Comparison of raised beds, sheet mulching, and tilling: a three-year study of home gardening methods

The short version:

In the past 7 years, I have built three types of garden beds at my current location.

1. Plowing of local soil, with horse manure added.
2. Sheet mulching.  Corrugated cardboard over sod, covered with a variety of materials.
3. Raised beds. Sides made of composite decking or 2x8s, placed directly on sod, filled with garden blend soil and/or horse manure.

I’ve had all three types for at least three years now, and I’m ready to proclaim a winner: raised beds. Plowing was an abysmal failure.

The details: (After the jump)

Plowing

Year 1

I had a neighbor plow under sod and weeds in a 12’x100′ strip of our clay-filled yard and smooth it out for planting.  think I also put about a foot of horse manure over the top of this.  A few tomatillo plants and peppers were planted the first year on part; compost crops like alfalfa and radishes went on the other part. The greenhouse ended up on part of this strip in October.

Year 2

Planted corn and squash and harvested rye.  The rye yielded 1 lb per 100sf and the corn was completely annihilated by raccoons and deer. Weeds were starting to get out of control, especially in the rye area, so I covered the entire area with cardboard and a foot of horse manure (sheet mulch). Planted winter wheat, one patch in rows and one patch broadcast.

Year 3

Planted sugar beets (in a patch we weeded by hand to remove sods of weeds) and potatoes (in a fairly weed-free area) The sugar beets never emerged from the straw; I blame slugs because I mulched them with straw that had overwintered outside.  Replanted the beet area with squash, which did OK but were never really robust.  The potatoes yielded about 1/2 lb per sf (half or a third of the rate of my “good” garden beds). The row-planted wheat succumbed entirely to weeds.  The broadcast wheat made it just about to harvest and the deer ate every last kernel.

Year 4

I looked at the grass and Canada thistle taking over the beds and declared the area a total loss.

The upshot:

Plowing was an abysmal failure for me. I’m never plowing again if I can possibly help it, and I’m never making a garden bed with that many linear feet of edge backing up to grass and thistles with no solid border between them.

Sheet mulching, a.k.a. “lasagna gardening”

Year 1 – fall

Laid down huge sheets of cardboard directly on the sod (20’x20′). Covered cardboard with about 8″ of fluffed-up straw, then a foot of horse manure (half composted and mixed with straw), then maybe 6″ of garden blend soil.  Possible the dirt was on top; I forget exactly.

Year 2

Made paths of straw and planted potatoes, sweet potatoes, brassicas, squash, and cantaloupe.  The squash and cantaloupe all succumbed to squash borers but the other things did pretty well.

Year 3

Added another 20’x20′ bed: cardboard with horse manure (this time mixed with both straw and wood shavings).  No layer of straw, and no dirt added. This section got a Three Sisters garden and the 2 year-old section got potatoes, brassicas, and black-eyed peas.  The three sisters garden was 2/3 failure and 1/3 OMG success: the squash went nuts and produced something like 300 pounds of winter squash.  The beans never thrived, and critters got the corn again, despite the thicket of squash vines and a nylon stocking placed over each ear.

Other crops did quite well: potatoes yielded 1.5 lb/sq ft and black-eyed peas grew 7′ tall.  It was a bad year for brassicas, meaning we had enough to eat all summer but not enough to share or freeze.

Creeping Charlie and grass (something with big rhizomes…quack grass?) were really starting to move into the beds by the end of the season and it became clear that beds with no edge would be succeptible to weed infiltration annually.

Year 4

Converted entire area to raised beds, with cardboard in the bottom and filled with garden blend soil.  Put down weed block fabric and stone chips on the paths.  Goal is to completely eliminate edge where grass and other rhizomes can creep in.

The upshot

Building up is better than plowing down.  The main difference between sheet mulching and raised beds is simply the absence or presence of wooden sides to the beds – and the sides make a huge difference in maintenance time.

Raised beds

These are still my all-time favorite.  I won’t go year-by-year, but I will say they that after 7 years, they continue to offer the best yields with the least work year by year.  Weeding is truly very minimal, and almost never resembles sodbusting.  Just put the frame on a flat area and fill with at least 8″ of heavy material – soil and/or manure – and that will kill the sod. Weeds can get in at the corners, so try to keep the sod away from the boards.

Framing materials

I’ve used composite decking material and untreated pine lumber.  I will not use the decking material again. It bows too much and has gaps that make it easier for rhizomes to come in at the corners.  Instead, I like 2″x8″ lumber, in whatever length makes sense.  I’ve had untreated beds holding dirt for 6 or 7 years and am not noticing any rotting.  Tacking a 1″x2″ “sacrificial” furring strip on the bottom might be good for longevity; if that rots, you could flip the bed up, pull off the 1×2, put a new 1×2 on, and carry on with the same 2×8.

I usually just drill pilot holes and use 3″ deck screws to hold the corners together – to braces, blocks, or brackets.  Most have held, but a couple have pulled apart.  I just push those back together and re-screw, or, in a couple cases, add an external L bracket to bandage it back together.

Filler materials

Beds need some dirt and some manure; just manure isn’t good enough to support many crops (beans, tomatoes) though some seem to appreciate it (squash).  If you fill a bed with just manure and put tomatoes in it, you’ll have blossom end rot until the tomatoes get their roots into the soil below, at which point they’ll do OK because they can access the minerals in the soil.  However, after a year, 8″ of manure compacts down to 5-6″ of soil, and the worms and roots and such have done a pretty good job mixing the manure and soil even if you don’t get in with a digging fork to help. Still, when I top off the beds, I try to use something with some mineral content (i.e., dirt, not just more compost).

Summary

As much as I admire the idea of building up soil, and as much as I feel bad importing materials to garden in, and as much as I grumble about paying for garden soil and framing wood, I really stand by my decision to use raised beds.   Now that they are installed, I can keep up soil fertility with compost produced on-site.  I don’t have to kill myself weeding or preparing beds each spring – a 4’x8′ raised bed can be completely weeded, composted, and ready to plant in about 10-15 minutes.  I don’t have to own, rent, or use a rototiller or tractor; I can maintain the gardens entirely without burning fuel other than elbow grease.  Yields are fantastic – I’ve raised squash and potatoes in multiple kinds of beds and the tilled area yielded only about 1/3 the weight of produce of an equal square footage in the other kinds of beds.  I know not everyone can afford to buy materials for a raised bed, but you can often scavenge them (wooden pallets for sides, perhaps?) and they make such a difference in ease and yields that I strongly recommend them to anyone trying to carve a garden out of a lawn.