How to Grow Your Own Kumara (Sweet Potato)

Propagating kumara slips from tubers for planting

This is a full circle lifecycle growing guide on how to grow your own kumara (sweet potato) plants. We’ll start with different methods of propagating kumara from tubers for those of you planting from scratch, tips for planning and prepping your kumara beds, planting, and care through to harvest. To take things full circle, I’ll also share tips and tricks for those of you who (like me) would like to carry over your kumara and start growing again in the next season.  Let’s dig in!

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Options for Propagating Kumara

Tubers vs. Slips

Much like true bulbs, tubers like kumara can be planted straight out into the soil for growing a new plant. But with a little extra effort pre-planting, there are options that let you propagate many plants from a single tuber. Or propagate your kumara and eat it too! 

In addition to getting more plants (or more eats), propagating with slips also helps to reduce the risk of passing on any unwanted nasties that might have come with the soil or surface of the mother kumara, if you’re not starting with your own. But it’s totally ok to just pop a kumara into your prepared garden bed or planter and let nature do it’s thing!

Sourcing Slips

Whether for planting direct or starting slips, you can use a kumara that you’ve overwintered as the mother (see below) or buy a healthy-looking specimen from a grocery shop or market. Easy as! I’ll also show you how to use cuttings as an alternative.

If you’d like to plant slips but aren’t keen to start your own, you might be able to buy ready-to-plant barerooted slips in springtime. Here in New Zealand, you can buy them from garden centres, specialist suppliers, or from gardeners and garden groups. You might even be lucky enough to have a friend or neighbour who always propagates too many for their own use (like me) and would be happy to share free slips with others. 

Close-up of kumara (sweet potato) slip with roots

Different Methods for Starting Kumara Slips

Kumara slips can be sprouted using whole kumara or portions of cut kumara. Whole kumara should be used for the sand bed propagation method (or soil bed, if you’d prefer). Whole or cut kumara can be used with the water propagation method. Water is a great option for trying to start a few slips from an end or offcut from cooking prep instead of sacrificing a whole kumara to propagation. Little to lose, perhaps lots to gain!  

Time Keeps on Slippin’ Slippin’ Slippin’…

The time to start your kumara slips is roughly two months before you plan to plant out the kumara, but you can start a little earlier and hold them over if needed, or start a little later and hope for a long frost free stretch before harvest. For our NZ climate, that means you’re probably starting things off in late winter (warmer areas) or in early spring, depending on how late your last expected frosts might be. 

Sand Bed Kumara Propagation

A covered container of clean soil or sand in a warm dark place makes a perfect set-and-forget starter for kumara slips. I use an old kitchen storage container, clean propagation sand, and our linen cupboard as my nursery. The hot water heater keeps that cupboard nice and toasty, and the kumara don’t need light until the sprouting is underway. 

The mother kumara are nestled into moist sand in the container. The covered container is kept in the linen closet, checking periodically for moisture and growth. Once the slips come out, the container is opened and moves to a warm indoor location with indirect light. Tenting is optional to help maintain humidity while the slips continue to develop. 

For the examples in the collage below, the timeline of starting through to removal of the tipu for additional rooting and/or hardening off to plant was roughly 1.5 months

Step-by-step photo collage of propagating kumara slips in a sand bed

Water Propagation

Kumara can be sprouted whole or cut in water, either suspended partially submerged or set in a water-filled dish or tray. It’s not always as reliable or prolific as the old-school sand bed, but this method has the advantage of letting you keep an eye on the action.
Unlike the sand bed which is best for whole mother kumara, this method also works with cut pieces. It’s a great way to grow new plants from gnarly old ends or other offcuts from your cooking prep. You can sprout your kumara and still eat (most of) it, too!
Time lapse collage of sprouting kumara slips from offcuts in water

Whether or not the slips develop roots at this stage depends on whether they’re growing on a covered position of the mother and a little bit of chance. If a slip has roots, separate those with the slip when the time comes. If it doesn’t or they’re tiny, don’t worry. They’ll grow quickly enough if the slips are placed into water before hardening and planting out. 

Using Cuttings to Propagate Kumara

Cuttings from kumara vines will root readily and can be planted like slips. If you’re getting a late start on planting and have a friend with established plants who’s will to share a few cuttings, you can root them and plant them out much quicker than starting from scratch.

Cuttings from healthy kumara root readily in water, easy as! You can also use cuttings as your slip source or over-wintering back up plan (more on that below). The same process applies if you’d like to grow more roots on slips before transplanting them out.

Growing a Kumara Vine Indoors

This is one of my favourite tricks for winning the over-wintering battle. If you can’t cool store kumara from your harvest long enough to replant or propagate for the following spring, consider growing a plant over winter instead. You can use that as your mother source for taking cuttings. Kumara vines make lovely houseplants. 

You can also start indoors for transplant if your growing season is too short or has too unreliable a frost-free window to grow your kumara all the way through for enough development outdoors. We wouldn’t usually recommend transplanting root crops, but careful transplanting might give your kumara plants enough of a head start to crop well, even with a shorter growing season in the garden. 

