How deep should my Bog [Marsh] Garden Be?

njbiologyApril 21, 2006


I'm building a 32 x 21 large linned garden pond with a 3' wide strip of linned bog surrounding the entire circumfr. of the pond. How deep should I make the escavation that will be linned with EPDM liner and back filled with soil; will be growing irises, watercress, and other marginals in the soil. NOTE: I live in Zone 6b and this past winter, I had a small kiddy-pool filled. with only 6" of soil and had irises, water calla, water cress, forgetmenots, cardinal flower, and other flowers in the shallow 6" of soil and not only did they thrive densely all last growing season but they all made it through the winter, having been frozen solid in moist soil. This leads me to believe that I do not NEED the suggested 2 feet of depth for a margin marsh-bog; i'm thinking that 12" would be sufficient for more marginal's root systems and for the purpose of overwintering. Please, give insight - thank you.

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Why make it all the same?

Make different areas of different depths.

Maybe you will run across some plants that might like
a deeper footing? Just be sure you know where what is!

    Bookmark   April 21, 2006 at 3:21AM
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kwoods(Cold z7 Long Is)


Could be trouble.

    Bookmark   April 21, 2006 at 11:08AM
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I see what you are saying.
You can have a 6" edge around the pool and set the body of
your bog plants behind say a one foot wide berm that lies
6 inches below the water surface and then have a wider or even a variable width and depth dug out. It would be more work so maybe a general depth would be easier to deal with. I was just thinking of marshlands I have seen. They are not round or oblong spheres of water with plants around them.
So to my eye I would get bored by the commonality of the scene! I am into serendipity!

    Bookmark   April 21, 2006 at 2:50PM
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How could it be trouble? KWoods Zone 7?

I don't see why i would need more then 12" - maybe then ill make it 18" but not more

    Bookmark   April 22, 2006 at 12:26AM
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? anyone have expertise with this?

    Bookmark   April 22, 2006 at 4:50PM
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njbiology, 6 to 8 inches is all you need. There is no reason for it being real deep since the roots don't go down very deep.

    Bookmark   April 24, 2006 at 1:24PM
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My cattail marsh garden is only 8 inches deep. It is 7 ft wide and 10 ft long. It has narrow leaf cattail and arrowheads in it. There are also some other types of plants, but those are the ones that have done the best.

Here is a link that might be useful: Cattail marsh garden

    Bookmark   April 24, 2006 at 2:39PM
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Excellent marsh garden.

1. What zone are you in, that the arrowhead should overwinter in only 8" of frozen mud? If you live, as I do, in zone 6b or colder: do you need the soil to dry out so that they can over winter; as an experiment, I overwintered arrowhead "bulbs" in only ice and they died, but survived when frozen solid in mud and even in dry gravel, outdoors - zone 6b.

2. Do you think that your typical native marsh plants [marsh marigolds, blue flag iris, cardinal flower, monkeyflower, bog arrum, arrow arrum] would over winter AND thrive year to year well in only 8" of mud?

3. If they would, why do you think every marsh garden site I find online states that 18" is a minimum for a bog/marsh garden and 24" ideal?

Note: I am concerned about the ideal growth of rootsystems over a number of years, about water retention [which might be why they recommend 18" for those with smaller bog gardens, and myself have noticed that, with the exception of arrow arrum, it seems that all of the above mentioned marginals/marsh plants only, as you said, go down a few inches]; that's why I want to know what's all this about 1.5+ feet.

If you could, let me know. Thanks, and love the very full, natural garden there.



    Bookmark   April 24, 2006 at 9:23PM
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njb, there is a lot of bad advice put out about marsh gardens by people that have no practical experience. Two feet just seemed about right to someone and others picked up that figure. Why dig deeper though if shallower works just as well?

Most hardy bog plants will do fine at 8 inches of soil. Mine do not dry out during the winter although I do stop putting fresh water in just because it is not necessary. As long the soil is moist, the plants will survive fine.

I live in Colorado a zone 5 and we get some nights below zero so it is plenty cold here during the winter.

If you dig up a bog plant, you will notice that the roots don't go down very far. Most are only 3 to 5 inches. One of the exception is broadleaf cattail that will put down roots deeper. In a real dry spell under natural conditions, this might be helpful, but not necessary under a mash garden.

