Thursday, 15 October 2015

Ipomoea batatas vs the Swedish weather: 1 - 0

Buds and more buds, but no seeds
Time for results! On 3 October I decided that summer was irrevocably over hence that it was time to find out what the sweet potatoes had been up to this paltry summer. Let's start with the bad news. Despite some pretty eager hand pollinating from my side and profuse flowering in at least 8 sweet potato varieties by the beginning of September, my plants failed to set a single seed. Not one seed. I'm not quite sure why this is, though I have some ideas. Sweet potatoes are self-incompatible, but with 8 quite genetically distinct parents that should not have been an issue. Lack of pollinators is a possible factor, and I did indeed see very little insect activity around the flowers at the time, even though bumblebees are generally quite abundant in my garden. Perhaps I have a species of bumblebee with a very finicky taste in pollen, or perhaps there were simply more exciting flowers around at the time. The most likely saboteurs in the grand sweet potato breeding scheme though, are environmental conditions. I need to look into this in more detail, but given the failure of hand pollinations it seems possible that succesful seed set was inhibited by low temperatures (something we've had plenty of this summer) and/or inadvantagous daylength. Just today I was reading Ottawa Gardener's post on how she managed to get a few seed from Georgia Jet and a purple variety in her Canadian garden, so I'm convinced that it should be possible to produce seed here in Sweden as well, provided I can create the right conditions for it. I'll be putting some more thought and effort in this for next year, perhaps growing a few particularly floriferous varieties on clear plastic to expose them to higher temperatures. Last year at least I had some plants flowering as early as July, which seems like it might be a more sensible time to produce seeds here.

Unfortuntately, vine growth is not a good indicator for
tuber formation
On to the good news then. As my pitchfork and me made their way through the first sweet potato row that sunny Saturday at the beginning of October, I became increasingly jubilant. I won't be winning any yield contests anytime soon, but my expectations, admittedly quite low, have certainly been exceeded. As could be expected, the majority of varieties failed to produce any roots at all. Sadly, this includes some particularly promising, and tasty, varieties like Orphan and Mukekuru tarya bibiri, both of which produced very vigourous vines but had roots that were no thicker than a pencil. Nothing worth saving there. The Euro/American varieties overall did quite ok, though Georgia Jet was, again, a disappointment. This is a mystery to me, though I only had two plants and both came from cuttings that I was forced to keep alive throughout the winter, so they might not have gotten the best of starts. I'll be looking for a new source of Georgia Jet next year, ideas and donations are warmly welcomed.

As last year, Nordic Purple produced very long but fairly thin roots that are hardly worth eating, but since it is the most prolificly flowering variety I have I will hold on to it. Burgundy produced some good-size tubers, as did Bonita, Nordic White and (somewhat less so) Nordic Orange, so I will be saving some of those for next year, particularly since all of them flowered as well. By far the best variety was T65. This is the variety I had expected most of and it didn't disappoint, generally producing two or three supermarket-standard tubers per plant. Sadly enough, it doesn't seem to produce flowers, not in my garden during the last two years, and not in Frank Van Keirsbilck's for quite a bit longer. T65 is a Taiwanese variety that came out of a breeding project there, so perhaps any fellow sweet potato devotee knows of an interesting, flower-producing sibling out there?

Though the success rate amongst the African varieties was considerably lower, there were some pleasant surprises as well. Burundi and Bunduguza both bulked up enough to make me slightly excited. Bunduguza gave a fair amount of smallish roots, while the three Burundi plants actually produced roots that nearly rivalled those of Euro/American varieties like Nordic White. Others that I thought were worth saving for the moment were Kitekyeru, Alira, Kwezi Kume, Mushemeza, Kipapari and Rwabafurugi. By any standards, these are still very small sweet potatoes, but I think there might be something here to work with. With their high dry matter content, the African varieties are very different from the ones you would normally find in Europe, so it seems worth hanging on to as many of them as possible.

Since the objective this year was to find the most suitable varieties for my climate rather than to maximize yield, I'm judging the harvest by the thickness of the roots rather than overall productivity. This criteria roughly gives the following classification, from thickest (about 3 fingers wide) to thinnest (somewhat wider than a thumb):
And the winner is...
  1. T65
  2. Nordic White
  3. Burgundy
  4. Bonita
  5. Nordic Orange
  6. Burundi
  7. Bunduguza
  8. Kipapari
  9. Alira
  10. Mushemeza
  11. Nordic Purple
  12. Kitekyeru
  13. Rwabafurugi
  14. Asian Yam 2
  15. Kwezi Kume

15 varieties is considerably more than I have place to continue working with, so I will probably make a further selection later. A lot of the tubers are damaged by wireworm, which might provide problems with storage and cause additional casualties later in winter. For the moment, though, I'm very happy with this year's results. With an average mean temperature of 15.86 °C we had a fairly cold summer this year, particularly during June and July, which make me optimistic that this year's results can be replicated under most summers here. For anyone looking to grow sweet potatoes under similar climatic conditions, I'm convinced it's possible to get fairly respectable yields, consistently, provided one grows a variety like T65. I also grew the plants on ridges, which might have had a positive influence on soil temperatures as well. You could certainly increase your chances by using a plastic mulch, as advocated by Ken Allan, which is bound to boost yields considerably. Personally, I'm going to continue searching for varieties that do well also without plastic ground cover, though I might cheat a bit in order to induce early flowering and seed production. Yes, I'm just that stubborn.

You might be able to guess what I would like for christmas this year: Some more T65-class, early maturing and cold tolerant sweet potato tubers, and why not some seeds to top it off. Please?

Temperature data for Malmö, June through September 2015