Beginner - 24" Deep Bed Questions

I a total gardening beginner and I am taking the plunge and building my first raised bed. It will be 8' x 4' and 24" tall. It is being built into the side of a hill. Half the box will essentially be buried on the high side of the hill.  I won't use any existing soil in the box, and plan to have a bottomless box. I don't know much of the soil below the box,  but it is in a generic suburban backyard in the middle of Iowa. 

My question is what it should be filled with. I want to use Mel's Mix, but I don't know if it makes sense to fill the entire depth of the box (given cost and drainage concerns). In the SFG book, Mel mentions filling with sand below 6". However I'm not sure 18" of sand is what he had in mind. Can I just use Mel's mix for the top 6" and fill the bottom 18" with regular potting soil?  Or should the bottom 18" be a mix of potting soil and sand or possibly pea gravel?  Also, is play sand what is needed, or do I need something specific?

Thanks for any advice you can offer.

Hi TheFife.  Welcome from Atlanta, GA!  We're glad you've joined us.

Question:  Why are you making the bed 24" deep?  Will you be growing potatoes or carrots in that bed? 

It would certainly give you that option!  But I'd hate for you to have to spend money & effort when you don't have too...

I'm sure some of the others will be pitching in soon.

Welcome again!

How much of a slope is in your hill?  Will you need to have some type of implement in place to keep your growing medium from eroding downhill?

If your slope is of little consequence, I would advise putting small gravel in the bottom 6 inches and then wood chips on top of that before you go with Mel's Mix on top.

This will give you extra strength on a hopefully smaller slope and terrific drainage while also giving you organic matter to slowly decompose as extra compost.

If you are on a severe slope, then I would guess you are building your containing device up to guarantee a level square.  In this case, I would fill the bottom up with cheap topsoil and humus, which you can frequently find for less than 2 bucks per 40-pound bag.  Then, you can add the Mel's Mix in the top.

I fill the bottom of the boxes below 10 inches with potting soil.  But beware, that soil will eventually become mixed a little with the Mel's Mix on top when you pull up an old plant with roots going down or mix in new compost.  It doesn't seem to have hurt my Mel's Mix, but who knows.

The slope falls approximately 1ft over the 4ft width of the planned box. I have leveled the ground where the box will be to ensure it is stable. This will put the box at ground height on the low side of the hill, and it will be buried 1ft on the high side of the hill. I am making the bed 2ft tall so that it is still a workable height on the high side of the hill. 

I do plan to grow potatoes and carrots, so the deep bed should be beneficial.  My main concern is proper drainage (and avoiding drying the soil out too much) as well as the cost to fill the bed. I am planning 3 or 4 beds in total, but am starting with 1 to make sure I get it right.

Welcome to the forum.

For fill on the bottom, you could use logs, plastic bottles, broken concrete chunks, cinder blocks, bricks, rocks, or anything else you can find for free. Fill in the spaces with existing dirt, and cover with weed cloth to keep the MM from settling.

Sounds like a lot of work though! I would build my boxes with a TT bottom, and then raise the downhill side to level with cinder blocks. Do a search for "TT" (table top) to learn more about them.

TheFife,  Welcome to the Forum from California!  

Just make sure that none of the box is buried into the hill side.  I tend to agree with Boffer.  Leave the hill and its slope alone, and raise the bed instead so it is above the ground.  Using three layers of 2" x 4" (10 1/2" high bed) or two layers of 2" x 6" (11" high bed) should be fine for root crops. Support the plywood bottom every 2".

boffer wrote:Sounds like a lot of work though!  I would build my boxes with a TT bottom, and then raise the downhill side to level with cinder blocks.  Do a search for "TT" (table top) to learn more about them.

I would be tempted to borrow an idea from hugelkuture and fill the bottom with wood of various sizes and some soil. Or with manure. There are some very experiences here with filling the bottom of a deep bed with manure, tamped down, and then MM on top.

If I had 2' deep boxes, I think I, yes, even I, the SFG advocate, would consider a wood chip and horse manure base of 18" with Mel's Mix on the top.    Eventually the base would decompose and the bed would sink some.  Then add either more MM or new compost and mix to the depth of the original MM.

sanderson wrote:
TheFife,  Welcome to the Forum from California!  

Just make sure that none of the box is buried into the hill side.  I tend to agree with Boffer.  Leave the hill and its slope alone, and raise the bed instead so it is above the ground.  Using three layers of 2" x 4" (10 1/2" high bed) or two layers of 2" x 6" (11" high bed) should be fine for root crops.  Support the plywood bottom every 2".

boffer wrote:Sounds like a lot of work though!  I would build my boxes with a TT bottom, and then raise the downhill side to level with cinder blocks.  Do a search for "TT" (table top) to learn more about them.

Thanks Sanderson. Quick question. Is there any harm in having part of the box buried into the side of the hill? I have seen pictures of raised beds built this way and assumed it was a fine practice. Is the concern just the work and expense, or are there other issues to be aware of?  Thanks!

I'm thinking soggy mix and rotten wood on the buried end when it rains. Also, the weight of the hill over time as it tries to fill in the area you cut out.

