Single- and double-digging: how and why

Digging is included in the RHS Level 2 Certificate in Practical Horticulture. Why, and how is it done? 

Often, when you say to people who don't garden, "I'm training to be a gardener", the first thing they think of is spades and dirt.

"Is it a lot of digging?" they ask. Well... yes. But digging is falling from favour. 

Double-digging, in particular, is now firmly believed by many to be a harmful practice, and I should say now that I'm one of them. However, it's still part of the RHS exams, and is still used in plenty of workplaces for particular circumstances.  

Is digging still a thing?

Soil is life. It's partly mineral, but it's also composed of water, air, and living and dead organisms. These aren't "in" the soil: they are it, and they're vital to plants. 

Our understanding of how soil works has advanced hugely in recent years. Traditionally, gardeners have believed that digging or tilling soil improves it. Initially, the practice can provide results. However, it also kills organisms that make up the soil and disturbs the balance of other elements. Over time, it effectively deadens the soil and harms the foundation of ecosystems. 

If you want to watch a really interesting, easy-to-follow, non-depressing documentary about this, I recommend Kiss The Ground. It shows how continued tilling could be disastrous.... but also how looking after our soil better is good for our economies, the climate, humans, and the life we share the planet with. 

Awareness is spreading amongst gardeners, farmers and policymakers, thanks to the work of no-dig pioneers like Charles Dowding. Even Jeremy Clarkson showcased regenerative farming practices in his TV show. 

Double-digging, in particular, is not only a harmful practice, it's really labour-intensive and hard on the body. So why do people still do it? 

Compacted soil in my garden. You can see the hard pan at the bottom of this clump.
(I moved Mr. Worm to a safe area!)

RHS students do need to know how to single- and double-dig

The RHS isn't ignorant of these developments - far from it. Its website says:

"...single digging, no-dig methods or avoiding digging altogether by growing plants on bare soil (crops or green manures) usually provide similar or equal benefits. For this reason, double-digging is rarely practiced or necessary these days."

Although unnecessary digging is widely understood to damage the soil, there are circumstances in which it's still common practice. Right now, it's still a task that could appear in your RHS Level 2 practical exam.

For example, when you want to break up areas of compacted soil. Compaction is when soil has been squished and become hard, so air and water can't pass through it. 

This can happen in very busy public parks, where the pressure of many feet compacts the soil. Or in new-build gardens, like mine, where the action of heavy machinery has created a hard pan in the clay, 7 inches beneath the surface. To create a new bed, I needed to break this pan up and remove any nasty stuff the builders had left behind. I also needed to practice the method for my course, so on this occasion I double-dug.

How it's done

There are several diagrams of single- and double-digging on the internet, some of which make it look very confusing. It's actually incredibly simple. This is the explanation I gave to some of my classmates who missed the lesson, with photos from my garden. Hopefully it helps you too!

You will need:

• Steel-toecapped boots
• 4 canes or pegs
• A spade
• A fork
• A wheelbarrow
• If you're double-digging, you'll need organic matter like manure or compost

1) Set out the patch you want to dig and get your tools ready.

For the exam, you'll dig a patch which you'll mark out with pegs or canes. In 'real life', you'll dig whatever area you need to dig.

Place a cane or peg in the ground. Lay your spade down with the handle touching the cane and place another cane at the tip of the blade. Do this twice more. You're marking out a square, using the whole length of your spade (blade and handle) as a rough measuring stick. This is your patch. 

Set the wheelbarrow at the top left of your patch, just outside your markers, with its side facing you and its handles towards the top of the patch. 

Stand inside your patch, a foot or so from the top of it.

Your spade and fork should always be placed safely when not in use - that means not lying on the floor, no fork prongs pointing out where they could catch an ankle or be trodden on, and nowhere they present a hazard. Either stick them into the ground where you can see them (never behind you), or lean them between the handles of your wheelbarrow with the fork prongs/spade blade facing in towards the barrow. 

Remove canes/pegs as soon as you can, as they are a trip hazard, and it's good practice to put flowerpots on the ends of them if you can.

