Double Digging

Like bastard trenching, double digging is a version of trenching. It’s simply trenching to loosen the soil two feet deep instead of the three-foot depth of standard trenching. As such, the trenches dug—to gain access to loosen the soil an additional foot with a digging fork—are a foot deep instead of two. Otherwise, the process is the same.

Even though it doesn’t prepare the soil as deeply as standard or bastard trenching, it’s still very effective for creating extraordinarily healthy plants, while involving less intense work gardeners. For these reasons, double digging has undergone a revival of late, being described in several more recently published garden books, most notably John Jeavons’ excellent How to Grow More Vegetables. Although less intense than the other forms of trenching, it still requires a lot of work but is the easiest way to loosen one’s soil and add compost to two feet deep.

The Method

First, loosen the top foot of soil in the area you intend as a bed with a digging fork and clear it of weeds. If sod is present, smother it far enough in advance that it sufficiently degrades into the soil, or remove it. If you have enough lead time, you have more options for the former. For more information, see our post on dealing with sod.

Second, amend the entire area of the bed with a two-inch layer of cured compost, applied right on top. Don’t worry about mixing it into the soil at this point; it’ll get mixed in naturally during the trenching process.

Third, starting at one end of the bed, dig a one-foot-deep and one-to-two-foot-wide trench that runs the width of the bed. Pile the removed soil just out of the way at the other end of the bed. You want it near enough to be easily shoveled back in to fill the final trench but out of the way enough to give room for you to walk and step while digging trenches on that end of the bed.

A wheelbarrow and tarp tend to make this and a couple of the latter steps easier. The wheelbarrow makes moving the soil to the other end easier. Buckets, garden tubs, old galvanized washbasins, or the like work fine; they just take a bit more hefting. The tarp makes moving the last several shovel-fulls of soil a little easier when you add this pile back to the bed toward the end of the process since it allows you to simply drag them to the bed and dump them on as soon as they’re light enough. Using a tarp also makes the final clean-up easier. Once you drag the last bit of soil to the bed and dump it on, you’re done with cleanup. There’s no final soil to be raked out of the grass or that has filled the spaces between your mulch, essentially ruining it.

Fourth, add a two-inch layer of compost to the bottom of the trench.

Fifth, thoroughly loosen the soil between one and two feet deep with a digging fork, naturally mixing in the cured compost added in step four in the process. Note, that the tines on a typical digging fork are one foot long, making the loosening another foot deep possible. Be sure the compost and existing soil are mixed well and the soil is thoroughly loosened while you have the chance. Gaining access to this depth of soil to loosen it and mix it with compost is the most difficult part of this method, so, once you’ve done the hard work of digging down to this level, do your utmost to improve this soil while you have it exposed.

Sixth, dig a second one-foot-deep trench immediately adjacent to your initial trench. This time, however, instead of removing its soil to the other side of the bed, simply shovel it directly into the first trench, filling it with freshly loosened soil and naturally mixing in the cured compost placed on top in step two in the process. Break up and loosen any clumps that need extra help.

Just like you did in the first trench, once the second trench is dug to a depth of one foot, add two inches of cured compost to the bottom of it, and thoroughly loosen its soil to a depth of another foot with a digging fork (i.e. loosened to a depth of two feet below the original soil surface), mixing in the compost naturally in the process.

Once the second trench’s soil is loosened and mixed with compost, dig a third trench, and so on, continuing the above procedure down the rest of the bed—moving trenches over to gain access to the soil one foot deep, adding two inches of compost, and loosening the soil an additional foot while thoroughly mixing in the compost naturally in the process—until you reach the final trench. Once you’ve added compost to and loosened the soil in the final trench at the far end of the bed, use the soil set aside from the initial trench to fill it.

Finally, use a bow rake to even out the soil in the bed and create a gently sloping shape to greatly lessen erosion and make planting easier.

The Benefits

For most of us, double-digging provides all the benefits we need (and is intense enough). It takes more time and effort upfront than the less-intense methods that follow—spading/forking, tilling and mounding, tilling alone, etc.—but its effects are so pronounced that it actually saves a considerable amount of time in the long run, even when just considering just the first year.

This method pays true dividends, especially for deeply rooted plants. Plus, it can be done weeks or months beforehand in the late fall or early spring—or even during the winter in the south—at times when one has more time and energy and fewer garden tasks requiring attention than is typical of a gardener’s life in spring, or any other time when one is burning to get into the garden but other methods require one to wait. Then, having put the energy into double digging, one’s plants benefit so much that there’s often little to do after planting. For the most part, one can simply sit back and enjoy the fruits of one’s efforts.

Most importantly, plants in double-dug beds are much healthier. Their abundant foliage, stemming from their massive root systems being able to spread widely to find ample water and nutrients in loosened soil filled with cured compost and getting massive amounts of sunlight, is strong and healthy, with thick protective coatings and robust defense systems. As such, the plants are much more capable of defending themselves from pests and diseases. They typically don’t need a gardener’s constant doting—or triage—and applied chemical “fixes” to “cure” them of pests and diseases, fixes that don’t actually create healthy plants but merely kill whatever’s attacking them. Instead, the healthy, rich, loosened soil of a double-dug bed saves the gardener a great deal of time nursing unhealthy plants, diagnosing problems associated with weak plants, attempting solutions that are like patching a house with a crumbling foundation and trying to keep their plants alive. Besides, merely being alive isn’t what we want as gardeners; maximum health is.

Knowing what we do about our plants’ roots—especially your tomatoes, broccoli, cauliflower, and peppers—it becomes obvious to double dig, at least for them. The resulting plants become strong enough to take care of themselves and outgrow any attackers and most diseases. And they’re also much larger and much more productive, compensating the gardener far many times over for the initial effort involved in bounteous harvests of scrumptious fruits and vegetables.

Additionally, weeds are much less prevalent in double-dug beds. The few weeds that do emerge pull from the highly loosened soil with such ease, with a couple of fingers, with so much less soil to shake off, that weeding time is cut to a minimum, typically becoming a brief side-task able to be accomplished in a few minutes while planting or on a brief foray into the garden. Likewise, since water storage capacity is so greatly enhanced by the dramatically increased number and size of pore spaces in the soil, greatly increasing water absorption and retention, time spent watering is greatly lessened if not completely eliminated.

There are, however, a few finer points, options, and differences of opinion within double digging that are helpful to know, especially when starting out. Plus, it might be beneficial to know how someone else deals with these, often contradictory, issues as a starting point.