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What on earth is ‘earthing up’?

As I greeted my emerging potato plants with glee a month or so ago, I wondered about the whole earthing up business. What’s it all for? When and how should I do it – and why??

The last two seasons, since I started growing potatoes, I’ve been dutifully chucking earth onto my potato plants as my Dad told me to, but I’ve never really been sure of what I was trying to achieve and if I was achieving it! (Except that I did have a pretty good harvest last year)

So anyway, I thought I’d do some investigating into the whats, whys and hows of earthing up, and share with you, lucky reader, in as simple and unbamboozling way as I can.

What is earthing up?

Earthing up, aka hilling up, is basically adding earth around the base of a growing plant – in this case, a potato plant.

Why is earthing up a good idea?

Potato plant

In my research I’ve come across these three benefits:

  1. It prevents light from reaching the potatoes

  2. It increases yield

  3. It protects plants from frost

1. Blocking out light – Potatoes that grow close to the surface can become exposed to light. If this happens they will start making chlorophyll (which turns them green), as well as a toxin called solanine. We all know we’re not supposed to eat green potatoes, but the green-producing chlorophyll isn’t the problem, this is just an indicator that the potato has been exposed to light and therefore also contains solanine – which is a problem as it’s poisonous to humans (not likely to kill you, but could definitely make you ill if you eat too much). Therefore, if we periodically cover the surface near the base of the plant, we can block out light to any potatoes that have decided to grow up near the surface.

2. Increasing yield – I guess it stands to reason that if a lot of your potatoes are green when you harvest them and you have to throw them away, then this decreases your yield. However, some sites say that earthing up can actually increase yield by ‘increasing the length of the underground stems that will bear potatoes’ (this from the Gardeners World website). I haven’t been able to find a more detailed explanation of how this works, but this infographic (follow link) does help a bit. It shows how with each earthing up more of the stem is buried, and once buried, the stem sends out shoots for more potatoes to grow on.

3. Protection against frost – This one is easy to understand. Spring plants need protection from late frost. I don’t think this is strictly a reason for earthing up (there are other ways of keeping plants warm, like fleece), but it’s a useful side benefit.

Different approaches to ‘earthing up’

The traditional method – The traditional method is to pull the surrounding soil up around the plant. If you’ve got your potatoes in rows you’ll end up with ridges as in this picture.

Potatoes earthed up into ridges

In my first two years of potato growing I’ve done roughly this, but with a couple of differences:

  1. No ridges – The potatoes were in a block, and I imported new soil rather than pulling it from between the plants as there wasn’t enough room between plants for that,

  2. Covering the plant – I followed my Dad’s method (which, from my research, seems a bit unusual – but not unprecedented), which was to almost cover the plant entirely, just leaving a little bit of green leaf showing. It never seemed to damage the plant and they just pushed on up and out.

This method seemed to work really well last year – no photo, but I promise you, it was a great harvest!

Potato bed 2020 just before harvest – Earthed up the traditional way

Mulching/No Dig methods – Many sites I have looked at now advocate using an organic mulch such as compost, straw or grass clippings to earth up instead of soil. Besides feeding the soil, this also means the potatoes are growing in a looser medium, and can easily be pulled out without the heavy digging.

In this video No Dig guru Charles Dowding shows how he does it, calling it ‘composting up’ rather than ‘earthing up’ (fast forward to 8:05 for this).

Some sites, like this one, Growveg, suggest that the seed potatoes are simply laid on the top of the soil and then covered in straw which is regularly topped up instead of earthing up.

My 2021 Earth Up Mash Up

I ended up doing a bit of a mash up this year – more by accident of having mixed up the methods in my head, rather than by conscious design. I planted the seed potatoes a few inches down into the compost layer that topped the beds. Then, instead of ‘earthing up’ I have been experimenting with ‘grassing up’. We have such an abundance of grass clippings at the moment – plenty to feed the compost bins and lots left over for the potatoes. Here in this pic you can see the first week’s grass on the left – the plants have pushed out above it during the week – and the newly piled on grass on the right.

Week 1 grass on left, Week 2 grass on right

I missed out a week last week (week 3) and applied more again today – today more of an ‘around the base’ approach today rather than covering them up completely, as they are so tall now.

Week 4 – Grass just around bases rather than on top

A few more weeks and I’ll be able to share whether or not the potatoes are a good as last year – taste and yield! But as far as earthing up goes, this method is a winner for me for a few reasons:

  1. Grass clippings are free and plentiful at the moment

  2. They’re light and so easy to put on (no heavy work heaving around soil or compost)

  3. They’re keeping moisture in really well

Something that may be a drawback is the heat that newly cut grass can generate. I noticed that it was quite hot when I put my hand in (I’m familiar with this because on the compost heap it can get up to 7o°c after grass is added). I also noticed some discolouration on the stems which I wondered if might be caused by too much heat (see picture below).

Discolouration of stem – due to heat damage?

Today I’ve read that potatoes grow best in cool temperatures, lower than 20°c, so maybe letting them get too warm is a mistake. As a precaution, when I added new clippings today, I mixed them up with the dry ones from last week so that more air will flow through and less heat get generated.

I’ve got three potato beds on the go growing three first early varieties – Nadine, Pentland Javelin and Maris Baird – so I could have tried out a few approaches side by side. Having said that, this way of earthing up has been so easy, if the potatoes are good I’ll stick with it in future for sure.

How do you earth up? Let me know in the comments section.

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