It'll rot down fairly quickly but hay or straw would be easy to handle and cost effective.

Thanks for the reply, Veggievicki. I am also worried about ants and bees, since water in New Mexico attracts things. So I'm pretty sure it will be filled with bugs of one sort or another.

Our local garden store said to fill it with mulch and then put dirt on top. I think that is my wife's plan, unless I get strong advice otherwise.

Thanks again!

3.5' high is pretty high for a raised bed. How high does the water get in your yard when it floods? In New Mexico it seems like the water would go down really fast. So you could probably make it a lot shorter. Like mentioned above hay would make a good cheap filler in the botton just put whole bales down in the bottom then your soil on top. As it decomposes and sinks down you could add more soil on top. The mulch would work it would just be more expensive.

Hay or straw bales would be my suggestion too and as an even less expensive solution check out the thread here on keyhole gardens (a hot climate specialty) where the base is filled with branches and cardboard.

My only concern with using the mulch - other than cost - is it can bind up the nitrogen in the soil above it as it decomposes.

Dave

If you fill it will anything that will allow ground water to wick up to the top, you have defeated the purpose. Things like concrete and sand wick very well.

So there is no point on your property that is higher than flood water?

You could build a giant planter and put it on concrete blocks to get it off the ground. I am not sure if I would use plywood for the bottom. If you do, you probably need to line the whole planter with a pond liner or something (if the plywood stays in continual contact with damp soil, it will rot out pretty fast). You will need to provide a bunch of drainage holes, also to keep the soil from getting waterlogged. If you are going to buy soil to fill it with, consider the cost of the soil. The deeper you make it, the more the soil will cost. I would think a full 12 inches would be deep enough for almost anything. Soil is very heavy (even the lightweight potting mixes add up, especially if they are wet), so you will need a lot of concrete blocks to distribute the load. And once you fill this thing you aren't going to be moving it, obviously. If you envision it this way, as a planter raised up off of the ground, then you will certainly be safe from the ground water contamination. Also, it may actually be quite convenient to work at a nice high level when planting and weeding, etc.

--McKenzie

Hence we have built a huge box, 16' x 4' x 3.5' high.
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That box will waste a lot of soil (IMO). For (most) plants you don't need more than 1.5 ft depth. If I had to do it, I would make the box about 18" deep, with a bottom and support it on (like) cinder block every couple of feet. This way the box will be protected from the flooding under neath. JMO

If standing water is really that common a problem for you, then I can understand why you'd like to put down crushed concrete at the base for drainage. Presumably you could use gravel or pea stone as well - whatever is cheapest. I'm not sure that 1 foot of soil is enough to grow veggies satisfactorily. Maybe it would work for some shallow rooted crops, but I'd definitely want a thicker layer of soil than that.

If you are going to layer soil on top of recycled concrete (or gravel or whatever), you'll need to put a layer of good quality landscape fabric between. Otherwise the finer soil particles will wash down into the gravel and take up all the void space, which would defeat the purpose.

I'd say 8-12 inches of gravel at the bottom, then landscape fabric, then 2 feet or so of soil, leaving room for a layer of organic mulch such as straw or whatever at the top.

I can't envision insects being a major problem, but I've never lived in NM, so maybe critters are different down there than what I'm used to. If you wind up with an ant problem, maybe just treat with an organic insecticide?