In getting started with your sustainable garden it’s important that you choose the right sizes for the beds or boxes in which you’ll grow your plants.  Spacing your plants within your beds is just as important (see page 262 of The Mittleider Gardening Course book), and we’ll discuss that another time. For today I’ll explain why you will want to plant in soil-beds or containers of 18″ wide, or 4′ wide.
There are important production and efficiency-related reasons for these sizes. Do not make the mistake of thinking any size is just fine, or you will discover that you are not getting the yields you expected, and/or wasting water and fertilierd.  Remember, the “poor man’s hydroponic system” Mittleider Method is a recipe!
Bed widths narrower than 18″ put most plant rows too close together when planting two rows. They also crowd the roots in some larger crop varieties. There’s less available water, which can lead to water stress, and the soil mix in the boxes can dry out faster in hot weather, making it even worse.
Widths wider than 18″ make watering and feeding more difficult and less efficient. For example, placing fertilizers down the center of a box or bed that’s 22″ wide will leave young plants hungry, because their roots haven’t spread far enough to find the food. Applying two bands of fertilizer doubles the work and may still not solve the problem, depending on how well the watering system dissolves and distributes the fertilizers. Also, the water will not reach young plants’ roots as well, and they will suffer from lack of moisture.
Even the size for the 4′-wide beds or boxes has been worked out for maximum yield and efficiency. This size allows for planting 4 rows of most plants, and two rows of vertically-grown varieties.  Some folks mistakenly think they can get by with a 3′-wide box, and they pay heavily in lost yield unless they’re planting carrots or onions. The reason is that most crop varieties need the 2 feet of space between the inside rows for light and air. Always plan for the space your plants will need when they’re mature!  For recommended plant spacing see the Garden Planting Details Schedule, page 262 in The Mittleider Gardening Course book - www.growfood.co/shop.
The 5′-wide boxes demonstrated in Jacob’s first book, Grow-Box Gardens, are no longer recommended for several reasons. First, it’s difficult to reach into the center of the box.  Second, efficiency requires planting across – rather than lengthwise in the bed – and then watering becomes a problem. Watering must be done by hand since the automated watering system doesn’t work well for planting across the width of a 5'-wide bed.
Remember also that aisle widths are important! We recommend 3 1/2′ widths – especially for soil-beds. You can do alright with 3′-wide aisles if you prune properly and continuously.  Aisles less than 3′ usually do not provide sufficient light and air for large crop varieties, and thus reduce yields.  It’s also difficult to get equipment down narrower aisles.
The container depth of 8″ works very well – especially if plants can send their roots down into the native soil. For a patio planter with a bottom – or if planting on cement, etc. a deeper box can be good, to give more room for root growth and to avoid overheating in warm weather. Remember, however, that a deeper box takes more material to fill, which adds to your expense. It also requires more water, and keeping the soil mixture moist is critical to your success. And finally, the fertilizers are distributed throughout a greater volume of soil-mix, so young roots have to search for them.
Benefits of having a deeper box include aesthetics if you’re using your Grow-Boxes in a landscaping scheme. It also makes it easier for folks who have difficulty bending over to work near the ground. Some people have successfully used Grow-Boxes as deep as 3 feet.
Once again, remember that the 8″ depth is least costly to build and fill, and is most efficient for watering and feeding, and then govern yourselves accordingly. For more details, illustrations, and lots of pictures check out the Mittleider gardening books at www.foodforeveryone.org.