A kind reader asked why it is necessary to hill potatoes. It's a good question and one I've heard asked many times.
Potatoes (the tubers we eat) grow very close to the surface of the soil in which they are planted. Even though they may have a thin covering of soil, the sunlight that gets through to the forming tubers causes them to develop solanine. That's the green coloring you sometimes see on potatoes. And which is toxic to humans in large doses. (That doesn't mean if I come across a green spot on one of my potatoes, I toss it in the compost. Most likely I'll cut the green area off and use the rest of the potato.) But generally speaking, finding potatoes that are green because of the solanine is not a good thing, and renders the potato inedible according to most potato growing experts.
Be aware that the toxicity of solanine affects some people more than others so you do not want the potatoes you grow in your garden to become green.
How to avoid this? By hilling your potatoes.
Hilling keeps the light away from your developing tubers. Potatoes form close to the surface of the soil so you want to "bury" them to a good depth. Also, the more soil you pile around the plants, the more the formation of tubers is encouraged.
When I plant my potato eyes (or sets as they are also called) I go down the row of tilled soil with a broad hand hoe and make a furrow a couple of inches deep. I place the eyes in the furrow about 12" apart, then using the hoe again bring soil from each side of the furrow over the eyes making a slight mount which I tap down lightly with the flat side of the hoe. That's all I do until my first hilling.
You can grow potatoes without hilling. This can be done by planting your potato eyes in a trench at least 7" or 8" in depth and then covering with soil leaving a slight mound on top of the row. This method, to me, entails much more work both when planting and harvesting (and produces a smaller yield). When harvesting you literally have to dig down into the (often compacted) soil to get to your potatoes. If you hill by pulling loose soil up over the growing plants, you harvest the potatoes by turning over the looser soil which is much easier.
You can also grow your potatoes by placing a thick mulch of straw up around the potato plants in the same way you would the soil. There are two advantages of using this method. 1) You don't have to "dig" the potatoes at all at harvest time. You just pull the straw away to gather the potatoes. And, 2) the potatoes are much cleaner than if they were grown in soil.
Over the years, I've experimented planting my potatoes in both straw and soil. Each time I've done this I've found the straw mulched potatoes yield only 50% of the quantity of those grown in the soil.
When is the proper time to hill your potatoes if you choose to grow them that way? (Or when do you pile more straw on the emerging plants?) Check five different sources on the subject and you'll find five different recommendations.
Although my plants got away from me this year because the rain and muddy soil kept me from working in the garden, I like to do my first hilling when the plants are small . . . about 6" high. I nearly bury them in the soil I bring over them.
Then my second hilling is done again when the plants are showing approximately 6" more of growth. The final hilling is done about two weeks later, although by this time the plants are so tall that I have a hard time finding more soil between the rows to use, and it's also hard to keep the soil from falling right back down the steep slope now formed up to the top of the plant.
Other sources say to do your first hilling when the plants are 8"-10" high and a second and final hilling 2-3 weeks after that.
As with so much of gardening, you have to experiment to see what works in your particular soil and climate conditions. For instance, if you can get potatoes to grow well using the mulch method, what a joy to have clean potatoes straight out of the garden, but if you have a wet year, the mulch makes a desirable environment for the proliferation of slugs, slugs, and more slugs. (Slugs which will dine voraciously on your potatoes!)
This information certainly isn't meant as "the-end-all-be-all" of why or how to hill potatoes but rather my personal experience I have to offer after many years of successfully growing potatoes.
Just don't ask me how to grow a full-sized, red, ripe tomato up here near the tundra!
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