ChrisInLA
Newly Registered
Posts: 2
Joined: Thu May 05, 2016 9:52 pm

How Deep Should the Soil in My Raised Bed Garden Frame Be???

Hi,

I've been vegetable gardening for a few years and want to re-do my raised beds. My questions is about the optimal soil depth for the new garden frames.

I want to buy or build three boxes of approximately the following dimensions: 3' wide, by 10' long, by 30" high. I want that kind of height so that I won't have to kneel to tend the garden but I'm also interested in the depth of the soil I put in.

I have read Edward C. Smith's Vegetable Gardener's Bible in which he describes the root depths of vegetables often going as deep as 3' so I feel that filling boxes 30' deep with an optimized mixture of loose, hummusy soil would be ideal. But I find Mr. Smith seems to give some conflicting answers as to how deep the loosened bed soil should be and I notice that most commercially available boxes or frames run between 6' to 20' of soil depth. Even the very tall boxes seem to have false bottoms to accommodate a soil depth of no more than 20". So I'm wondering if I should go to the trouble and expense of building a box that has 30" of soil? Will that be beneficial for my vegetables? Is that kind of depth pointless? Or even harmful? Am I, for instance, missing some issue of drainage or soil management?

Thanks for any advise you can share.
P.S. My garden is in a temperate, near coastal strip of Los Angeles (USDA Zone 10B, Sunset Zone 24B) and I want to continue growing a very wide variety of fruits and vegetables.

gumbo2176
Super Green Thumb
Posts: 3065
Joined: Mon Jul 19, 2010 6:01 am
Location: New Orleans

Re: How Deep Should the Soil in My Raised Bed Garden Frame B

A lot of that depends on what you want to grow. Typically, larger plants will require deeper soil to allow root development. Root crops like carrots, kohl rabi, beets and smaller plants like lettuces, chard, spinach, etc. don't require as much soil.

My only raised bed is set on the ground and it is 12 inches high and 4 x 12 ft. in area. In it I grow mostly root crops like onions, garlic, and the few mentioned above.

I know things like tomatoes and okra need much deeper soil. My okra in the ground sends out massive tap roots and side roots that likely reach 3+ ft. in the ground and I would think eggplant and peppers need deeper soil too since they can also get pretty tall and full by seasons end. To add to that, many larger plants need support to keep them from falling over and if grown in a shallow environment, it would be difficult to achieve that. About 90% of my garden is directly in the ground and I stake tomatoes, all pepper plants and eggplant for my spring/summer garden. If I don't do this, the plants will fall over and the fruit will be on the ground and subject to more bugs and rot.

I can't imagine a raised planter 30 inches deep in the size you want to make without being made of some very substantial lumber.

Is there any reason you can't direct sow some stuff in the ground other than the fact you may have some issues with that format?

User avatar
Allyn
Green Thumb
Posts: 485
Joined: Tue Mar 03, 2009 10:38 pm
Location: Mississippi Gulf Coast - zone 8b

Re: How Deep Should the Soil in My Raised Bed Garden Frame B

I so get that you don't want to kneel. I have a knee that I can't put down, so that means no kneeling and it makes it very difficult to get up if I do to get down on the floor/ground. I can manage but it ain't pretty. I was investigating raised beds just as you are, and as gumbo pointed out, a lot depends on what you want to grow. For me, I was considering 16 inches. That would accommodate most of what I wanted to plant and that left me with filling the bottom 16 inches or so with a filler -- sand most likely or gravel, whatever I could get cheap. I didn't want to spend the money to fill the whole thing with planting mix. I never did follow through with that. I discovered the joys of container gardening, though I still have those raised bed plans and there may still be one in my future.

imafan26
Mod
Posts: 11269
Joined: Tue Jan 01, 2013 1:32 pm
Location: hawaii, zone 12a 587 ft elev.

Re: How Deep Should the Soil in My Raised Bed Garden Frame B

If you want to make a high bed for convenience, I get that, it gets harder to bend every day. I suggest you build the bed out of hollow tile instead and fortify with rebar. It can handle more weight than wood and it does not rot. Mine is dry laid so the water seeps out between the tiles. Cap tiles make a good bench.

It is harder to work though since I do have to climb in it to weed and work it. I have a couple of tiles on the outside of the wall for a step.

