Besides the sheer cost of trying to put 20-24" of soil into a box you're going to need some serious support for that lumber to hold all that weight pushing on the sides...especially after a big rain.

Most plants rarely get past the 18" mark with a the bulk of their root system except for taproot veggies and stuff like corn (though corn, with support in windy areas, will do fine in less than 18").

Will it work...yes, but it's overkill and you need to be careful about choosing lumber and support for your boxes that won't need replacing in 2-3 years from splitting.

...and don't forget to leave a good 2-3"+ in your boxes for adding mulch because they will loose their water fast in the dead of the summer.

Btw, for a general refenence...whenever you read something about how corn, tomatoes, etc. can set roots 3' or more in the soil almost all the bulk of the root's work is going on in the first 1' with bleed-over into 1-2' area if given room.

For most plants anything after the 2' mark is negligable and many plants can get away with 1' if given enough horizontal space.

Hmmm.... hadn't thought much about the boxes warping and needing more support when they're that full, but that makes sense. I'm using smooth redwood 2x12's and they're going to be 4' by 8' max. One box will be 4'x4'.

But you make a compelling argument for just 12" deep. I supppose I could do 20" deep boxes, and fill them to about 15-18" deep. I am not growing corn, but lots and lots of tomatoes which I know like to grow deep.

Thanks so much for giving me something to think about.

Actually, just thinking through your feedback, I am probably going to just do 16" beds (2 rows of 2x8's), or maybe 20" beds (2 rows of 2x10's). They'll still be somewhat deep, but hopefully not overkill.

Structurally speaking, they'll be attached to 4x4's in the corners and the sides are only 8' long... so I am thinking these won't bow out too badly. If they do I can always put a support post half way through the 8' length.

Thanks again for your feedback.
-k

"Btw, for a general refenence...whenever you read something about how corn, tomatoes, etc. can set roots 3' or more in the soil almost all the bulk of the root's work is going on in the first 1' with bleed-over into 1-2' area if given room."

On the page 585 of the Sunset W. Garden there is a table showing root depth of some of the garden plants. It shows that tomato roots go to 10 feet, deeper than some fruit trees and even grapes.

"On the page 585 of the Sunset W. Garden there is a table showing root depth of some of the garden plants. It shows that tomato roots go to 10 feet, deeper than some fruit trees and even grapes."

That's a pretty extreme example. A lot...if given enough space and the right soil...can easily hit 3-4+ ft. into the soil as a mature plant. However, almost all the water/nutrient uptake and the root mass is in the top 1-1.5 ft. of the soil.

Main roots and their laterals can go many feet...those little wirey roots which uptake most of the plant's goodies don't tend to produce in huge amounts after 2' down compared to what's up top.

It might also be worth mentioning that in sandy soils common in the Southwest a plant like a tomato would be prone to sending down a larger set of roots because of how much faster the soil drains.

The soil you start with plays a big role in how much searching out the roots do in some plants unless the plant is already prone to growing in hot/dry areas (like okra which has a shallow root zone spreading more wide than deep).

A box taller than maybe 18" becomes a retaining wall. You could line the bottom of your beds with 1/2" square galvanized fence to keep burrowing animals out, but allow roots through.

kristimama

Another thing you might do to lessen the over-all depth of your bed but provide extra root space for plants that need it is to use five-gallon buckets (or other containers with good depths such as large Totes) with the bottoms cut out and placed on top of your soil in the beds. Then fill the containers to the top with more more of that good soil and set deep-rooted plants into them. There would be only a few vegetables that would require this kind of root-room, tomatoes being one of them, and the rest of your plants would probably be fine in 12 inches of soil.

Let me say this about clay soil. (Here in Georgia we have red clay soil that can bake to bricks in the summer.) If clay soil is under a soil mix or is heavily mulched and stays moist, there is no soil richer in plant nutrients or so vigorously burrowed into by many vegetable roots as long as they have sufficient root-branching in friable soil above the clay. You still have to fertilize if a soil test indicates it's short on something, but it almost always has lots of micronutrients that other soils don't. If you can find a place where you know the soil is uncontaminated, I wouldn't let the fact that it's clay deter me one bit from putting a raised bed on top of it and letting the roots burrow in it all they want. Bring in earthworms, too, since they help make clay soil porous and more friable.