Not the neatest, but here's my best effort at a trench.

2) Dig one trench, moving soil into wheelbarrow.

Dig a trench along the entire top of your patch.

You want to try to dig in a methodical way, working in blocks about the width and depth of the spade's blade. This approximate measurement is called a "spit". Sink the blade into the ground at the top, left and bottom edges of your block, pulling it back slightly to loosen the soil if it's heavy. 

Then do the same in the right-hand edge, and lift the block up into the barrow. It may not come up in a neat cube. Don't worry about this.

 

Remember not to twist or overstep as you move the soil - this is bad for both your back and your exam! Hold the spade with one hand near the end of the handle and the other closer to the blade - this improves your leverage, making the weight easier to lift. 

As you remove each 'block' of soil, cut the edges neatly by holding the spade at a sharp angle (right-angle). You want your first trench to be nice and straight and even - this will set you up well for the others to follow.

In this soil, I like to make all the vertical cuts first, then do the bottom cuts last and flip each block into the first trench.

Once you've removed the main 'blocks' of soil, plant your feet apart a bit more, hold your spade at a flatter angle, bend your knees to get low to the ground and scoop soil out of the trench into the barrow. You're aiming for a neat, oblong trench, one spit deep, one spit long and one spade wide, with a flat bottom. 

Use your fork if you encounter any tricky stones, but be careful with it. Lever it forwards and backwards, never sideways. 

Move your barrow to just outside the bottom end of your patch. Be careful not to trip over your canes!

3) Dig a second trench, moving soil into first trench

If you're double-digging, there's an extra step here. When a trench is done, fork the bottom of it, enough to crack and loosen the soil, to the full depth of the fork. 

Using the fork to break up compacted soil

Then add in your organic matter - about a bucketful per square metre. You don't need to do this if you're single-digging. For clay soil like mine, 'long compost' (with long pieces of fibre still in it) is best. In the photos, I've used green manure and a bit of darker soil from a raised bed, just so you can easily see what I've done. 

Starting the next trench, with the previous one filled with organic matter

Whether you're single- or double-digging, start your second trench just below the first. It's exactly the same as the first one, except you're not moving the soil into a barrow. You're moving it into the first trench you dug. Keep your second trench the same dimensions as the first. Be especially careful not to let the edges wander and become wonky.

3) Repeat step 2 until you've dug your whole patch. 

Tip the soil in your wheelbarrow into the last trench you dig, remembering to fork and add organic matter if you're double-digging.

The result

You should end up with a neat, mounded square of earth. My first attempt on the Capel Manor College plots produced something like this (a bit wonky):

My home efforts, on a bigger patch, produced a neater, but much chunkier result:

The soil I'm working on is very heavy clay which comes out in lumps. In your exam, you'll likely be working in a college garden with nice loamy soil. It will be easier to work, but you may find it slips back into the trench from your pile as you dig. If this happens, just keep working, don't pressure yourself, and try not to pile it high. You want your 'patch' to be generally even in height. 

Further improving the soil

My soil was very rough and full of builder's rubble, so I spent a lot of time forking and hauling out giant rocks and bits of concrete. I found bits of plastic, bottles, petrol cans and other unpleasant things under the turf. The hard pan under the top layer of soil was really hard - think a thick layer of dried-out Plasticene. Forking it was a serious workout, and the whole thing took me four separate mornings. 

It doesn't look great right now, but I finished it off by adding a product called Clay Breaker - this stuff is great for loosening clay soil over winter. 

Clay Breaker in action.

It's important not to just leave soil exposed, which can result in erosion. I'll now mulch my new bed and leave it until spring. Frosts and thaws, coupled with the Clay Breaker and the organic matter I added, should slowly create soil that's easier to work and friendlier to roots. 

Double-digging is not good for soil health and doesn't usually need to be repeated. Once is enough to relieve compaction... which I'm very pleased about!

I'm charting the transformation of my bland new-build garden into a tiny wildlife haven. If you're interested, read along!