It will be costly to fill it. unless you plan on filling it slowly. You can lasagna the bottom and just fill the top twelve inches with a good garden mix. After that you just keep adding to the top.

My dad can't bend much in the garden and he has a raised bed so he now plants most of the small things like lettuce, herbs, beets, in a half barrel on a stand and only large plants like the eggplant, cucumber, beans, and bananas are in the beds or in the ground. The bed is 16 inches high (two tiles high). When they need help taking out the bananas or weeding under the trees they call my nephew and he does the digging and pulling and flaming under the citrus tree.

Most of the yard was cemented years ago and there is a 4 x 6 ft patch of grass that is getting smaller all the time. My mom had it for the dog, but they are both gone now.

My dad uses a mini tiller/cultivator to till the soil in the beds.
Last edited by imafan26 on Fri May 06, 2016 2:40 am, edited 1 time in total.
Happy gardening in Hawaii. Gardens are where people grow.

User avatar
Lonesomedave
Senior Member
Posts: 118
Joined: Thu Apr 03, 2014 2:21 am
Location: NASHVILLE, TENNESSEE- zone 6B - 7A on USDA plant hardiness map

Re: How Deep Should the Soil in My Raised Bed Garden Frame B

well....in a perfect world, where money, space, materials and amount of soil are not an issue, yeah....i guess thirty inches would be nice....but we don't live in that world... :mrgreen:

it should be as deep as you can make it, with all those things factored in....i am not really a raised bed gardener, so much as i am a big container gardener, and my problems will be different from yours

for one thing, when you use raised beds, you have the advantage of building your bed on the ground...that means you don't have to worry nearly as much about it drying out....however, within reason, the deeper the soil, the better

one reason for this, especially if you have access to good soil, compost, composted manure etc, is that the plants roots can go however deep they want without any struggle at all....also, you can look at a big bed sort of like a flywheel on a car...that is, it is more forgiving, you can coast a little bit, you have all the nutrients you need and then some, so that you don't have to constantly worry about anything...although by no means am i advising going on auto-pilot

i believe that the average bed, with average good, composty soil, can be as little as 6 inches deep, although i would not recommend that....i say, minimum on one foot, and 18 inches-2 feet even better

if you do that, and fertilize with composted manure, leaf compost, fish extract, seaweed, etc, you can make yourself a bed that, theoretically, you can use forever without your soil getting tired

maybe have it checked by your local ag extension every couple years, and follow their advice on whatever it needs

my one bed is made of concrete blocks, so that is how deep it is....one block...with soil mounded up higher in the middle so i can grow strawberries.....i think i am going to add another just like it (although longer) and add about 2-3 hundred more strawberry plants....but i am using nursery bought soil and composting heavily with leaf compost...and i don't believe strawberries have especially deep roots

one foot to 18 inches would give you some headroom, and basically allow you to grow whatever you wanted.

if kneeling is your problem and you can do it....go for 2 feet....that is about how tall all my containers are and they are a joy to work with....if you want to go for 3 feet, do it...just be aware that that may make tall crops, like tomatoes a little harder to reach

you are wise to make it narrow, as you have to reach in from both sides...i think you could even go to 4 feet and still work it easily from both sides

i think 4 feet wide X 2 feet deep X however long you want would be ideal and allow you to grow whatever you want

/dave/
Fertilizer...Kelp Extract...Compost Tea...Fish Emulsion....Manure (tea)...etc....A little all the time is better than a lot at once... thus endeth the lesson....

ChrisInLA
Newly Registered
Posts: 2
Joined: Thu May 05, 2016 9:52 pm

Re: How Deep Should the Soil in My Raised Bed Garden Frame B

Wow! Thank you all for those very thoughtful, thorough and timely posts. I think I'm going to go for it w/ the 30" depth!

User avatar
Lonesomedave
Senior Member
Posts: 118
Joined: Thu Apr 03, 2014 2:21 am
Location: NASHVILLE, TENNESSEE- zone 6B - 7A on USDA plant hardiness map

Re: How Deep Should the Soil in My Raised Bed Garden Frame B

go get em!.... :mrgreen:

you'll be very happy, and literally, the only drawbacks i see are the tall vegetable issue i mentioned earlier ( and, of course, the cost).....with thirty inches, the plants will think they're in the ground, and drying out should never be a problem.....please use great soil, and you will be set up for years,,,, one foot of great soil is better than 3 feet of chirt (or is that churt?....chert?)

