I recently shared a post in some gardening groups I belong to on Facebook about our kumara (sweet potato) harvest and some growing tips. I thought it might be helpful to turn it into a blog post, which may be helpful to readers. I have adapted it slightly to take account of the fact that some of my audience might live overseas.
I live in Auckland, New Zealand. We are on a suburban section in an area called Manukau, I think the section is about 1/5 acre but the house and garage take up some space. I’m not sure about the dimensions of all our garden beds but I dug up our entire front lawn. I have been gardening for a decade. We are mostly self-sufficient. This is our best kumara harvest to date but to be fair I did devote much more space to growing kumara and planted more slips this summer. This is how I have gone about growing kumara but that does not mean that it is the best, only or right way of doing things. Other people may have had a lot of luck – or even more luck – using other techniques. I don’t know everything and I’m not always right. Don’t forget that there are many articles, blogs, videos etc on the subject too which offer detailed and maybe even better advice. Although I have learnt how to propagate and grow most veggies, I am also learning and for kumara that involves curing and storage. In the past I only planted a couple of slips so our harvest was maybe just a couple of kilos, which we consumed quite quickly. I didn’t bother to cure and store it properly.
I only grew red kumara this season. In the past I have experimented with heirloom kumara from Koanga, as well as gold and orange kumara from the supermarket. The harvested tubers were very small. I found that the standard red variety from the supermarket performed best so I only grow that now. But that is not to say that other people might not have had better luck with those other varieties. That is just my experience. I certainly don’t want to deter anyone from trying to grow them.
Around the shortest day last year (21st June for us) I placed a 60 litre Sistema crate filled with some old potting mix in our greenhouse (a converted spa pool room which we didn’t use for that purpose). I half-buried 10 red kumara I purchased from the supermarket in the dirt. I did not cut the kumara or do anything to them. I know it is possible to propagate slips by putting a kumara in a glass of water. The reason I didn’t do it this way is because I asked Henri, the owner at Awapuni Nurseries, for his advice as to the best way to propagate slips and he advised me to place the kumara in some dirt. Awapuni sell kumara slips every spring and are very knowledgeable about growing kumara so I followed Henri’s advice. I placed the lid on the crate as it is cold in winter, even in the greenhouse. Every day I sprayed the crate with a bit of water in a spray bottle to keep it moist.
By spring, shoots had begun to develop. As the slips grew taller and it became warmer, I could no longer cover the crate with the lid.
I waited until the slips were about 10-15cm long before I carefully picked them off the kumara and potted them up in a plastic pot filled with some potting mix. I kept them outside in our patio (my little “nursery” where I keep my seedlings, which is nice and sheltered) and watered them. In about a fortnight they developed roots and were ready for planting. I did try planting some slips out about the third week of November but ended up losing most of them. The survivors were mostly planted out about mid-December. At least in our microclimate it is just too cold to plant kumara outside any earlier than that. While daytime temperatures are warm, don’t forget that plants are also outside at night when the temperature drops. Also in spring temperatures can fluctuate a lot, which stresses plants out and can cause them to die. But when you can plant kumara depends on where you live. Up north you can probably plant slips much earlier than mid-December.
Kumara needs a long hot summer in order to form decent tubers. Experts have advised 4-5 months. Mine were in the ground for close to four months.
The site I used to plant the kumara was in full sun and was next to one of our dahlia gardens (I am also an avid flower gardener). I measured the area. It is 2.3m wide and 4 m in length. As a root crop, kumara needs to be in full sun. I’m sorry I don’t know how many slips I planted out. I lost some and replaced them, and squeezed in some that developed later on so I lost count. But I used slips that formed from 10 kumara tubers. As for spacing, I didn’t really measure but I think the slips would have been around 15cm apart. Henri from Awapuni recommends making a J-shape with the roots of the slip when planting it to help it form roots so I did just that (as opposed to planting the slip with the roots straight down as you would any other plant). Some gardeners recommend planting kumara slips in mounds but I didn’t do this. I left the soil flat when I made the hole to insert the slip.
It's very important not to dig the bed over before planting kumara (something which you would ordinarily do with potatoes and most other crops). If you do this, the roots will run down forever without forming tubers. Kumara needs to hit a hard surface in order to form roots. That is the advice Henri from Awapuni gave in a Youtube video I watched. But other gardeners may have had good experiences with growing kumara having dug the bed over beforehand. I used a little superphosphate fertiliser to help with the development of strong roots (I sprinkle a little in each hole prior to planting the slip) and mulched the plants with pea straw once they were planted in the ground. I do not add compost to the soil because it contains too much nitrogen, which would encourage the plants to develop lots of leaves at the expense of forming decent tubers. So as you can see I actually do very little to the soil. I am sorry but I do not know what type of soil we have. I have never had it tested.
