Bog design considerations


terraglen

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HI, new to the forum. In our new house we inherited a ~750 gallon pond that I'm embarking upon renovating. I've been doing a lot of reading, including the amazing, 7 year long, 41 page opus founded by Addy1. I'm trying to distill things into a single post for my own clarity. As you can see, I'm sort of compulsive about understanding a project around my house before starting it.

So here goes, my picture of how a bog filter works, and what factors matter most in the design:

First and foremost: everything that goes into the pond has to come out of the pond. If you have a lot of leaf drop, you should design a leaf net into the pond in advance. Wet leaves suck. Pieces of stick. Bugs. These will just clog up the bog plumbing; it's a biome, not a magical transporter of crap. There must be a mechanical filter in the system, even if it's just a leaf basket over the pump. But a skimmer is better. More on that in a minute.

There are two kinds of things in the water: gas, and material stuff. Most of us understand oxygen and carbon dioxide, which are gas molecules in the water. These are easy; you manipulate the concentrations of these gases by putting the water through a skimmer/waterfall/fountain, anything that churns up the surface in contact with air. This makes the water concentrations be the same as what's in air: more oxygen, but also (and very important for this filter) very little CO2.

Stuff consists of everything too small to be mechanically filtered: dust, pollen, fish food/poop, bird poop, etc. The carbon in stuff can wind up as CO2 and eventually leave the system by surface exchange as outlined above. The nitrogen is another matter, because it is poisonous in its eventual liquid form (ammonia, NH3) and has to be converted (via two bacteria) into nitrate, which is plant food.

So the bog filter is the final step in managing nitrogen that gets into the pond. You remove nitrogen from the system by harvesting the plants. If you allow dead leaves to fall back into the bog, that nitrogen goes back into the system. It also means: The most efficient bog filter system will include fast growing plants that can be harvested frequently.

Nitrogen could also leave the system by conversion of nitrate into nitrogen gas, which would bubble out. This last bacteria requires an anaerobic environment, which occurs down in the muck of a real bog. Unlike everything else in this discussion, which happens best in an oxygen rich aerobic environment. I have seen advice all over the net to the effect of 'you can increase your filtration by making your gravel bed deeper' which is probably true, because you are creating a slimy, anaerobic muck deep in the gravel bed.

Yuck. Do NOT make your filter too deep!

In fact, we want all the stuff to wind up as leaves without allowing a muck zone to build up. That means that the ideal depth for a bog filter is determined by the size of the root ball of the plants within. If I had a sloping gravel bed I would put bigger root balls in the deeper part. Perhaps people who harvest their plants whole could post size of plant and root ball? This might be a valuable resource in choosing bed depth.

But back to the other two aerobic bacteria, which we want to have in our system. The product of one is the feedstock for the second one, which is why it takes some time for the pond to 'cycle'- the plants can't grow until nitrates are present, which requires two bacterial populations two grow in succession. These two bacteria die at water freezing temp, so if your pond freezes it will have to cycle every year. It also means that a brand new pond will benefit from some stuff instead of a totally clean setup, to get it to cycle faster. This might be as simple as keeping the soil on the root balls of the plants and just knocking off the pots.

These two bacteria can also secrete slime, and in fact can turn dissolved CO2 directly into slime (this would be a cool superpower, if you had to pick from what's not already taken). I think this is why people who have aerators of some kind between the pond and the bog describe having clean gravel beds- it's not more oxygen, it's less CO2. A skimmer performs this function, as well as mechanical filtration. Thus, to me, there should be a skimmer between the pond and bog.

Someone on here described a negative edge waterfall into a gravel bed sump, which then feeds the bog piping. This design combines skimmer, waterfall aerator, and mechanical filter into a single, aesthetically pleasing entity where you can just spray leaves off the bed with your hose. I love this idea.

What else is left... oh. If we are going to make the bog inhospitable to our friendly, nitrogen-fixing bacteria then it has to grow someplace else, ie in the pond. These bacteria do not like light and will produce slime to hide themselves. There is CO2 in the pond (from the fish) and so there needs to be nooks and crannies in the pond that are never in the sun. Plumbing is one place, but I personally am going to add shady surface area with rock.

Wife is giving me the evil eye. Have to finish up with size in another post.
 
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Someone on here described a negative edge waterfall into a gravel bed sump, which then feeds the bog piping. This design combines skimmer, waterfall aerator, and mechanical filter into a single, aesthetically pleasing entity where you can just spray leaves off the bed with your hose. I love this idea.
That's me. We love our skimmer less pond design. We get skimming action but never have to worry about a skimmer basket overflowing or needing to be constantly emptied. In the spring, our crabapple drops a ton of blossoms all at once it seems - we can clean it out or just leave it alone.

