French Intensive and 24 inch deep raised beds?

kristimamaMarch 27, 2009

Hi everyone,

I realize a lot of people here are devoted to Mel's Mix and Square foot Gardening ... BUT, I am hoping that someone who has tried or uses French intensive or deeper beds can give me some insight into their gardening, too.

I tried SFG last year with mixed results. I love the idea of intensive growing. But overall I think the 6" of planting depth simply wasn't enough for my particular growing situation (i.e. Raised beds/plywood bottoms on wheels over extremely hot concrete patio that got to 100 throughout summer.) I also think I didn't water effectively, so there's also the user error component. :-)

I also generally like Mel's mix but prefer more than 1/3 compost.

Anyway, I'm getting ready to build more boxes over actual ground (i.e. in contact with native soil) and am debating how deep to make them.

Reading a bunch of the "French Intensive" references, they say to "double dig" to a depth of 24" so roots have a lot of room to grow down. Even when you build a 12" box/frame, the assumption is that the roots grow down into the native soil below the wooden border as they need to.

Well, in my case, for a variety of reasons, I really don't want to use my native soil. Not the least of which is a serious gopher problem, so I need a distinct "box" above ground that I can attach the hardware cloth to. But also, we did some excavation of the yard for some construction last year, so the soil has been compacted and is a lot of hardpan and compacted clay. And finally, long story short, there may be a smidgen of a chance of site contamination... our house was owned by am old jeweler who did a lot of work here on site and I think he may have "tossed" some of his waste into the ground out there over the decades. It's just a suspicion, but I'm reluctant to grow too close to that soil.

So here's my thought for trying more of a "french intensive" approach, but above ground. I would like to build 24" deep boxes (2 rows of 2x12's), so the boxes are deep----plus the 24" is a nice height for my back.

I KNOW Mel advocates shallow depth and 24" might seem like blasphemy, but I'm wondering if anyone has tried it? Or anything more than 12". (I'm getting contractor's pricing on the lumber so the price difference between 2x12"s and 2x10"s and 2x8" is minimal.)

For the soil, I'm bringing in a rich mixed compost from a local soil company that sells by the Cubic yard, which is a very affordable option. I may or may not mix in the vermiculite... I may see after I grow in it a season.

I know the bottom 12" will compact over time, but I can keep the top 12" loose and compact with regular amendments of compost.

So, am I completely nuts? (OK, don't really answer that. LOL) What I really am asking is, can boxes be "too big"? Are there any potential pitfalls of a deep box? I realize some people will say it's wasteful and unneccessary, but aside from that fact, will it work?



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24 inches seems excessive. Roots rarely go that deep (except for trees). I would remove any sod, loosen the existing soil with a garden fork, and lay chicken wire over the entire bottom to keep out critters. Then build a 12-inch raised bed with lots of compost. Be sure to do a soil test to chech the pH. I got a great deal on 2 x 10 untreated spruce from a reclaimed building supply yard.

    Bookmark   March 27, 2009 at 11:56PM
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My beds are about 15-16" tall and filled lasagna style. I don't know for sure how deep roots go, but I think most annual plants don't put down really deep roots. I pulled a sampling of plants in the fall to see how deep they went but mine were all pretty shallow. I don't think I was watering deeply enough though. I can say that there were moments when I was filling them that I wished I had built shorter ones... :-)

I think you'd see a huge difference from planting in a 6" box over concrete to about any depth on ground.

Here is a link that might be useful: Tales of a Transplanted Gardener

    Bookmark   March 28, 2009 at 12:23AM
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I know many here have excellent success with the 6 inch beds, but I went with 8 and things didn't really go well for me. I increased some to 12 inches this year and things are really taking off.

    Bookmark   March 28, 2009 at 7:19AM
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dan_staley(5b/SS 2b AHS 6-7)

. Roots are genetically predisposed to go as deep as soil tilth allows. The more soil, the more of a water bank you have.


