are edible and delicious.

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What we generally refer to as sweet potato tubers are, technically, storage roots. Whatever you call them, they are tasty and nutritious and come in a wonderful array of colours and shapes. The four colour groupings are gold (orange skin, orange inside), red (reddish skin, white inside), purple (white skin, purple or white flecked with purple inside) and white (white skin, white inside). These can also be divided into white fleshed drier and starchy ‘staple’ types and orange fleshed moist and sweet ‘dessert’ types. People will argue all evening on the merits of one type or colour over the other but it’s all down to personal taste.

In Australia, about 90% of the sweet potatoes grown commercially are the gold variety ‘Beauregard’. There was no mention of particular varieties that do well in this area. They all do. Just grow the one/s you most enjoy eating. Sweet potatoes only set seed in tropical climates, so there is no danger whatsoever of crossing varieties here. We have to propagate by ‘tuber’ or cuttings. Grow them all!

The leaves of sweet potato also come in a wide array of shapes, from heart shaped with pointed tips to multiple finger shaped lobes. The leaves are a great warm season, warm climate spinach. They can be eaten raw or cooked. Snip them off the stems and off you go. They wilt quickly with storage so eat them as freshly picked as possible. There is a belief that the young leaves are better for you but I could not unearth any solid evidence to support this. The theory is the young leaves contain fewer antinutrients which is a general truism of edible leaves.

It’s not just us who enjoy sweet potatoes. If you share your patch with rats, bush turkeys, bandicoots, or wallabies, you will inevitably be sharing your sweet potatoes as well. They love them. In fact there may not be so much sharing with you going on. You may need to get inventive to be able to enjoy sweet potatoes. I have seen them grown enclosed in a metal bath, with chook wire covering above. A little extreme, but it worked. Not everyone has to go to this much trouble.

Also note that flea beetles help themselves to sweet potato leaves, munching holes all over the place. They tend to be more of an annoyance than a problem, at least for sweet potato. Flea beetles like decimating eggplant, basil, mint, and potato leaves so take note to protect these plants if you spot sweet potato harbouring flea beetles.

Incidentally, sweet potato is NOT related to potato. Not closely anyway. It is simply a sweet flavoured root vegetable looking similar (ish!) to a potato. One of their closest relatives is kang kong / water spinach (Ipomoea aquatica), and they are also related to a range of inedible flowering garden weeds all commonly known as morning glory.

Growing sweet potato for the roots requires a bit of attention. You should easily have enough leaves. This plant grows long vining stems that will run for metres and will send out roots at any leaf node that touches the ground. As it’s a sprawler, it touches the ground in many places. There is a tendency for people to plant sweet potato as a ground cover and walk away, say in a food forest situation. But what happens is that the plant makes roots everywhere. They don’t tend to be big sweet potatoes as the plants’ energy is so divided with creating a full cover of leaves and as many storage roots as it can manage, and the sweet potatoes are difficult to find and dig up. It’s not that this approach doesn’t work, you can uncover a hidden trove of huge sweet potatoes created with no effort on your part, but if you want to know how to grow them reliably in a known location, there are some key points to keep in mind.

Grow sweet potato on a wide mound. They have no tolerance for waterlogging and will simply rot away if left to sit in soggy soil. A mound will drain well. A wide one will retain enough moisture to keep the plant happy. Skinny mounds dry out too quickly. Sweet potato doesn’t like drying out, this stresses the plant and substantial splits in the root can result if it has to endure wild moisture fluctuations. Of course the bonus of growing on a mound or wide raised row, is it makes unearthing the sweet potatoes so much easier than digging downwards. Mounds need to be at least 20cm high and at least twice as wide.

Knowing that this plant will root wherever a node touches the ground is another key.
Part one is preventing excess roots. Once planted, don’t let it ramble and root all over the place. Snip off the wandering tendrils at about a metre long (eat the trimmings!). Don’t let them get too long. Encourage the plant to put its energy into making a few storage roots only, not tendrils and leaves and a whole bunch of roots. Some people grow the tendrils up a trellis to prevent them laying on the ground and making more roots but you can just keep pruning.

Part two is to use this habit of rooting from nodes to your advantage. When you plant your sweet potato (more detail below on the options), bury more than one node per planting. A group of storage roots will grow from each node. You will end up with one plant, multiple nodes underground and multiple groups of sweet potatoes in a contained area.

Don’t feed sweet potato lots of nitrogen unless you really want a lot of leaves and bugger all roots. This is really important.

To feed sweet potato for storage roots you’ll want to prepare the soil with plenty of well rotted organic matter. If it’s not fully broken down it will tend to have all sorts of nasties that mess up the roots. It likes lots of compost and friable soil to 30cm depth in total. pH 5.2 – 6.7 (acidic). This plant needs a good supply of potassium (potash) to make those big storage roots. A dose of mineral fertilisers is recommended (crusher dust). And they have a relatively high requirement for boron. Don’t add this without getting a soil test. The dose is tiny and too much is toxic to almost everything you want to grow. Use seaweed tea if soil testing is too pricey. It’s a safer option for adding trace amounts of boron.