I'm a little confused by your concept of "harvesting" your plants. Are you assuming you have to remove plant matter in order to remove the nitrogen from the water? If so, that's a misunderstanding. The plants just keep growing all summer long. (I am in a climate that freezes.) Nothing in my bog dies back until fall. I remove most of the dead plant matter in the spring when I start to clean up the gardens. I do thin out my bog from time to time (and you'll hear @addy1 talk about that too) but that's more a function of needing to keep the water flowing freely. Plants will completely fill the space they have available if you let them and eventually your bog will start to overflow.

Yuck. Do NOT make your filter too deep!
I don't agree with this either. Our bog is four feet deep - which many people will tell you is too deep - and we've never had an issue. We used varying sizes of rock and gravel by layer so the bottom layer is softball sized rocks, then a layer of golf ball sized, and so on. This is year 8 for us - we've never had to clean our bog our had any problem with anything getting clogged. I don't think you NEED to go as deep as we did - we were building ours to accommodate a vault and centipede plus a layer of Aquablox. As you said though, we send water that is free of any solid matter through the bog. Everything gets skimmed before it gets to the pump. @addy1 pulls her water directly from her pond - she just makes sure not to pull from the very bottom.

There's definitely more than one way to achieve the same goal, but you've hit on some of the more important design principles for a successful manmade (or in many of our cases - woman made!) planted filter.
 

terraglen

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No, I am saying one needs to harvest plants to remove nitrogen from the whole system. Leaving dead stuff in the bog is counterproductive, because the nitrogen eventually gets broken down and added back into the water. A secondary possibility is that I bet there are fast-growing plants that can be harvested several times per year, which may being more nitrogen out of the water than just letting things grow all year (not so sure about this secondary point).

Your pond design continues to impress me. You are correct that I was assuming pea gravel all the way down; having all those big rocks near the bottom would control slime-forming bacteria by reduction of surface area. I would love to know how far down the roots of your plants go in this system. If the roots go down further than in a 12 inch gravel bed, that would be a big win for filter efficiency.

Another good thing is that the gradual decrease in rock size would allow better flow through the filter, by reducing turbulence where the pipes meet the pea gravel.

Much food for thought. Thanks.
 
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Leaving dead stuff in the bog is counterproductive,
What do you imagine is dead in the bog? During the pond season, there's literally nothing dead. The plants grow like wildfire. They do die back in the winter, but the plants stay in the bog and come back again year after year. They continue to spread and get divided and "yanked" as @addy1 says - but nothing is dying during the pond season. What zone are you in?

I'll show you my roots on my irises in a few weeks here - I'm waiting for them to bloom and then we're gonna have a little chat about who's in charge out there! I can tell you from past experience the roots fo 18 or more inches into the gravel. The irises on my pond edge have a floating root system that almost hits bottom by the end of summer.
 

terraglen

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Apologies. I should have enumerated that I am looking for feedback and advice, not to challenge people who have demonstrably been successful in this hobby. I'm considering a challenging biome design for which filtering efficiency is crucial*. Your feedback has already got me thinking about digging the bog deeper and setting up something like you have, rock-size-wise, so thanks. If your iris roots get down that far, wow.

All I am saying that if you don't remove the dead plants, whenever they die, water quality has to eventually suffer because the nitrogen buildup will overwhelm the converting capacity of the bacteria. That is, removing the dead plants before springtime each year is functional, not just aesthetic. This was not as obvious to me as perhaps it should have been. I am not expecting plants to die during the year, except through my ineptitude.

Does "get divided and yanked" happen multiple times during the year? I am in 10a (greater Houston area, which is itself a very large bog filter). And I would love love love to see your irises when you pull one.

* I am aware that there are other filter designs which might be more efficient than a large bog filter. All of these require frequent removal of gook from come kind of container. Non-starter for me; it's a bog filter or nothing.
 
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Oh, no worries! I love the conversation and am not one bit concerned about differing viewpoints. Like I said, there are definitely many ways so achieve the same goal.

Does "get divided and yanked" happen multiple times during the year?
Yes! And no! Certain plants are much more invasive - anything in the "wort" family for instance, or the watercress that I plant in my bog and waterfall - those get thinned out multiple times per year. If I didn't, my bog would be blocked by plant matter and water would find another way! Other plants - irises and cattails and rushes and grasses for example - I generally divide every year or sometimes every other. I learned the hard way that more than two years will cause you to say swear words in front of your kids... and even if they are full grown they will still be shocked. So it's best to keep an eye on them and use a heavy hand. @addy1 has far more surface area to her bog - mine is only 4x6 so I have far less wiggle room when it comes to a plant take over!

