Plan for a raised bed off ground?

Hello, I have been reading around the forum and saw a thread about tree roots getting in to the bed. Is there a best loved plan for raising a 4' x '8 x 11" bed off the ground? I have three shrubs and a desert tree just waiting for me to put a delicious bed on the ground. I see trouble I would like to avoid. Thanks!

I use table top beds for that reason.  I would make two (2) 4'x4' beds instead of a huge heavy 4'x8' bed.  I started with 3/4" plywood bottoms, tried another material, and then came back to it.  I have a 3.5' x 3.5' bed that needs repairing within weeks.  Another method I have experimented with is using "cementy" cement board on the ground with the bed frame well upon the board.  It comes in 3'x5' sizes which is actually a good size for a bed.  Two pieces of 2" x8" lumber required for the frame.


Just 2-4' away are my neighbor's Italian cypress trees that wanted the Mel's Mix.
The bed was starting to fail so my husband remade the 2'x4' bed and cut the cement board to fit.  It's working very well.    

First experimental bed with cement board:


I lined with durable weed fabric and filled with Mel's Mix. Trimmed the weed fabric to fit.

Would putting 3/4" plywood on the bottom without elevating the bed be enough to keep tree and shrub roots at bay?

The plywood will eventually rot and be infested with termites due to constant wetness.

Ok, the plywood on the ground is out. The cement board is intriguing. I found I could get 4' x 8' boards of that exact board at HD. I also like the idea of a 4' x 4' bed up on three or four cuts of 4" x 4" lumber. Not sure which would be best in my situation. Shrubs and Desert Tree nearby and lots of HOA trees and shrubs on the other side of my wall. Plus there's that pesky bermuda I'm trying to kill. After watering I see many new shoots coming from the darkside. Tried to spray again today but my sprayer decided to go kaput almost as soon as I started.   That grass is pushing my stubborn button.

The only size cement board that I have seen in HD or Lowes is 3'x5'. Looking at the product website, I see they do make a 1/2" thick 4'x8'. It should be the cement board with obvious cement and metal mesh, not the interior backerboard.

Plywood should be supported every 2'.

I see, and it's good to know exactly what to look for. I am very interested in the cement board at this point. How long have you been using it, have you had any problems with it yet, and what do you cut it with?

I posted a photo of the "cementy" cement board I used. It worked well on the ground as an invasive root barrier. I don't like it for the bottom of table top beds. When wet, there is a little sag even though it is supported every 1'. It can be cut with a box cutter just like drywall. Cut deep to score, blend, and finish cutting through it.

I prefer 3/4" plywood with 1/2" drainage holes. I have the plywood supported every 2'.

I am still working on getting my beds filled, trellised, irrigated, and gridded. The first two are built and in place. At this rate I might get the both filled over the next couple of weeks. I will probably build the next two beds in the fall for 2024. They were fun to build. I learned how to use a drill and a battery operated screw driver, how to pick wood from what is out there, and to use gloves when using wood. Ouch. I opted for two 4 x 4' beds up off the ground due to shrubs/trees. I have just spent a few hours this morning filling one bed, but have yet to mix in the compost/manure/worm castings. It's still somewhat hot out there so it's slow going. I do want to thank everyone here who has helped me with all the information for this project. I really appreciate all the help and direction. 

Hello Gimme,

What part of Arizona do you live in ? My concern in S. AZ (Phoenix) would be the bunnies, which are everywhere.

Happy gardening!

Hello Mark, I'm in Chandler. We don't have bunnies that I have ever seen, but I did see a coyote running down the street once. I thought I was crazy until a neighbor mentioned it. 

So, since you live here, where do you find compost? All I have been able to find so far is Chicken Manure, worm castings, and JB Stone compost. I would use a little cow manure but I am a little scared of persistent herbicides. Have you ever used alphalpha pellets?

GimmeShade wrote:
So, since you live here, where do you find compost? All I have been able to find so far is Chicken Manure, worm castings, and JB Stone compost. I would use a little cow manure but I am a little scared of persistent herbicides.

No, sorry I don't live there. But I visit Phoenix (Sun City) quite often. It has bunnies. And coyotes.

Looking at the map, I see that you're just the other corner of Phoenix. I'd really be surprised if you don't have bunnies and coyotes. Do you ever camp out, either on a tent or a porch? You're likely to hear coyotes at night.

My theory is that cow manure is hard to come by across the South West because of all the organic farms in CA buy up the good manure before it ever reaches the retail market.

If you do have a source for cow manure, you can test it by growing beans in it. There's a youtube on it somewhere. If the manure has grazon, then the bean will grow poorly, in a particular manner (ill formed leaves).

