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I've heard that I should be piling dirt onto my potato plants as they grow so that I end up with little mounds.

My potato plants have started to poke through the surface. Should I totally cover them? By how much?

What is the purpose of this, anyway?

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up vote 16 down vote accepted

The whole point of 'earthing up' potatoes is to promote as much growth of potatoes along the stem while also protecting the potatoes from being exposed to light. If the growing potatoes are exposed to sunlight, they will produce the toxin solanine and become inedible. You know that spuds have been exposed to light because they also turn green due to the production of chlorophyll.

I start earthing up my potatoes when the leafy bits are just under 12 inches tall, making sure I leave some of the leaves exposed to the sun so they can carry on photosynthesizing - so not completely covered. I then do it once again a week or two later (depending on how fast they're growing) essentially until I run out of soil. The potato mounds in my garden are about a foot high or so.

Another way to ensure your young potatoes aren't exposed to light is to use black plastic to cover the soil, although this method requires you actively water your plants as the rain can't get through. A benefit of using plastic is that you have to do very little digging to get to your potatoes. This year I used a combination of earthing up and putting a thin layer of newspaper and grass clippings over the soil. We'll see how that goes...

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Growing potatoes in mounds is a great way to grow them because it saves you from having to dig into the soil to pull up your potatoes.

Potatoes never grow any lower than where you planted your original seed potato, but you can grow them as high as the growing season will allow.

There are methods for growing potatoes up columns of tires which you might want to investigate.

Someone else will have to answer definitively how high to mound them, you should be able to cover most of the plant without hurting it.

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ok, I'm going to have to look up the tire thing; it appeals to my inner redneck.. –  Chance Jun 10 '11 at 13:34
    
bing.com/images/search?q=potatoes+tire pretty much tells the story. We do this and it's simple and straightforward. –  Kate Gregory Jan 17 '12 at 19:45
    
agreed on the timing for burying them, BUT, don't use tires. would you eat off a tire? seriously toxic stuff. tires are great for non-edible plants, but please don't use them for food. –  user1154 May 4 '12 at 15:46
    
Tires are fine - ignore the "would you eat tires?" logic.. I would not eat dirt or cow poop either, but I do grow my veggies in both –  user1164 May 7 '12 at 12:43
    
Another minus for tires: you waste a lot of soil in the air part of the tire. Tires are free -- that's about the only plus I see. You can easily make shallow boxes that stack on top of each other to fill the role of tires, with less wasted soil and no chance of toxic residue. –  Jan Steinman Jun 18 '12 at 18:21
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