Indoor tubers? You never know your luck! In theory, you should be able to if you give the plants enough room and the right conditions. Personally, I’ve never gotten any substantial tubers from my indoor kumara or from my experimental glasshouse winter kumara crops. The leafy portions grow well, but below the surface nothing much. Our glasshouse is likely too cool and perhaps not consistently bright enough through our wet and dreary winters for the plant to move into production mode. Indoors, I usually keep the plants in smaller pots and wouldn’t expect them to produce anything substantial.  

Planning and Preparing a Kumara Garden Bed

Growing Depth

Good news, lazy gardeners! Kumara are best grown with a hard base to stop them from burrowing too deep. That can draw resources away from main crop production, or leave you with a bottomless bed of kumara that you’ll never be able to fully harvest. Haha!

In all seriousness though, around 30 cm is a good growing depth for kumara. You can either use a raised bed or mound on a hard base (our beds are on hard packed clay soil) or grow in large containers with solid bottoms. 

Aspect and Spread

If your space allows, kumara plants like to grow in full sun. 

It’s a vigorous sprawling vine, but reasonably tolerant of being trained or clipped to stay in it’s space if it starts to get a bit too adventurous. If you’re short on space to let it sprawl enough to feed the roots, it can also be trained upwards on a trellis or frame. As a bonus either way, they’re also quite pretty plants. 

Soil Preferences

Like most root crops, they enjoy loose rich soil with good drainage to avoid rot. Set your depth and fill your beds (or refresh your existing garden bed soil) and get ready for planting after your last frost.

Planting out Your Kumara Slips (or Tubers)

Hardening Off to Outdoor Conditions

Your indoor started slips might need a little time to adjust to the ambient outdoor conditions. Like hardening off seedlings, you can take them outside for a few hours in a protected spot and then slowly extend their time and sun exposure. 

In my experience, kumara slips are pretty resilient and they’ll bounce back if they get a bit of shock or sunburn from being planted out without adjusting first.

Stretching Things Out

Unlike a typical straight-up planting, I like to plant my kumara slips on their sides right up to the leaves, and around 5cm deep is plenty. Don’t worry if the leafy portion above ground looks a little funny at first, they’ll quickly straighten up, settle in, and then start spreading like mad. Space your plants around 30-40 cm apart to give them room for tubers. If you’re planting multiple rows in a wide bed, you can offset the plantings.

Skipping the slips? If you’re planting the whole kumara tuber, you can pop that into a hole in your prepared bed in which ever direction best suits the shape of the tuber and your bed. Sideways is my go-to for this type of tuber shape unless sprouts say otherwise.

Caring for Kumara Plants in the Garden

Watering Kumara Plants

Once established, kumara are generally low maintenance plants other than making sure they get enough water through the hot summer weather. Keep your young plants well-watered as they establish, and then carry on with watering on an as-needed basis through the growing cycle. With any luck, nature will water them for you.

Avoid watering from above, if possible, especially is sunny weather where resting water droplets might cause sunburn on leaves. I tuck the nozzle of my garden hose up and underneath the leaves to water at soil level. 

Feeding Kumara Plants

You can feed your kumara with natural boosters, like seaweed solution or compost tea, and/or use a general-purpose veggie garden or potato fertiliser. Follow the product directions for application. Like many root crops, too much nitrogen can cause them to grow great leaves but not produce great tubers, so prep your soil and feed with care.

Managing the Sprawl

As sprawling vines, they cover the soil surface creating their own natural buffer instead of needing mulching. As noted above, they can be trimmed (bonus: they’re edible!) or trained if the vines start to outgrow their space in your garden, but give them enough room for a healthy cloak of greenery to be fully productive.

You can periodically lift and shift any vines that are resting on the ground to reduce setting more roots, keeping the energy focused on your main crop.

Watching for Pests and Problems

General purpose pests, like aphids or caterpillars, can make a move on your kumara plants. Fortunately, in most cases, the damage is to the leaves and, unless they decimate your plants, the kumara themselves still grow fine underground.

A few key exceptions for New Zealand gardeners are weevils, wireworm, and other soil-dwelling larvae that will attack kumara, potato, and other root crops underground. For gardeners who have wireworm issue but don’t like using chemicals or drenches, crop rotation, soil cultivation, and test trapping can be used. Treating the soil with neem granules might help keep them away or limit their damage and spread.

Monitoring for Risk of Frost 

If you get an unexpected frost warning after planting or before you’re ready to harvest, cover things up as best you can to help protect your frost-tender kumara plants. 

Frost can burn and kill off the leaves of your kumara plants, which isn’t the end of the world at harvest time as long as you’re quick to dig the tubers out after the vines are frost damaged. But, unfortunately, cold temperatures can also affect the kumara tubers below the soil. Kumara are often grown in mounds or in shallow raised beds where temperature fluctuations can hit extra hard. Affected kumara may split, soften, or rot.