/dave/
Fertilizer...Kelp Extract...Compost Tea...Fish Emulsion....Manure (tea)...etc....A little all the time is better than a lot at once... thus endeth the lesson....

gumbo2176
Super Green Thumb
Posts: 3065
Joined: Mon Jul 19, 2010 6:01 am
Location: New Orleans

Re: How Deep Should the Soil in My Raised Bed Garden Frame B

ChrisInLA wrote:Wow! Thank you all for those very thoughtful, thorough and timely posts. I think I'm going to go for it w/ the 30" depth!

You would do better cost wise if you bought the garden soil in bulk from a local nursery or company that sells topsoil, garden soil and mulch products. But not all companies are going to process this stuff exactly the same as I found out a couple years ago. The first place I got garden soil from had some fairly large organic matter in it that took a while to decompose in the raised bed and I found it lacking in nutrients when some of the plants were struggling a bit.

I removed some of it and spread it over my in-ground garden and topped the raised bed off with a small load of better soil and that made a big difference.

If local nurseries don't sell garden soil, then they probably have a leg up on the places that handle quality stuff and it would likely help if you checked with them.

User avatar
jal_ut
Super Green Thumb
Posts: 7480
Joined: Mon Jan 19, 2009 3:20 am
Location: Northern Utah Zone 5

Re: How Deep Should the Soil in My Raised Bed Garden Frame B

I am not sure if you are going to put a bottom on the raised bed or leave it open to the ground underneath?

What is the ground under like? Will grass, trees and weeds grow in it? If so your garden plants will too. Plant roots go deep so it is best to not put a bottom on a raised bed and let the roots go as deep as they will. The roots of many of our garden plants go down 3 to 6 feet deep. Have fun!
Gardening at 5000 feet elevation, zone 4/5 Northern Utah, Frost free from May 25 to September 8 +/-

imafan26
Mod
Posts: 11269
Joined: Tue Jan 01, 2013 1:32 pm
Location: hawaii, zone 12a 587 ft elev.

Re: How Deep Should the Soil in My Raised Bed Garden Frame B

If you have an open bottom like Jal-ut says, the roots can always go deeper into the soil. Unless you are growing something like gobo or parsnip, most of the vegetable roots will be in the top foot of the bed because most of the vegetables are annuals and they are short. The tall things like tomatoes, peppers, eggplant, cabbages and long root vegetables benefit most from deep soil. Remember too even if you make the bed 30 inches deep, it will compact over time and most of the time you will only be able to till and amend the top 6-8 inches and the rest of the soil gets nutrients by trickle down. It is also why most of the feeder roots are near the surface and what goes deep are usually support roots.

I talked to my friend who is an arborist about how deep tree roots grow. He said even big trees have most of their roots in the top 3 feet of soil but they spread out as wide as the unpruned canopy and beyond to provide a stable base. He said below 3 ft there are fewer nutrients so the roots stay in the zones where there is the most nutrients and water available. He said roots will go deeper if there is deeper water which is why you don't plant trees near utilities and why so many sewers are infiltrated with tree roots.
Happy gardening in Hawaii. Gardens are where people grow.

gumbo2176
Super Green Thumb
Posts: 3065
Joined: Mon Jul 19, 2010 6:01 am
Location: New Orleans

Re: How Deep Should the Soil in My Raised Bed Garden Frame B

imafan26 wrote: He said roots will go deeper if there is deeper water which is why you don't plant trees near utilities and why so many sewers are infiltrated with tree roots.

My now late father-in-law called me once when his sewer lines were backing up. He was in his late 70's at the time and couldn't handle all the digging in his hard clay soil to find the problem. We first tried to unclog it with a snake and would always run into something in the same spot in the line.

Out came the shovel and pick and I dug the line until we found the problem. A nearby tree had its roots infiltrate the terra cotta drain pipe and totally clogged the line. I wound up taking out several feet of terra cotta pipe and replaced it with heavy duty PVC drain line and not more problems. Roots love that old terra cotta pipe. Of course this just had to be the main line from the toilet and that was one nasty experience. I don't blame plumbers for charging what they do.

Return to “Vegetable Gardening Forum”