Folks? Comments?

I made my boxes 24 inches high so I wouldn't have to bend over to work on them and put a ledge around them to sit on.  The first 12-14 inches was a local garden mix and 8 inches Mel's mix was layered on top.  I add compost each year and am able to mix it all together without pulling any of the Garden mix which tends to compact.  This is a picture when first constructed

Nosmok, Really nice looking boxes. I like the idea of 8" of MM on top of "local garden mix."

sanderson wrote:Nosmok,  Really nice looking boxes.  I like the idea of 8" of MM on top of "local garden mix."

I also like that ledge.  Thoughts for my future.

nosmok wrote:I made my boxes 24 inches high so I wouldn't have to bend over to work on them and put a ledge around them to sit on.  The first 12-14 inches was a local garden mix and 8 inches Mel's mix was layered on top.  I add compost each year and am able to mix it all together without pulling any of the Garden mix which tends to compact.  This is a picture when first constructed

I really like the look of these beds. I too am planning a ledge/seat for the top of my beds. My question for you is about the width of the seat. I was planning to use a 2x6 as the seat. Is that what you used?  If you had it to do over, would you make them the same size?  I am just worried the 2x6 might be a touch too narrow. 

Thanks!

I finished my first bed and now have seeds in the ground. I thought I would share my results for those interested. It will also be a good reference for me when I make my next bed later this year. 

I started by cutting away the grass and leveling an area the size of the bed.  I dug the ground and raked it extensively to make it easier to get level. 











I used nearly exclusively with 2x6 cedar boards. Most were 8ft long. This was pricey for a bed this tall, but I hope the rot resistance will pay off. The sides are 4 stacked 2x6 boards.  After cutting the boards to length, I did a quick layout in my garage as a reference to use while building it. Most of the building too I place in my garage, and then I transferred the whole unit tithe back. At that point I got it leveled and then added the top 2x6 boards to act as a seat for the bed. 

Once it was all put together, I took it to the back yard, got it leveled, added stakes to keep it from ever sliding down the hill, and installed the top.






Then I filled it with 10cu ft  of pea gravel. It ended up being around 4 inches of rock.  I did this based on the advice found in this forum. Hopefully it will help drainage.  Here is what it looked like. 




After this I added 25cu ft of generic potting soil. I chose something fairly cheap that appears to have a lot of peat moss. I figure this dirt shouldn't have much impact on what I grow, since there is nearly a foot of Mel's Mix that will be added on top of it. This potting soil filled the planter up to around 12 inches. I then watered the mixture to see if I would get any significant settling of the soil into the pea gravel. There didn't seem to be.

Finally I made the Mel's Mix. I used the following ingredients:

• 11cu ft Coarse Vermiculite (purchased at Earl May)
  
• 11cu ft Sphagnum Peat Moss (purchased from Earl May)
  
• 3cu ft Omagro composted lawn refuse (purchased from Earl May)
  
• 1.5cu ft Mushroom Compost (purchased from Menards)
  
• 3cu ft composted cow manure (purchased from Earl May)
  
• 3cu ft Dr Earth Compost (lots of ingredients) (purchased on Amazon)
  
• 1cu ft Lobster Compost (purchased on Amazon)
  

I mixed this in 3 batches on a tarp. It was hard work mixing this much soil. Next I added the soil to the planter and did a little additional mixing with a rake. After filling the planter, I added eye hooks and garden twine to make my grid.   My last step was to plant some carrots, potatoes, and watermelon. I marked them with some colorful sticks found on Amazon. 









Now to wait for the first sprouts.

That looks gorgeous and bomb proof, both good things.
Keep an eye out for some rather high nitrogen compost (Chicky doo doo or something similar) to top dress any greens and the melons if they seem to need it. The carrots and potats should be fine and happy.

That looks wonderful, TF!  Nice & high, level, a place to sit if necessary...

Planting on a slope is such a pain, but you pulled it off beautifully!

Can't wait to see how your garden grows. 

That's a beauty, Fife! Be glad you leveled it like that. Sometimes I wish I had leveled my beds. My SFG is on a slight slope, not as much as yours, but the MM moves downward by year end and ends up about 2-3" higher. lol And THIS year all the plants at the lower ends are growing faster than the ones at the higher ends.

CC

CapeCoddess wrote:... And THIS year all the plants at the lower ends are growing faster than the ones at the higher ends.
CC

Interesting development! Are you thinking about making changes accordingly?

AtlantaMarie wrote:That looks wonderful, TF!  Nice & high, level, a place to sit if necessary...

Planting on a slope is such a pain, but you pulled it off beautifully!

Can't wait to see how your garden grows. 

+1

boffer wrote:

CapeCoddess wrote:... And THIS year all the plants at the lower ends are growing faster than the ones at the higher ends.
CC

Interesting development!  Are you thinking about making changes accordingly?

Nope.  Although I suppose I could plant my favorites at the low ends in the future.

I echo others admiration, TF.  I really must have been a lot of work but it will save you a lot of work in the long run which is what matters.  How many more are you going to make?