Which material is best for building Grow-Boxes? And what about gravel in the aisles?

Q. I was telling my husband I needed 8″ Redwood or Cedar for the grow boxes and he asked if it wouldn’t be better to use “Trex” as that doesn’t seem to ever wear out.  Is there a reason to NOT use Trex?  (web site about Trex https://www.trex.com/)  Is there an advantage with Redwood/Cedar that is not there with Trex?

Also, we are getting dirt to put under the boxes and level the area.  They tell me it would be much less expensive to get crusher tailings (rather than topsoil).  Would this be just as fine since I’m putting grow boxes on top or should I get top soil since the roots may go down into the dirt?

A.  Redwood and cedar are good for Grow-Boxes because they last a long time, and do not deteriorate like pine.  However, they are expensive, and it costs much less to use pine and paint it, or even to use treated lumber.  Trex looks great, and will last indefinitely, but I suspect it is very expensive.  We encourage people to do things without incurring any more expense than necessary, so I probably wouldn’t recommend Trex for that reason.

Anything you put down on your soil will be there a LONG TIME.  Unless you want to deal with crusher tailings forever, I wouldn’t put them there.  Paying for topsoil isn’t necessary, however.  I would get any clean fill-dirt that’s available.  On the same subject, sometimes folks want to put gravel or wood chips in the aisles.  Unless you get a lot of rain and your aisles would be muddy most of the time, I recommend you leave them alone.  Just do the nominal amount of weeding it requires to keep them clean, and with no watering, they remain dry, clean, and pleasant.  Gravel, wood chips, etc., can be a real pain to push tillers, wheelbarrows, etc. through, and even walking isn’t as nice as on clean, hard, dry dirt.

Using Dirt in Grow-Boxes – or Growing Right In the Dirt

To further clarify my last post, most of the time we grow right in the existing dirt, with no soil amendments – no matter how “worn out” or “bad” it is.  Whether you have heavy clay or straight sand, or enything in between, unless the soil is diseased, toxic, or under water you can grow a great garden right in the soil.

However, “If for some reason you really must use Grow-Boxes, because you have no ground, but only a driveway, patio, or rocks, (or if you just want the many benefits offered by Grow-Boxes) then go for it. But DO NOT USE dirt in the Grow-Boxes!”  Following are a few of the many reasons not to use dirt in Grow-Boxes.  I recommend you save this information if you are contemplating that method of growing.

Grow-Boxes are designed to give you an excellent growing environment if you can’t grow in the dirt.  They involve an investment of both time and money, and therefore you should obtain the maximum benefit from their use.  If you put dirt from the garden in Grow-Boxes you defeat some of the reasons for having them in the first place, such as:
1) There are always multiplied thousands of weed seeds lying dormant in the dirt, just waiting for conditions favorable for sprouting.  Putting dirt in a Grow-Box creates that favorable environment, and you will have thousands of weeds, instead of NONE as you should.
2) The likelihood is also high that there will be some bugs and/or disease organisms in the dirt you use.  Put it into your Grow-Box, and suddenly instead of a pristine environment, you have the same problems of fighting bugs and disease as if growing in the soil.
3) By putting dirt in your Grow-Box you have the problems of clay soil, too sandy soil, etc., etc. – again minimizing the benefits of the Grow-Box environment.
4)  The weight of dirt in the box is between 2,000# and 2500# in a single 18″ X 30′ X 8″-high Grow-Box if used exclusively, and much more when saturated with water.  This will put substantial outward pressure on the box, and you will end up re-building your box many times over the years.  On the other hand, using sawdust and sand, I’ve seen Dr. Mittleider’s Grow-Boxes still perfectly usable after 25 years!
PLEASE, don’t try and figure out the best methods by trial and error!  It’s been done for you by the best!  Follow the procedures accurately as outlined, and you will have great success.