Gimme, I love your table top / elevated beds. I want to save the photo to my collection. No way will roots invade your Mel's mix. Is that 3/4" plywood for the bottom? And are the 3 horizontal rails 4"x4"? I made one bed using 4"x4" as a demonstration bed.

Just a caution. If I understand correctly, you mixed the fluffed peat moss and vermiculite and put them in the bed without the compost. ?? How will you add the compost ingredients and mix thoroughly AND get everything really wet?? Folks seem to fall into 2 camps of filling the beds but both have all 3 ingredients mixed first. I make small batches in a cement tub and get it really wet. Dump and repeat. It drains enough to plant the next day. Others mix and put in 1-2" layers and water well. It takes a monsoon or hours of watering and mixing if the bed is filled without being wet first. Remember that peat moss is hydrophobic and repels water until it gets initially thoroughly wet.

GimmeShade wrote: All I have been able to find so far is Chicken Manure, worm castings, and JB Stone compost. I would use a little cow manure but I am a little scared of persistent herbicides. Have you ever used alphalpha pellets?

Alfalfa pellets are not considered compost until they have broken down. I know some folks use them but they are not listed as Mel's Mix ingredients. The fact that you were able to find E B Stone Organic Compost is a major score for AZ. Also, worm castings and chicken manure (hopefully composted chicken manure) is fine. All manures count as one type of compost (manure-based) so you don't have to buy a second one. Poultry manures are higher in phosphates the cow manures so use sparingly.

I'm guessing you weren't able to find Malibu Bu's Blend which is 60% organic cow manure by volume. I am in love with the stuff and I am comfortable with using it as I can't imagine organic ranchers using Grazon-containing grasses for their cows. The stuff is expensive though. $$ I use Bu's Blend as 1 bag with 2 bags of E B Stone to keep the phosphates down.

sanderson wrote:

GimmeShade wrote: All I have been able to find so far is Chicken Manure, worm castings, and JB Stone compost. I would use a little cow manure but I am a little scared of persistent herbicides. Have you ever used alphalpha pellets?

Alfalfa pellets are not considered compost until they have broken down.  I know some folks use them but they are not listed as Mel's Mix ingredients.

Alfalfa Pellets and Alfalfa Meal is a nitrogen amendment for the soil or compost, and not used as part of a soilless grow medium.

Sanderson, please feel free to take the picture. After all, you had a lot to do with the bed building plan. I used a lot of the advice I was given here and generally followed this video https://www.youtube.com/watch?v=KRAieQI4SWo&list=PLtL-Fi5liR5rlpNgpJBP3YeIOpR-nh5Gk&index=57, mostly because I didn't know what the heck I was doing. 
My plan is to mix the composts thoroughly in. I did really soak everything before dumping it in. I was not able to get true Mel's Mix. it was either get what I could afford or not garden. I ended up getting bags of Vigoro garden soil (not full of wood at all and very friable), peat moss, and the composts. I couldn't find the Malibu Bu's Blend, but that's not a surprise anymore. Compost is so hard to find here. I am hopeful that I will be able to grow vegetables in what I could get. It would cost me over $700 to fill the beds and that was a no-go. I am sad about it, but I have to do what I can.

Gimmeshade, would you do me a favor and take another photo of your 2 beds but with the sun in a different position? The shade of the sun just happens to be where the support posts are in the shade and the beds are in the sun. The posts are invisible against the dark weed fabric when viewed in a small photo. I'm using your current photo in a discussion with the small working group of the SFG Foundation. Thank you.

Is this better?

Yes, thank you.

May I ask a favor of you? Would you post a picture of the irrigation set up in a bed? I saw a picture you posted of potatoes laying in a bed. I would really like to see the whole bed. If you are able to do that, I would really appreciate it.

Also, what are you discussing? What not to do? 

Discussing invading grasses, like Bermuda, and tree roots that will come up inside the bed regardless of weed fabric.  The only way to stop them is to create an air gap = elevated beds, even short ones like yours.

Or using Roundup to kill vegetation in the bed area.

Or, using the "cementy" cement board on the ground with the bed set on top of the board.  You checked into it but went with the simple elevated beds.

Edited: vegetation instead of vegetables.

That is a GREAT discussion to have. I went this way because I didn't want to use herbicides, and I know that pesky bermuda, the tree and the shrubs would get in there if there was a way. The landscape fabric on the ground seems to have smothered it. I left it over our very hot summer. I still expect to see a piece of that grass give it a try, if there's any left.