Pruning potatoes - an experiment

Did your potatoes rise to great heights this season?  Did they block paths and make your potato patch an untidy mess?  A visitor to my garden suggested that I prune my potatoes.  Never before had I  heard of anyone doing this. Would it increase or decrease yield?  I decided to conduct a potato pruning experiment.

Before we discuss the experiment, let me first say that in soil that is absolutely perfect for potatoes, potatoes will not rise to great heights.  They do so when soil is richer than is ideal.  If there are too many nutrients in the soil, above-ground potato stems and foliage will be prolific, but this might detract plants from forming potato tubers below-ground.

It is a good idea to have a 4-year crop rotation plan for your food garden.  Potatoes belong in the fourth year of such a plan because by then three successions of crops have been grown and the soil is no longer rich in nutrients.  In ideal circumstances the soil still holds enough nutrients to give a good potato crop, but potato foliage does not become very high.  Read more about why crop rotation is important and how to set it up for your food garden here.  

After the visitor to my garden made the novel suggestion to prune my potatoes, I looked up on the internet whether potato pruning might increase or decrease yield. Opinions seemed divided.  I did not find anyone who had conducted an experiment and compared yields, so I decided to do this myself.

I had two areas next to each other of very similar size, with very similar soil.

In early Spring I had planted 10 King Edward potatoes from the same batch of seed potatoes around 15cm deep in each area.

Throughout the season both areas received the same amount of water.

I let one area do whatever it wanted without pruning any potato stems. In the other area I pruned the stems of all potatoes to half size in early Summer.  Each potato stem in that second patch still had some leaves, but was a lot shorter and the area looked a lot tidier than its unruly neighbour.

The plants, that had not been pruned, died off first in mid summer.

The plants in the area that had been pruned kept growing and they stayed alive a lot longer.  Would this result in more spuds forming and growing?

The potatoes that were not pruned have already died off (in the background)

When, at the end of the season, the potatoes in both areas had died off, it was time to see how the yield compared.

Some of the potatoes from the plants that were pruned

The result for the area that had been pruned was 4.2 kg of potatoes.

Some of the potatoes from the plants that were not pruned

The result for the area that had not been pruned was 5.8 kg of potatoes and on average they were bigger.

My conclusion from this limited experiment is that pruning potatoes may have a negative effect on yield.  

This could be because pruning encourages the plant to grow new foliage and therefore energy is used for that, rather than for forming spuds.

This was an interesting experiment, but the result does not encourage me to prune my potatoes again next season.

There is always more to learn in food gardening!