    Bookmark   March 28, 2009 at 10:24AM
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snibb(Salt Lake City)

Just because you have 12 or 18 inches of soil doesnt mean your roots will go down that far. Roots are predisposed to going down as deep as they need to go for water and nutrients, hence the old traditional single row mentality. Thats the reason SFGing works only in 6" of soil-but its got to be the right kind of soil. Why does a root have to go down more than 6 or 8" when all of the needed water and nutrients are right there? Like I said, I guess I just must be a lucky gardener-I only grow in 6-8 inches and have done so for 9 year very successfully....

    Bookmark   March 28, 2009 at 10:39AM
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Snibb, do you grow parsnips?


Here is a link that might be useful: Annie's Kitchen Garden

    Bookmark   March 28, 2009 at 11:25AM
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dan_staley(5b/SS 2b AHS 6-7)

Why does a root have to go down more than 6 or 8" when all of the needed water and nutrients are right there?

It doesn't. It is not about need. And if there is 24 in, there is that much more water and minerals in solution for the plant. And the volume is a better temp buffer.


    Bookmark   March 28, 2009 at 12:17PM
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snibb(Salt Lake City)

Granny...sorry, I have never grown you? I just dont know what to use them for.

"It doesn't. It is not about need. And if there is 24 in, there is that much more water and minerals in solution for the plant. And the volume is a better temp buffer."

Some folks like to make it complicated. Thats one of the points of SFG-80 less water and 100% of the yield. Its "reduce" in SFG. You can go and plant your 24" while I stick to my 6-8 inches. And, your garden will not grow any better than mine, but, I will use less resources. How many pictures of successful SFG's do you need to see before you come to the conclusion that 24 inches is not necessary? Like I said, I must be a lucky SFG'er.

    Bookmark   March 28, 2009 at 1:06PM
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KMama, a good resource for intensive gardening is anything by John Jeavons or Eliot Coleman. Jeavons is an advocate of double digging which addressses the 24 inch question and both are proponents of sustainable agriculture. I've tried Mels method, and about all the others I can think of, and think that Jeavons approach gets better results for me. Double digging is a lot of work however, especially in the clay soil out here on the plains.

    Bookmark   March 28, 2009 at 1:28PM
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Snibb, I just asked, because parsnips can get way longer than 6-8 inches. I'm going to try them in 11" of soil this year, but in bottomless beds so the roots can go deeper if they need to. I just don't think 6-8" of soil depth would do it.


Here is a link that might be useful: Annie's Kitchen Garden

    Bookmark   March 28, 2009 at 4:45PM
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snibb(Salt Lake City)

If they get that long, they probably wouldn't do well. But, what you could do is use one of those "high rise" boxes that Mel talks about on his site. Its a way of still using 6 inches for most of your bed and then it gives you an option of putting in things like you want to grow-parsnips, or carrots as well...

    Bookmark   March 28, 2009 at 6:22PM
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Victory Garden had a thing a couple of years ago on parsnips. There are contests in England to grow the best ones. The gardeners had them in chimney tiles about 36-40 inches long. The individual parsnip filled the whole length. I have grown them. They may get so long that you will have to break/cut the root to harvest.

    Bookmark   March 28, 2009 at 6:32PM
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I have seen them grown in sections of sewer pipe filled with sand. I'm not looking for huge ones, just nice sweet ones. They have to stay in the ground until after it freezes to be really sweet, but they are oh so good! Like I said, my soil is 11" deep, and it is sitting on sandy loam, so I'm going to try it.

    Bookmark   March 28, 2009 at 10:06PM
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If you are not going to have your beds on concrete, you don't need them that deep. Also, plants main need is water, that is probably the reason they didn't do well more so than the heat from the concrete.

    Bookmark   March 29, 2009 at 7:59AM
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Well, I don't grow parsnips, (boy those get big!) but I do want to grow bigger carrots, and tomatoes, and squash.

Anyway, I do appreciate all your feedback. The more I talk to gardeners, I learn there's no consensus on any one approach. So I decided to make them 16" high (2 rows of 2x8's). I also mixed in a rich amendment about 6-12" below the boxes and took away the horrible crust of construction debris sitting on the surface. (It was compacted after a year of construction, and would NOT have drained well under the boxes.)