Sweet potato is a warm season plant. Their ideal temperature range is 24° to 30°C. They suffer from cold stress and go dormant below 15°C. They do not tolerate frost and will die back in our winters. Although they can rot in the ground over winter, some survive and keep growing when the weather warms up again. Older storage roots can become quite large, but also quite tough and fibrous. This is why they are better grown as annuals and are fed well to encourage large roots in one year.

Sweet potato can be grown in this area from storage roots, slips, cuttings or sprouts.

Storage roots – just plant an entire storage root in the ground, 5 – 7 cm deep. This is not recommended as any pests or diseases from last year will be propagated too.

Slips – the stored storage roots will shoot from various eyes (like a potato does) when the weather begins to warm. Much home gardening advice says to cut the storage root into pieces, each with a slip, and plant these 5 – 7 cm deep. But this method has the same problem as planting entire storage roots. Any pests or diseases from last year will be propagated too.

Cuttings – these are pieces of the vines with a growing tip. This is a recommended method. No pests or diseases are transferred. You will need vines already established to take cuttings from.

Sprouts – very similar to cuttings and also recommended for the same reasons. These are essentially slips minus the storage root attached. The leaf nodes tend to be very close together which is handy for getting as many nodes underground per transplant.

In the case of cuttings and sprouts, take a length of at least 20cm and snip off all but the last 3 tip leaves. Leave the transplant in a shady place with at least it’s cut end in water for 2 – 4 days to begin making roots. They are often also covered in damp hessian. To plant out have just the tip leaves, about 10cm of growing end, above the ground and hopefully about 3 nodes underground.

To take your transplanting to the next level, the advice from Paul of Barefoot Fruit and Veg is to take a cutting about a metre in length and gently tie it in a figure of eight shape with both a loose knot in the middle and at the end. Leave the growing tip loose about 20cm. Bury the entire figure of eight flat underground with the growing tip poking up out of the earth about 10cm. This enables a very large number of nodes to be planted in a contained area. Whenever the aerial parts get longer than about a metre, cut them off to prevent them touching the ground and rooting.

Sweet potato needs a long frost free growing season of 4 – 6 months / 16 – 25 weeks with warm days and warm nights. If the weather is cool the storage roots will take longer to be created than when it is warm.

The first 5 – 6 weeks after transplanting is when the plant is growing leaves and basic roots to become established. Initially the transplants will need easily available water as they won’t have a root system to speak of. They will be prone to heat stress. Once the roots have begun to develop, back off on the water, letting the surface dry out, to encourage the root system to grow deep and wide in search of water. The roots can reach 60cm deep.

From 5 – 8 weeks after transplanting is when the storage roots that we enjoy eating begin to be created. The plant goes nuts with a massive growth spurt above and below ground. It is particularly sensitive to lack of water at this stage. Give it plenty all the time. Uneven water availability results in big cracks in the roots. They need about 500mm over their growing season.

1 – 2 weeks before you expect to be digging the roots out, reduce the amount of water available to toughen up the skin a bit. This makes it easier to dig them up undamaged which means they will store for longer.

Just before digging, wet the soil to soften it and make pulling the roots easier. You’ll know it’s time when the leaves begin to turn yellow and the vines begin to die back in autumn. Dig before frost arrives otherwise the roots begin to rot. Avoid damaging the skin of the roots if you can, so they can be stored for many months in a cool, dry place.

When it’s time to plant again, pick a different location in the garden to avoid the build up of diseases. Don’t plant sweet potato back into the same spot for at least three years.

Big thanks to:
Paul of Barefoot Fruit & Veg, Ian of The Gourmet Garden School

Carole & Phil Helman, Tim Hill, Nick Radford, John Hodgkinson, Jeff Alcott, John Vernon, Pete Bufo

Supported by Bellingen Shire Council via the Bellingen Shire Disaster Recovery and Resilience Grant Program Funding

Notes

1. Fanton M& J. The Seed Savers Handbook. The Seed Savers’ Network; 2008.

2. Growing the crop – Sweet potato information kit – 2000 reprint [Internet]. Department of Agriculture and Fisheries Queensland; 1999. Available from: http://era.daf.qld.gov.au/id/eprint/1689/4/3growswp.pdf

3. Problem Solver – Sweet potato information kit – 2000 reprint [Internet]. Department of Agriculture and Fisheries Queensland; 1999. Available from: http://era.daf.qld.gov.au/id/eprint/1689/6/5probsweet.pdf

4. Sweet potato information kit – 2000 reprint [Internet]. Department of Agriculture and Fisheries Queensland; 2000. Available from: http://era.daf.qld.gov.au/id/eprint/1689/5/4keyswp.pdf

5. Sweet potato. In: Wikipedia [Internet]. 2021 [cited 2021 Dec 2]. Available from: https://en.wikipedia.org/w/index.php?title=Sweet_potato&oldid=1056744325

6. Cultivars [Internet]. Australian Sweetpotato Growers Inc. [cited 2021 Dec 2]. Available from: https://www.aspg.com.au/cultivars/

7. Sweet Potato growing information [Internet]. Green Harvest. [cited 2021 Dec 2]. Available from: https://greenharvest.com.au/Plants/Information/SweetPotato.html

8. Sweet Potato Leaves [Internet]. [cited 2021 Dec 2]. Available from: https://specialtyproduce.com/produce/Sweet_Potato_Leaves_11487.php