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ideal depth for raised beds

 
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has anyone done any experimenting with what would be the ideal depth for raised beds? how deep should they be to give plants optimal root growth and not allow any weeds from popping up from native soil from bottom. ive had such serious problems with native grasses and weeds was thinking of trying raised beds filled with bales of potting soil
 
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Our raised beds are based on the avaiabilty of the size of lumber.

Two 1' x 10' will make a 12" high bed that is 5' x 5' x 12".

Twelve-inch deep beds are plenty deep for most vegetables.

Most garden beds are built over a base of dirt so most bed heights still give the plants room for their roots to go deep down.

We once had a bad idea of using a kiddie pool for a garden bed.  Bad ideas as the plants were root bound so the next year we cut the bottom out of those kiddie pools.

Another consideration for garden bed height is age.

Older folks may have trouble getting down on their knees to work on low beds.

Then there are folks with back problems that need higher-height garden beds.

Unless making beds over concrete or bedrock most garden bed heights are okay.

I am looking forward to what others feel are good garden bed heights.
 
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Six to twelve inches is the standard for good root room for your vegetables.   Avoiding weeds coming up from the bottom will depend on what weeds you are fighting.  My worst weed is our grass and we find we need to dig weed barriers six to eight inches underground and a few inches above the soil all around the garden.  We use aluminum flashing for this purpose and a rock border to protect the easily bent raised portion. That border also protects our easily cut skin from a rather sharp edge.  

These are in ground beds so weeds are struggling through compacted lawn to get deep enough to rise again.  I have seen our grass send runners 5 feet straight up through a mulch pile which is why most lasagna gardens start with cardboard. If you aren't clearing the ground before building the bed you might want to consider that also.

Edit:  inches underground not feet! We're near bedrock here.
 
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I've done a few different types/heights, & it really just depends on what you want to grow there for root depth.  I do like the 1"x12" boards on edge as mentioned above.  I've successfully grown potatoes in this setup for a few years.
 
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I think everyone's advice is right on.  A foot is plenty.  I am experimenting with building some beds that are 3 feet high just to keep from bending or kneeling as Anne mentioned.  These beds are kind of a long term experiment for me.  I'm making them by building compost piles and continuing to add material as they sink.  I have no idea how long it will be until the are the proper height.  My plan is to make a couple hugel kulture style to save on materials.  I don't know if wood in the bottom will hold more moisture than pure compost will.  I guess I'll find out in a few years :)
 
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Trace Oswald wrote: I am experimenting with building some beds that are 3 feet high just to keep from bending or kneeling as Anne mentioned.  These beds are kind of a long term experiment for me.  I'm making them by building compost piles and continuing to add material as they sink.  I have no idea how long it will be until the are the proper height.  My plan is to make a couple hugel kulture style to save on materials.  I don't know if wood in the bottom will hold more moisture than pure compost will.  I guess I'll find out in a few years :)

I decided on 30" high for exactly this reason - that's standard table height, and I'm not a particularly tall person. The two newest beds I built are ~4'x6.5' because I based it on some pallets I was able to get. One of them has quite large rounds of punky wood in the bottom, the other has a bunch of miscellaneous scrap wood. They both have compost mixed with crappy dirt on top and both sank a fair way over last season and I didn't manage to top them up in the fall (last fall's weather was crappy here.) Where most people here have to water veggies once/day to keep them alive, mine will go 2-3 days generally. I suspect that's a combination of compost that's got some biochar mixed in *and* the punky wood.

However, what's significant to the OP - there are weeds that have still come all the way up - particularly Morning Glory, but also Himalayan blackberry. However, with this tall a bed, it's much easier for me to knock them back a fair way. They're both thoroughly invasive to my garden area, so it will be a constant job to try to catch them small and pull out as much of the underground bits as I can. I use various techniques to make sure they don't reproduce further, but I suspect that with the Himalayan Blackberry, it's a loosing battle. At least the berries make great jam, crumble, ketchup, animal feed...
 
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We have short beds and tall beds and I have a few rhizomenous (I don't think that's an actual word) weeds that regularly work themselves up through 3' of dirt. The short 12" beds do better against those weeds because they have raspberries in them and they outcompete the weeds.

My dad has 4' tall beds and he doesn't have any weeds coming up into his beds.
 
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Like everyone is saying the typical permaculture/garden answer is it depends.  What do you want to grow? How much material do you have. How tall do you want it, what climate are you growing in?  All info is relevant.  I can say I got a raised bed kit on clearance years ago, it was about 4" deep, and that isn't deep enough. I attached fence boards to the top making it about 12", and as others have said I can grow anything in it. That being said it's my least favorite bed.  I live in a dry, hot place, and it needs to be watered more than my other beds.
My favorite and what I call hugel beet beds. I dug 2' under ground, and two cement blocks high above ground making it about 40" deep.  They are filled hugelkulture style (large wood,soil, branches and small chunks of wood, soil, compostable stuff like leaves, kitchen waist, cardboard, stuff like that, soil, wood chips, topped with organic soil and compost.) I love these beds and will eventually convert all my inground beds.  They just outperform all the others and need less water.  
I also like the raised beds I built. I made them  about 3'  high I'm short, so it makes them so easy to use.
As far as keeping weeds out my inground beds rarely have weeds. I think I went deep enough I got most of the weeds. I know permies people will not like this, but I put down a good quality weed cloth under the cement blocks. This combination has saved me a lot of work.  I made the raised beds with hardwire cloth on the bottom so I can put it on bare ground if I want, but at this time it sits on weed cloth.
So the answer is decided what suits you best, and go for it.  Good luck.
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Pallet raised bed
Pallet raised bed
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