There is nothing wrong with making the marsh garden deeper, but there is no practical reason for doing so.

One thing I am questioning though is your plan to put a 3' wide strip of linned bog surrounding the entire circumfr. of the pond. That is not a great idea since it would make getting to your pond water very difficult and make viewing your fish very hard. You might consider limiting the marsh garden to the back side only so it leaves the front open to feed the fish, viewing the fish, and doing maintenance on the pond.

Of course, you could locate the marsh garden completely away from the pond, which is what I have done.

    Bookmark   April 25, 2006 at 10:44AM
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Now I have been talking about the regular marsh type plants. If you wanted to grow lotus in a marsh settings, you would need something deeper.

My lotus marsh garden is made from a 5 ft wide 2 ft deep stock tank. It has about 14 inches of soil and then 10 inches of water over that. Lotus roots will die if they are frozen so it is necessary to make sure they are deep enough so this doesn't happen.

Here is a link that might be useful: Lotus mash garden

    Bookmark   April 25, 2006 at 10:57AM
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WAITE!!! That lotus garden photo: the pond is one of the best I've ever seen, because it looks real and natural. Is it natural? Can you please post photos, particularly of the margin and edges. My concern with how you've done it, if its not natural, is that run off from the laws will get into the pond and increase the amount of free nutrients.

2. secondly, I'm curious. That Lotus garden: are you saying that you keep that 5 foot wide, 2 foot deep bog [not attached to the main pond] all winter, and in zone 5, it will not freeze to the bottom?

What about mosquitos - maybe you can keep some minnows and overwinter them alive in their? I'd like to do this.

EXCELLENT WORK! please, let me see more of that pond - i'd love to see even shelving, if possible - but i'd be happy with anything you have to put up.



    Bookmark   April 26, 2006 at 12:40AM
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I just figured out how to check out your photo album that you posted - it's great, now i see how you keep the run off out. The only problem is this: if I have that lotus setup you have, uncirculated, the frogs will go in their in the winter and suffocate, when the hole freezes and the mud decays and releases Co2.

A friend of mine suggests that I build an external filter, based on cost and efficiency, in the long run.

Do you think I can run my 25 foot long entrance stream and 9 foot exit stream throughout the winter months [i think we get as low as -15 F. it's the colder end of zone 6b. I'm concerned about supercooling - or else, i could use a cheap pump and put it in the pond to keep a hole in the ice - for the frogs and fish. I'd like to run the stream because i'm afraid that fish and amphibians will remain in the exit stream which is level with the pond and freeze to death solid in ice - i think we get about 6" of ice and the stream is 12" but i am not sure that the stream, even though attached to the pond, will not being narrow, freeze solid. could you give me advice on these few points?



    Bookmark   April 26, 2006 at 1:02AM
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steven, I turn my filters and waterfall off during the winter and the fish do fine. If you are concerned about them, then put a stock tank heater in the pond and that will keep an area open in the ice.

The frogs will find a place to dig into the mud and survive just like they do in the wild so I think you are worrying for no logical reason.

Running a stream during the winter would probably ice over and really cool the pond down so I would turn it off.

    Bookmark   April 26, 2006 at 10:47PM
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I know that marginal plant rootsystems will survive freezing if in mud, outside of the pond, even shallow soggy mud; and, also from experimenting, I know that the same marginals would die if placed in a tray of water left to freeze, bareroot in ice. My concern is this: I want to leave my marginals on the shallow shelf around the pond which will only be 6" deep during the winter [arrowheads, monkeyflowers, irises, bog arrums, and a large number of other shallow/bog marginals]. My concern is that, since they will be bareroot with only the sediment that collects in the stones used to support/anchor them, that if the ice covering the margin shelf gets down to 6", freezing in solid ice will be fatal to the plants.

1. Ponds around here get about up to 6" ice, but can get deeper, I would imagine;
2. I want to make my margin shelf only 6" deep, and fill the water up to 4" because they do better with less depth and fill the pond to the top in winter, so that 6" of water will be over their crowns;
3. There will be some sediment and gravel around their bareroots, which I hope will help, just as temporarily water saturated mud protects bog plants outside the pond;
4. and lastly, I do not want to remove the marginals from the shelf during the winter - plants,btw, do great bareroot without soil, except for lilies.


