Containers – Depth, Width, Materials & Drainage

Q. I’m building a large Grow-Box 3′ high 3′ wide 15′ long. We receive 10-12″ of rain per year; we have mostly clay soil. 1) I’d like to promote better drainage and have a 3″ soil augur. Is it better to fill my holes with a gypsum and soil conditioner mix or a gravel and gypsum? 2) I’m hoping to get some County compost to mix with sawdust, sand, gypsum and clay to fill the mankiller in disguise.

A.   A real drainage problem is almost unheard of when using containers, and especially in the dry country you describe.  If you must dig, fill auger holes with course gravel. More likely, you would want to preserve every drop of water for the plants, rather than expediting the drainage. For example, even without raised containers, by making level, raised, ridged beds right in the soil, and having the planting area an inch or so above the aisles, you will normally solve any drainage problem in the low rainfall area in which you live.

Why is your Grow-Box 3 feet deep? If bending over is a problem, I recommend 18″ or at most 2’.  If you like working in a Grow-Box rather than the soil, and low bending isn’t a problem, consider building it 8″ deep.  And for any depth box, fill with peat moss, sawdust, perlite, and sand, in equal amounts by volume.  Any combination is fine, so long as the sand is 25-35% maximum. 

Also, build it 4′ wide, rather than 3′, if you have the space. This size will give you 4 rows of most vegetables, while still allowing plants the light they need, while a 3′-wide box only gives you 2 rows.  Excellent detailed instructions for building Grow-Boxes are in most of the Mittleider gardening books, available at

You speak of gypsum as if it was a major ingredient in your soil mix.  In a Grow-Box 8″ deep X 4′ wide X 15′ long, apply just 2# of gypsum evenly over the surface of the soil under the Box before you fill with the planting mix. Then, after filling with the planting mix, apply another 2# evenly over the surface and work it into the soil mix.   After each crop apply 2# to the soil mix and work it in.

Perhaps you only meant to use the gypsum in the holes you propose drilling into the clay soil beneath the Grow-Box.  If that’s the case, I would use sand and gravel in the holes.

The Mittleider Gardening books and Manuals teach all you need to know about this subject, and can be purchased in the Store section, or as digital downloads. 

A digital copy costs 30-40% less, and is available instantly!  I HIGHLY recommend you look here for the best gardening books available anywhere!  Get one NOW and be gardening TODAY!!