Now I just can't wait to get things planted.

    Bookmark   March 30, 2009 at 11:36AM
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sinfonian(U8b A2 S5 SeaWA)

Goodness I missed a whopper of a thread. And all I wanted to say was this:

1) Make sure you mix your end mix well and then don't double dig it. You will already have the desired results. I know, duh.

2) 16 inches (three 2x6s) is how high I went because I wanted too. I did get bigger than 6 inch carrots so I guess all that Mel's Mix is being used. Oh btw, I went about 40% compost because I rounded on the bulk order.

I hope you are satisfied with how they perform for you and remember that there is 100 ways to grow a tomato!

Here is a link that might be useful: Sinfonian's garden adventure

    Bookmark   March 30, 2009 at 1:53PM
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gardener_mary(6 MA)

Lots of veggie roots will grow alot deeper than you might think if they are no blocked. Soil does not have to be prepeared deeply but if you put down weed fabric or have layers of clay, roots will not be able to get through and grow as long and as strong as they would like to.

Good gardening, Mary

Here is a link that might be useful: root development of vegetables

    Bookmark   May 26, 2009 at 5:39PM
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I realize that this is a little late for this discussion, but being basically lazy, I made my bed out of corrogated iron measuring 2 1/2 feet by 10 feet long. The ends were also the same material but only 5 feet long. No bending over in this box! I could easily reach the middle from either side. It took a lot of compost to fill it, but I was able to feed a family of 4 with plenty of extras to give to the neighbors. This was in service for 14 years and when I moved, I dismantled it and reassembled it at the new location. In fact, when I moved again from California to Hawaii, I took the iron with me and it has been in service here for 10 years now and still going strong.

    Bookmark   August 16, 2009 at 7:08AM
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"Granny...sorry, I have never grown you? I just dont know what to use them for. "
Parsnips and carrots prepared in any number of ways is pretty excellent.

    Bookmark   August 17, 2009 at 11:57PM
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RE: 16 inch deep raised beds...We would like to share our experience gained this past year.

We have had good results with our 16 inch deep raised garden beds. One is a huge 4' x 10', the others only 1' or 2' wide with various lengths to fit in the back yard. The plants seem to love the depth and don't mind the intensive planting methods.

One thing we would pass on to anyone considering a raised bed of this depth or more would be to Never Let The Bed Dry Out. Our beds take a fair amount of water, more than we at first thought. This may be because our summers have above 100-degree temps, and our warm breezes tend to quickly dry the soil. Everything we read said Âless space and less water with SFGÂ, but we got it wrong before we got it right.

It is difficult to restore even moisture in the soil of the deeper beds once it has dried out. Water will then tend to channel out the bottom rather than spread moisture evenly for the plants, and the stress caused to the plants results in plant loss or diminished harvest. Keeping the bed moist deep down will help prevent this. We were saved by a drenching thunderstorm that re-soaked the beds. We have since been careful to maintain enough water to keep the garden happy.

We now water deeply, and more frequently than once a week. We water different sections at a time so it's not a long drawn out process any given day. Working in the garden is enjoyable and the harvest results have been worth the extra care!

Also, lining each bed with chicken wire or hardware cloth (our choice) is essential, we believe, if you have any sort of burrowing critters. We laid about 4" of cardboard over the wire as a weed deterrent (perhaps unnecessary with the depth of the beds, but easy to do) and filled with a mix from a local yard. We amended the mix with more compost, peat moss, and perlite or vermiculite, watered well and planted.

The added height of the higher beds is a boon for our over 50 bodies. That is a serious plus to taller planting beds and we highly recommend them!

We've had a nice harvest for our first year, and are looking forward to adding beds and variety to our SFG.