Harvesting Your Kumara

Time Until Harvest

Kumara are a long-growing crop, and the longer the frost-free season you have the better your chances of success. Generally, you’ll want at least four or five months of warm growing conditions in between planting and harvest. 

Early Harvests and Crop Checks

In late summer and early fall, when frost still isn’t a risk but the plants have been growing for a while, you can fossick around the soil under the vines gently by hand to feel for a few tubers that might be big enough to break away and dig out for an early meal before your main harvest. Young immature kumara can be tasty, but won’t store well, so let the crop mature before digging. 

Main Kumara Crop Harvest

The main harvest should be dug, cured, and stored away before your first frost, if possible. It can sometimes be a bit of a gamble on leaving things as long as possible for maturity vs. running the risk of frost. Checking by digging out a test kumara or two as above can sometimes help judge whether the crop is mature enough to harvest. 

If the autumn weather cooperates, harvest after a few days without rain or watering. You can cut back the leaves and then gently dig out the tubers. Rather than forking, I prefer doing this with my hands to reduce the risk of damaging the kumara.

Freshly harvested kumara are delicate and the skins are easily bruised or scraped. Handle them with care until they’re cured.

Collage of cutting back and harvesting kumara tubers

Inspecting for Damage

Damaged kumara won’t keep. Any damaged tubers, if otherwise healthy and firm, should be eaten or processed straight away for alternative storage methods, like freezing. 

Inspect your freshly dug kumara harvest carefully for damage, cracking (even with care, sometimes you break a few), or other damage, like growth splits on the skins. In many cases, these kumara are still totally fine to eat if you cut out the damage parts.

Storing Kumara Through Winter

Curing Kumara

After you’ve checked and removed any damaged tubers, the rest of the mature crop can be left to dry and cure briefly before storage. You can brush off excess dirt, but there’s no need to wash residual dry dirt from your kumara if you plan to store it. Washing can actually shorten your storage life. It goes against all my internal desire to clean my root veggies, haha! But if a little dirt helps them store a little longer, then, hey!

Cure them somewhere sheltered, warm, and dry, ideally out of direct sunlight, for a couple of days. Once cured, you can gently brush away any excess dirt (I use a soft paintbrush) before storage. Check them for any signs of softening or damage that you missed on the initial inspection. 

Wrapping for Storage

Kumara store best when they’re not touching each other. You can nestle them in a box with clean sawdust or shavings, or individually wrap them up. I use newspaper for wrapping and then gently place them into boxes. The paper keeps them apart and also helps with moisture. I keep different varieties in different boxes for ease of use. Any kumara that look like they might not store longer term go on top of their pile for first use.

Storage Conditions

Store your wrapped kumara somewhere dark and dry, and make sure that no pesky critters can get to them for nibbling. If you’re not using them quickly, check on them every now and then, and remove anything that’s starting to spoil or getting squishy.

Getting Ready for the Next Season

Propagating from Your Harvest

In most places, including here, you’ll have several months as a gap between your harvest and starting the next year’s slips. With any luck, you’ll be able to overwinter some healthy mothers using the process above and start again. If you’re worried that they might not last long enough, you can try overwintering as plants. You can do this with long-term potted kumara grown outside and moved to shelter for winter, or with houseplants that can be used to source cuttings or to propagate out in the future, as noted above.

Crop Rotation Plans

In an ideal world, you’d have space to shift around your kumara crop from year to year, alternating at least every other year, and ideally longer. This would reduce the risk of carrying over any pests or diseases, and give you longer to adjust the soil conditions. If you’ve had issues with pests, like wireworm, the longer the rotation gap the better!

But the world isn’t always ideal. If you need to reuse the same beds, be extra vigilant in monitoring for any problems and use the offseason to alternate or amend the soil.

Preparing Soil for Replanting

Make sure that all of the kumara (or as much as you can reasonably find) has been dug out and removed from the bed at the end of the season to reduce transfer risks.

Since the next year’s crop will require the same nutrients, you’ll want to make sure that you replenish the soil with amendments. You can use top up soils, compost, fertiliser, etc. and remember that kumara like lots of phosphorus for tuber development and not too much nitrogen. Working the soil over repeatedly in the offseason can also help to disturb any lurking wireworms, hopefully for an early demise. 

It will be too cold for effective solarisation in winter, but you could defer your next planting if you have a long summer season and a deep clean is needed.

If your weather and space allow, you can grow short-season winter veggies or grow a sacrificial winter cover crop in the bed. Try to pick plants that kumara pests, like wireworm, don’t enjoy. Supposedly, they dislike brassicas and those grow well in our winters. Bonus for not having to battle against white cabbage butterfly in winter, either!

Mustard is a good winter cover crop and it can have a soil cleansing effect to help with pests. We’re actually growing it right now in our winter garden. It’s quick to germinate from seed, tolerant of cold temperatures and light frosts, and low maintenance. As a cover crop, make sure that you compost it back into the soil (or your bin) before it has a chance to set seed. Otherwise, it might take over your garden. You can chop it as often as you need to, and it will keep regrowing until you take it out at the roots.

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