Last week, a couple of people contacted me about harvesting their potato variety trial plots, and that seemed like a good topic for this week. I’m guessing most of you are several weeks out from harvest, at a minimum. Let’s talk first about the maturation process in potatoes.

Though new potatoes are a much anticipated delicacy, potatoes are mostly grown as a storage crop. For optimum storability, it’s important to harvest potatoes when they are mature. Senescence (die-back) of the potato vines triggers the maturation of the tubers. As the vines begin to yellow and die, the transfer of carbohydrates from the leaves to the tubers slows, and tuber growth naturally slows also. Sucrose levels in the tubers drop as the sucrose is converted to starch. As the tuber stops growing, the skin begins to set. While the tubers are rapidly growing, the skin is thin since it is also growing and expanding – this is why new potatoes are so easily skinned during harvest and handling. During tuber maturation, the skin thickens and strengthens, mainly due to the incorporation of suberin into the skin cells. This “suberization” provides a protective barrier, minimizing water loss and tuber shrinkage in storage, and reducing tuber susceptibility to storage diseases. If the mature tuber is wounded, say during harvest, the wounded tissue will actively heal, producing a suberized layer that maintains the protective barrier. Harvested tubers are alive – they respire, producing heat and carbon dioxide, and consuming oxygen. Immature tubers respire more than mature tubers, causing tuber shrinkage, impairing wound healing, and producing ideal conditions for storage diseases.

In the Upper Midwest, the relatively short growing season means that late maturing potato varieties may still have green vines – and immature tubers under them – when fall frosts begin. Having harvested potatoes from partly frozen ground once, I can tell you it’s an experience to be avoided! Since tuber maturation takes 3 weeks from vine death, green vines should be killed 3 weeks before you want to harvest, to ensure that the tubers are mature. Organic growers can do this by chopping, mowing or scorching the vines. In a garden setting with a small number of varieties, you can just clip the vines at ground level. If you are growing the variety Australian Crawlers as part of your trial, I have to warn you that it will grow back! This variety just won’t quit.

When should the variety trial plots be harvested?

At our research station site, we harvest all our trial plots at the same time, within a 48 hour window. This gives the varieties equal time for production, since they are also planted on the same day. As a potato variety trial participant, you will ideally harvest your potatoes at maturity, and within the same week (I know it can be a big job to dig them all at once!) If you don’t want to kill green vines, you can pick a time when most of the varieties are dead or dying back, and dig them all at the same time. Varieties with green vines will have thinner skin on the tubers, which should be eaten first.

What about selecting for new potatoes?

To make fair comparisons between varieties, it’s necessary to give the different varieties an equal chance to perform well, and that’s why I harvest on the same day. However, as we get to know these heirloom varieties, I plan to select the most promising for more detailed trials, including a comparison of early and late harvests. Earliness is a great characteristic in these northern states, and I’d like to see which of our high yielding varieties are ready to harvest early in the season.

I’ll leave you with some tantalizing pictures of new potatoes – these are from a trial of new breeding lines that we harvested early. I’m hoping to see these again in future years – they are pretty and delicious. I like it when my research makes it from the field to the table!

 

 

This article was posted in Blog Posts and tagged heirloom, organic potato, potato variety trials.