    Bookmark   August 18, 2009 at 12:46AM
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We made our beds about 18 inches deep using three 6" cedar board for the sides. Since I was able to find compost for only $35 per cubic yard, it was easiest to just fill the entire thing with compost, but otherwise you could fill the bottom with cheaper dirt. It's the stuff closest to the top that really matters anyway!

Like jolo says, it's important with this high bed to keep it moist, as it will dry out much sooner due to more exposure to air on the sides. It's basically like a really big container garden, with similar pros and cons. But if you amend it and water it well, it should do just as well as a garden in the ground!

    Bookmark   August 18, 2009 at 2:36PM
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rj_hythloday(8A VA)

It's not any thing like a container garden, unless you have legs and a bottom keeping it away from the soil. Or if it sit's on a concrete slab maybe. Just sayin'

    Bookmark   August 19, 2009 at 9:21AM
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I just made two garden beds. They are 3x12 feet. One is 11" deep and the other is 16.5" deep. They are filled with good loam, an inch or so of peat moss and an inch or so of composted manure. I made them before I had found Mel Bartholomew and Square Foot Gardening. If I had read Square Food Gardening, I probably would've followed those plans, though I really like root vegetables and live in a community where no one else is growing root vegetables so it'd be fun to specialize.

I don't know anything about gardening. Any thoughts on what book I should use to get me started? My wife and I have Raymond Nones book and aren't happy with it. I *love* Eliot Coleman's book Winter Harvest Handbook and am excited to visit their farm, but it doesn't tell me step by step what to do with raised bed vegetable gardening.

Thanks for any advice!

    Bookmark   April 30, 2011 at 11:32PM
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I inherited raised beds between 20-24" deep when I moved into my current townhome, so I really didn't have a choice. However, I did regular gardening last year with tomatoes and brussels sprouts and am doing SFG with Three Sisters this year and I've had no troubles so far!

As for parsnips, I love them-- oven roasted or in winter vegetable soup!

    Bookmark   May 3, 2011 at 11:03AM
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If dryness is issue try Jerry bakers solution, a solution of common elcheapo dish soap in an over sprayer then spray the beds once a week. I did it for house plants when I lived in an apartment and then again on my lawns in our home. breaks surface tension. If you have lotsa peat in the bottom half it will retain more water. as you normally water the bed the nutrients will trickle down and the deepest roots will get nutrients. plus a layer of grass (half inch or so) really helps a lot.

additionally if you put a diaper of black plastic inside the bed allowing say inset of half inch all around so it can allow excess water to go over the top if you get heavy rains fill the outer edge with gravel and sand. (think pot in a pot)

all that peat moss or lawn clipping in the mix will wick upwards. If that seems chancy then put cotton fiber rope from the inner diaper/pan up into the mix in each "square" that will wick up to the plants root systems.

just a thought

    Bookmark   June 16, 2012 at 10:36PM
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The idea with double-digging is that the plants roots have space to grow down instead of out. This enables you to space plants very closely, without a competition for water and nutrients. With only six inches of space to extend downward, the spacing between plants would have to be much further to account for their shallower and wider root system. The sun and wind will consistently dry the top six inches of soil long before it can get to a depth of 18" or 24". So to me, a shallow bed would seem to require many more waterings. I water my plants occasionally and deeply. Furthermore, closely spaced plants can act as a living mulch for each other, shading the soil with their foliage, and thus preserving more water! If you can't tell, I'm an enormous advocate of the French Intensive methods. If it ain't broke... don't fix it.

I built two 24" - 30" deep experimental beds a couple years ago, and planted two varieties of tomatoes in them. They grew with a voracity which I had never before witnessed... They exploded to ten feet tall, with trunks almost as thick as my wrist... And how they produced!! We canned gallons and gallons. (This was in northern Maine, not usually a hospitable environment for most nightshades.) A deeper, stronger root system can support a taller, healthier, more productive plant!

I highly recommend John Seymour's 'The New Self-Sufficient Gardener' to any seeking information on traditional European intensive gardening methods. It was the book that sparked my obsession with growing food. Very cool stuff. =]

    Bookmark   May 1, 2014 at 11:59AM
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