No need to bring in earthworms. Just load on the organic matter (leaves, lawn clippings, etc) where you plan to build the bed and they will find it. When they do, they'll text, tweet, and Facebook all their friends to come too.

hi well your first idea of 24 inch high beds sounded great to me, but yes u will need to put in some support posts,,,here is my suggestion, why not take several of these above ideas and trying several of them out, then over the years you can actually observe if one method is out producing the others and modify the bed height to the best for your site,,, i do like the suggestion of putting wire under a deep bed, it might keep somethings at bay.

Hi everyone,
Well, I guess the more I talk to other gardeners, I realize there's not consensus on any one approach. But it's how I learn, and I hear all the "why's" and "how's" so then I can make up my own mind of what will work for me. So I really do appreciate your feedback and help.

Now, I just need to get out there and start doing. So I decided to make them 16" high (2 rows of 2x8's), and dig down with a rich amendment about 12" below ground. The deep growing things like tomatoes, they can grow down if the want or need to.

And I suppose if we want to make them deeper I can always add another row of 2x8s next year.

Thanks!
-k

You are planning the bed heights using the nominal lumber sizes. You need to know that the actual planed dry lumber dimensions are smaller.

4" ===> 3-1/2"
6" ===> 5-1/2"
8" ===> 7-1/4"
10" ===> 9-1/4"
12" ===> 11-1/4"

Since I am frugal, I would determine the cost of the lumber per aquare inch (sq. in.) for the different board widths. Use an 8' or 96" length for calculation purposes. That is, for a 2x4 --- 3.5" x 96" = 336 sq. in. Divide this into the cost of the board to detemine the cost per sq. in. You can also check on the cost by board length as well. Sometimes longer boards are cheaper than shorter boards on a per unit basis.

It is advisable to have some extra room in the box. That is, the soil should not be right to the top.

If you decide to use hardware cloth at the bottom to keep out gophers, then use a mesh size around 1". This should still keep out the gophers and be less costly.

Also think about having the corner and other supports on the outside of the box. The support will last longer and will not be in the way when digging.

kristimama, my only really sunny area is in a part of my yard that has solid rock ledge, so I went with raised beds. I used cinder blocks, and I love them. Pretty much ugly as sin, but very functional and the two levels I used provides enough depth for my plants. And they are quite sturdy/stable, even on my uneven ground. For extra stability you could drive some kind of post through the holes here and there if you wanted.

{{gwi:12838}}

Hope this helps!
:)
Dee
P.S. The one-level bed you see off to the left is in an area with more soil underneath, so I went with just the one level. It really is a bit easier on the eyes, if not the back, lol, to have one level. A bit less intrusive to the landscape, IMO.

What would be really nice is if you could get colored cinder blocks and put some sort of cap stone on top.

I have had only marginal success with the shallower beds

Why do you think this is?

jeremyjs, when I was looking into doing the cinderblock beds, I saw (I think here on GW somewhere) a post by someone who did not paint the cinderblock, but had terracotta-colored, flat capstones on theirs, and they (the beds) were arranged rather formally (I think it was a long, squared-off "C" shape. They were gorgeous! He may have even had an arbor in the middle of it.

Of course, his were in the middle of a nice lawn, and he used them I think for flowers, and he also had nice, flat land, unlike my uneven ledge, which made for a nice, even, level bed, but it looked unbelievably nice for cinderblocks.

I think I may have saved a photo of it, although I don't know the rules and/or etiquette of posting other people's pictures, so maybe I shouldn't post it...

:)
Dee

JaGuy... I've missed you. :-)

I completely admit it was probably user error. I prolly didn't water enough or evenly enough for the constantly hot temps on that wall to wall concrete patio with 6" boxes with the plywood bottoms. i just think the water was evaporating quicker than I could work with.

Like you have said on another post somewhere (see I remember everything you say)... intensive is intense.

Anyway, whenever I did have success in the 6", and then yanked out the roots to see how far they went, they almost always were colonizing all the way to the plywood bottoms, and it was clear they didn't have enough depth to grow effectively in. And since what I'm growing out there is tomatoes, beans, and squash... all deep feeders, I think they need more than 6".

What do you use for your depth?

Thanks,
-k