    Bookmark   April 27, 2006 at 9:23AM
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Steven, I grow my bog plants in plastic dishpans full of dirt and then gravel on top. They do excellent that way and I have never lost any. I have never tried growing them in gravel so I can't coment on that.

I wonder if a 6 inch shelf is deep enough? I would probably make a 12 inch shelf and add a few bricks to raise the bog plant containers if necessary. Then I would drop the containers down to the 12 inch level during the winter if the plants were not real hardy.

Pickerel Rush is one bog plant that should be lowered below the surface during the winter since if its roots freeze, it will die. So I just drop it down.

    Bookmark   April 28, 2006 at 11:36AM
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1. How many inches, at most, have you ever received on your pond?

2. If ice ever made it down, hypothetically, do the base of the 12" shelf where the marginals have been lowered to, would they take temporary freezing?

3. What if I make my first shelf 12", but only fill half way in summer and fill the pond to the brim, 6" higher, in winter - since I wont use the skimmer/pump in winter? This is going to be a big pond and my favorite part are the marginals - so i cant play around with lowering them, as their will be too many.

    Bookmark   April 28, 2006 at 11:24PM
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Steven, ice gets about 6 inches deep on my ponds.

It sounds like you have a good plan.

    Bookmark   April 29, 2006 at 3:08PM
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sheryl_ontario(Muncho Lake, BC z2)

I have had 6" shelves with bareroot marginals, iris, cattails, marsh-marigold, watercress, creeping jenny. I leave them as they are all winter to freeze in the pond, They always come back in the spring.

    Bookmark   April 30, 2006 at 7:55AM
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Are you certain that 6" ice accumulates on your pond, that all of the roots are incased in ice? Do you think i would get as much ice as you, zone 6b in New Jersey - and are the bare root plants definitely in only 6" water?

thanks, sheryl


    Bookmark   April 30, 2006 at 11:51PM
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kwoods(Cold z7 Long Is)

Not to be mean or dismissive but, steven you appear to be kind of a hard guy to help. Perhaps a good book or some research on the web would point you in the right direction?

Web page

Water Gardens book


Could be trouble.this is what I meant_____here _____and here

    Bookmark   May 1, 2006 at 12:03PM
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kwoods, I think Steven is smart to be asking these questions before he builds his pond. Too many people make decisions without finding out the facts.

That is why we are here-to answer questions.

    Bookmark   May 1, 2006 at 8:25PM
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sal_mando(z6a SW Ohio)

I think that Kwoods is pointing out that it seems difficult to convince the OP one way or the other on the depth.
This is a huge project to tackle if you don't have any previous experience. I think that njb had a good idea to start with plants in a kiddy pool. If I were going to do this I know I would want to know everything I could before I started.
I went through all kinds of angst when I was building my little bog. I can't imagine what it would be like if I were trying to build a complicated pond and marsh that required $700 worth of liner and the removal of 40 tons of soil.
I'd probably ask local professional pond builders for opinions.

    Bookmark   May 2, 2006 at 11:40AM
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kwoods(Cold z7 Long Is)

Sam, absolutely agreed. I just think that steven might want to look at other resources beyond this forum if he is continueing to have trouble making a decision. Depth is an important issue but it seems to be a real sticking point for steven, I wouldn't want it to prevent him from going forward and we did have this conversation last year.

Maybe as Sal_Mando says starting small and making mistakes might not be a bad thing.

    Bookmark   May 2, 2006 at 12:30PM
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I've been away from the forums for a spell working on other projects.

njbiology, I don't understand why you're obsessed with an exact depth. As Kwoods pointed out we've all discussed this last year. There is NO specific depth that is critical. My bogs were installed in 1998 and dug 18" that has worked well for me. You're a bit warmer in winter than I so perhaps shallower is fine. Also Like I said, the larger the bog, wetland, whatever the more stable the conditions will be.

We're trying to help you and willing to offer what's worked for each of us but let's not rehash it every time we read that someone else is doing it differently.

    Bookmark   May 2, 2006 at 3:52PM
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