In spring I liquid fed the garden weekly with a seaweed tonic or water soluble plant food so the kumara patch got that, too. Other than watering the garden I didn’t really do anything. You are meant to keep lifting the runners and turning them over on themselves to stop them from forming roots everywhere, so the plants can invest their energy in the actual root, but I didn’t do this. I have a very big garden and didn’t have any time. Besides, once the kumara did take off it was such a mess. The runners went everywhere and I didn’t want to stand on the plants and risk damaging them.
You are supposed to harvest kumara before the first frost. The advice a neighbour at our bach (beach house) gave us was to harvest in April. She is very knowledgeable. She is in her 90s and comes from a long line of gardeners in the Far North, who grew kumara. About a week ago, I had a little feel around a few of the plants at the edge of the patch to see if they were ready to be harvested. I recommend using a fork rather than a spade to harvest kumara. I worked very carefully so as not to spear any tubers. If that happens, they won’t store well. I loosened the soil some distance away from the root but most of the time used my hands to find the tubers. I wore both fitted disposable gloves (the kind you can get from the supermarket) and on top of that, thick gardening gloves that some German wwoofers (travellers with working holiday visas who stayed with us in exchange for some help in the garden) left behind back when we used to host them prior to the pandemic. The gloves are from Germany and I don’t think you can get them here. Wearing good gloves will help protect your hands. I am not into manicures so that isn’t an issue but I hate it when the soil gets lodged deep under your nails and you can’t remove it. It can be very painful.
Once I harvested all our kumara, I organised the tubers into different boxes according to their size. You can see this in the photos. There were some monsters which were absolutely ginormous. I have never seen such huge kumara! They went into one box. Then there were ones that were big but slightly smaller. Then the medium sized ones went into a few boxes. All the really small ones went into one box and the ones that got speared or cut by the end went into a trug which we are keeping in the kitchen. We will eat these first as they won’t store very well. After we finish those we will eat the small ones as they don’t store that well either. Interestingly I read in an article that the head of the Maori garden at Hamilton Gardens (a famous public garden in New Zealand) said that enormous tubers don’t store that well either, as they contain a lot of moisture. In that case, we had better eat those ones after the smallest ones. In his opinion, medium sized kumara store best.
I’m sorry I didn’t weigh our kumara harvest. I never weigh our harvest but it would be a good and interesting thing to do. A gardener I know in a place called Foxton had the goal of growing a tonne of veggies in a year, which she managed to achieve. Now that I have scaled the veggie patch down so I have room for growing dahlias, my new obsession, I doubt we would reach that.
In previous years, I only planted a few slips and our harvest was very small, so storage was not an issue. We got through them fairly quickly. We are new to the storing process. I have done some research, including looking at Koanga and Awapuni’s advice. The consensus is that the tubers need to be cured ie dried in the sun so that the skin thickens and they keep better. I am not sure how long I am supposed to do this for as I am new to curing and storage, but I am going to leave them in the sun for a week and turn them over half way through so they cure on both sides. After curing, there are different schools of thought but some experts recommend wrapping each tuber individually with newspaper and storing them in a dark, dry place. It is important that the tubers do not come into contact with sunlight as they will sprout. One source recommended storing them under your bed, which is what I’m going to do as there are no dark places in our house or garage.
Some people have asked me if kumara will grow where they live (outside Auckland). I’m sorry but I have no idea! I have only done gardening on our property here in Auckland. It’s best to ask this question in a local or national gardening group. Someone where you live might know the answer. What I do suggest is why not give it a try on a small scale at first? Buy one kumara, propagate a few slips and plant them to test it out. You don’t have a lot to lose. At the time I purchased kumara from the supermarket to propagate my slips, I think it was around $3 kg. I ended up spending $10 for all the tubers I used, so one kumara won’t cost that much, especially if it is purchased when prices fall. I have even seen kumara for $1.99 kg in the past but it might not fall that low again because of the pandemic, which has pushed the price of everything up.
I hope these tips are helpful! Good luck. And thank you to Henri from Awapuni Nurseries for your advice! If you don’t want all the hassle of propagating your own slips, are pressed for time or your own slips failed, remember that you can purchase some from Awapuni in spring.