This is a full 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!
The Kumara Question
I’m not sure why kumara (sweet potato) aren’t more commonly grown by home gardeners. Perhaps its because they’re not commonly available as seedlings.
But kumara actually rate as one of my top easy-going easy-growing edible garden plants. In reality, if you have a long summer, they’re super easy to grow, tasty, and great value. All you is a kumara tuber (or a few ready-to-plant slips), room to grow, soil, sunshine, and time. And they’re rather pretty plants, too. Bonus! Ready to try? Here’s our full lifecycle kumara growing guide.
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 for starting slips, you can use a home-grown 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 kumara slips in springtime. Here in New Zealand, you can buy them from some garden centres, through 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 slips for their own use (like me) and would be happy to share free slips with others.
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 getting ready for starting 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, or start a little later and hope for a long frost-free season 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/soil propagation tends to be quicker than water.
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. Comfy and neatly tucked out of the way for a few weeks.
The mother kumara are nestled into moist sand in the container. The covered container is kept in our linen closet, cosy and warm due to the hot water cylinder, 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 from the tubers into jars of water for additional rooting and/or hardening off to plant was roughly 1.5 months. Then they carried on in water until planting.
Water Propagation
Shoots and roots: 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 if 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 (nor tried to, they’re just for greenery) 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 dig out at harvest. Haha! Even with a shallow bed, some can be tricky to extract intact.
In all seriousness though, around 30cm of quality soil is a good growing depth for kumara. You can either use a raised bed or mound on a hard base (our beds are shallow over hard packed clay soil) or grow in large containers with solid bottoms. Our hard clay soil under the garden bed does a pretty good job as a semi-solid base.
Aspect and Spread
If your available garden space allows, kumara plants like to grow in full sun.
It’s a vigorous sprawling vine, but it’s also reasonably tolerant of being trained or clipped to stay within it’s designated space if it starts to get a bit too adventurous. If you’re short on space to let the kumara vine sprawl outwards at groundlevel enough to feed the developing 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, kumara plants enjoy growing loose rich soil with good drainage to avoid rot. Set your depth and fill your beds (or refresh your existing soil) and get ready for planting after your last frost. For soil prep, don’t go too rich on nitrogen (see feeding below) as that can increase greenery at the expense of tubers.
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. Keep the roots/stems in a container with water.
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. If your garden is windy or you’re worried about wildlife or pets knocking them about before establishment, landscaping fabric pegs are handy for a little extra security until they settle and secure their roots.
For group plantings, space your young kumara around 30-40cm apart to give them room for growing tubers. If you’re planting multiple rows in a wide bed, you can offset the plantings to make more efficient use of space. If you’re container growing, pick something wide but fairly shallow as per the notes on growing depth above.
Skipping the slips? Its not as efficient for maxing out propagation or yield, but it’s quick and simple to just follow natures way with a whole tuber. 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. Easy peasy.
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, then carry on with watering on an as-needed basis through the growing cycle. With any luck, nature will water them for you.
As with most plants, it’s best to avoid watering from above, if possible, especially is sunny weather where resting water droplets might cause sunburn on kumara leaves. Their leaves form a dense soil cover as the plants mature, so I tuck the nozzle of my garden hose up and underneath the leaves to water the kumara plants at soil level.
Feeding Kumara Plants
You can feed your kumara plants 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 for kumara, potatoes, and similar root veggies.
Managing the Sprawl
As sprawling vines, the kumara plants will eventually spread and cover the soil surface, creating their own natural buffer instead of needing mulching. As noted above, the vines can be trimmed (vines and leaves are also edible, if you’re keen to try) 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.
Getting a bit wild? 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 pest 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 don’t like using chemicals or drenches, crop rotation, soil cultivation, and test trapping can be used. Some cover crops or companion plants, like mustard, may also act as deterrants. Treating the soil with neem might also help keep them away or limit their damage and spread. See the notes later in this post on preparing for replanting for more information and ideas.
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 until the end of your growing season before digging out your main harvest.
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. Even with delicate digging, I always end up with some that are cracked or otherwise damaged. On to the inspection.
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 few days. Once cured, you can gently brush away excess dirt (I use a soft paintbrush) before storage. Check 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 for moving and storage. 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 from your harvest using the storage process above and start slips again in prep for spring.
If you’re worried they might not last long enough, you can try overwintering plants. You can do this with long-term potted kumara moved somewhere to shelter for winter, or with houseplants that can be used to source cuttings or to propagate out in the future, as noted above.
And, of course, if you don’t have any overwintered kumara or if you had a health issue in your previous crap, you can always start over again with new kumara, easy as.
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.
