Raised Garden Bed Height - 18 inches tall enough?

skinnylegs · Mon May 26, 2014 5:51 pm

I will be constructing my raised garden "containers" this coming week. I'm going with (6) 8 x 4 containers. They will be sitting on decorative rock over clay-compacted soil. Is 18" tall enough? Do I need to use a treated wood?

Thanks in advance!

rainbowgardener · Mon May 26, 2014 6:07 pm

18" is tall enough for most vegetables you would grow, including tomatoes, as long as you give them plenty of horizontal space.

Untreated wood is hard to find and treated wood will last a lot longer. Some people worry about the chemicals they are treated with, but they are relatively non toxic these days.

I made mine by stacking 4" x 4" pine fence posts. They are very solid that way and last a lot longer than boards, which warp and rot after a few years.

Here's a picture of a couple of them:

They are now about 12 years old and still going strong. Held together with steel rebar pounded down through the stack.

skinnylegs · Tue May 27, 2014 11:28 am

Stacking pine fence posts - I love it! Great idea! Thanks!

skinnylegs · Tue May 27, 2014 2:10 pm

I noticed that you have stacked (4) 4 x 4's, for a dept of 16". I was thinking of going 18", which would require a 5th 4 x 4. I'm wondering if this 5th 4 x 4 is even necessary? I'm guessing that 16" is an adequate depth for a raised garden?

rainbowgardener · Tue May 27, 2014 2:36 pm

I do grow tomatoes and any other veggies in mine and they sit on top of a concrete patio, so no soil underneath them.

feldon30 · Tue May 27, 2014 6:31 pm

skinnylegs wrote:I noticed that you have stacked (4) 4 x 4's, for a dept of 16". I was thinking of going 18", which would require a 5th 4 x 4. I'm wondering if this 5th 4 x 4 is even necessary? I'm guessing that 16" is an adequate depth for a raised garden?

I'm a little confused by the "decorative rock over clay-compacted soil" part of your original post. If the raised beds are built directly on the underlying soil (even if it is of poor quality), then you can get away with 12" high beds. But if you are using stone to segregate your raised beds from the underlying ground, then you may want to go the full 18" for tomatoes. I only have personal experience with raised beds over gumbo clay soil in Houston, and containers.

skinnylegs · Wed May 28, 2014 9:01 am

I guess what I was trying to say is that the containers may as well be on concrete because the soil they are sitting on has a lot of clay in it and the area is covered with decorative rocks. I think I'll go with (5) 4 x 4's just to be safe. This will give me up to 20" of soil depth.

Gardener123 · Wed May 28, 2014 3:36 pm

skinnylegs wrote:I noticed that you have stacked (4) 4 x 4's, for a dept of 16". I was thinking of going 18", which would require a 5th 4 x 4. I'm wondering if this 5th 4 x 4 is even necessary? I'm guessing that 16" is an adequate depth for a raised garden?

All mine are 12" tall, and even my tomatoes do well. So, if you ask, 16 is better, but not needed.

rainbowgardener · Thu May 29, 2014 6:58 am

Gardener123 are yours on top of concrete or dirt? Makes a big difference! Even if your soil is poor, having soil under there still allows for rooting down into it. 12" is plenty for beds that are sitting on top of soil.

Gardener123 · Thu May 29, 2014 10:54 am

Almost like concrete, lol. All the topsoil from my property was removed before they built our neighborhood..... The soil doesn't feel like soil, by any means.

skinnylegs · Fri Jun 06, 2014 3:44 pm

Constructed my first container last weekend. As noted in an earlier post, I used 4 x 4 posts and 2 x 4 siding. All redwood. Tomato plants went in yesterday. Time permitting, I will build the other 7 containers this weekend. BTW, I ended up taking the easy route and bought vegetable garden soil from Home Depot. A bit pricey but it should get the job done nicely.

Thanks to everybody for your help!

rainbowgardener · Fri Jun 06, 2014 4:27 pm

Your box looks nice, but I don't quite understand. You put the siding on over the posts? Why?

Sorry, but you might have asked first about what to put in it (soil). A box like that is essentially a giant flower pot. Like other flower pots, pure garden soil (aka dirt) doesn't do real well in it - kind of dense, tends to compact over time, holding too much water and excluding air. I would have mixed your garden soil, with some peat moss, and some kind of mineral element to keep it loose and free draining - perlite, vermiculite, crushed granite, etc.

skinnylegs · Fri Jun 06, 2014 4:37 pm

I put the posts on the inside for aesthetic reasons. I like the clean look on the outside. In terms of the soil - they had me at "bat guano." LOL

grrlgeek · Fri Jun 06, 2014 5:43 pm

That's the Kellogg's organic Raised Bed mix, a new product in 2014 and they're making it exclusively for Home Depot. It's really good and I use a lot of it. I don't think it's got the OMRI cert on it yet, but in talking to their rep, I learned the paperwork is in, it just hasn't completed yet. They had to sticker over the OMRI logo on all the bags until they get it certified.

It's got a mix of organic material, peat moss, perlite, and trace amounts of NPK. I still blend in more peat, perlite, and compost, and I don't use all Raised Bed; I add some of their Patio Plus mix and some of sandy soil from my yard too. Like all the bagged products, it's a little heavy on the forest products and I have reservations about it tying up N while they break down, but so far in this first season using it, I have been pleased with my plants' progress.

You definitely want to add some compost though, or pay close attention to nutrient levels. You will definitely have to feed your plants without any amendments. Along with compost, to keep it from getting too compact and soggy, I would add more perlite as well, there isn't very much in the mix.

A bed that high should compact down several inches after you get all the mix wet. Plenty of room to add compost and more perlite and work it into the top 6." Check out the EcoScraps compost if you don't have any homemade. The Kellogg stuff has way too many wood chips to be called compost in my book.

That's a beautiful bed you got there! Happy growing!

skinnylegs · Fri Jun 06, 2014 5:59 pm

Thanks for the reply! I have a buddy that has about 40 yds. of compost so I'll add that in as settling occurs. I will also let you know how the tomatoes do. Thanks again!

rainbowgardener · Sat Jun 07, 2014 8:20 am

OK... that's a product I didn't know about and might be good. You just said "garden soil" before. What they sell in bags as "garden soil" specifically says on it do not use in flower pots. If you are lucky it is good topsoil not just fill dirt.

You have plenty of room left in your box to plant some other stuff with the tomatoes, like basil and peppers.

grwrn · Sat Jun 07, 2014 8:24 am

Thanks for the info on the raised garden bed soil. I didn't know of it yet as I usually don't shop at Home Depot but I guess I will now.

skinnylegs · Sat Jun 07, 2014 1:23 pm

rainbowgardener wrote:OK... that's a product I didn't know about and might be good. You just said "garden soil" before. What they sell in bags as "garden soil" specifically says on it do not use in flower pots. If you are lucky it is good topsoil not just fill dirt.

You have plenty of room left in your box to plant some other stuff with the tomatoes, like basil and peppers.

I was thinking about adding cilantro and basil to the box. Like maybe 4 plants? What do you think? I don't want to crowd it too much.