I honestly don't remove "dead" plants in the fall. I somewhat see your point, but the plant itself doesn't actually die, as in it doesn't rot and decompose. The plant is still "alive" under the gravel and ice and whatnot - if it weren't it wouldn't come back the next year. If I have annuals - like I often plant canna lilies in the bog - those come out at the end of the seasons because they will rot if I leave them until spring. But irises and cattails and milkweed and all my other plants just die back and go dormant until spring. The dead plant matter stays put - I'm a spring clean up kind of gardener, as I have found that leaving the dead leaves and branches helps your plants gather leaves and snow around them and insulates them through the coldest winter. But you wouldn't know anything about that in Houston!

Tell me about your plant-life in Houston - do you have perennials and annuals like we do here? Do plants have a life cycle similar to ours? Managing your bog might be different if your pond never actually goes into a cold winter mode. This is my ignorance showing - I've never really thought how gardening would be different in hotter climates.

I will definitely get some photos of my irises when I do the dividing. The whole bog is getting a bit of a makeover in a bit here.
 
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OK - this picture almost scares me. These plants are out of control. That's a waterfall between those two columns of dark pink flowers. You can't see water because the watercress has completely taken over - climbed out of the bog and headed for the pond! I remember this happening in less that a week's time. I took this photo and then yanked out probably 15 lbs of plant matter - no joke. To the rear at the left you can see the irises and then cannas in the middle (white flowers) and more irises to the right. That's my bog. On steroids. Haha. Not really. Just fish poop! There are smaller plants in the undergrowth that you can't see from this vantage point. And about a bazillion tiny frogs that hide out up there so the big frogs don't gobble them up.


120611
 

terraglen

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Holy COW! Settle down there, bog!

With respect to plants that grow around here suitable for a bog, I have assembled only a list of those plants suitable for shade, since my pond sees comparatively little sun. One very good list has been referenced on this site before, published by Texas A&M which is right up the road.

Some more plants that I saw at a large local pond supplier here in Houston. They are big bog filter advocates (they first got me doing google searches that led me to this forum). The show nursery at this store has 50/50 sun/shade in the summer where the bog plants are.

Horsetail
Creeping jenny
Corkscrew rush
Water Celery
Taro (various)
Dwarf Papyrus
Aquatic mint
Dwarf Sweetflag Iris
Parrot's feather
Blue Moneywort

@Lisak1, what is the bottom substrate in your pond? All I can see in that photo are the giant fish.
 
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addy1

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Welcome to our group!

My bog is about 2.5 feet deep, 27 ish feet long, 4.5 feet wide. I draw water from a foot off the bottom of the pond to keep from pulling in tads, snails large debris etc. I do have a leaf basket, very seldom need to clean it.

I dug down into the pea gravel one year, after all the comments about the lower gravel being anaerobic, stinky etc, nope clean good smelling even after sitting all winter.

In the fall the bog plants die back, I leave them as they die, in the spring I take a rake and rake out the blow in leaves, dead plant debris, etc and let the bog begin to grow again.

I have not cleaned the bog, i.e. the gravel in the 10 years it has been running.

This picture is around early summer.
pond11.JPG


How it looks in winter before clean up in the spring.

9.JPG


this is current growth
Capture.JPG
 
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what is the bottom substrate in your pond? All I can see in that photo are the giant fish.
Gravel bottom. And sadly those big fish are not more... we lost them all this winter to a combination of the Polar Vortex and a plumbing failure.
 

pondlover

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OK - this picture almost scares me. These plants are out of control. That's a waterfall between those two columns of dark pink flowers. You can't see water because the watercress has completely taken over - climbed out of the bog and headed for the pond! I remember this happening in less that a week's time. I took this photo and then yanked out probably 15 lbs of plant matter - no joke. To the rear at the left you can see the irises and then cannas in the middle (white flowers) and more irises to the right. That's my bog. On steroids. Haha. Not really. Just fish poop! There are smaller plants in the undergrowth that you can't see from this vantage point. And about a bazillion tiny frogs that hide out up there so the big frogs don't gobble them up.


View attachment 120611
Beautiful!!
 
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LisaK1 ... that's an amazing set you there. It obviously working... but I don't get the basic physical mechanics of the whole set that aren't requiring your to clean the bog at all.

The inputs into a pond/bog scenario would be fish food, blown in debris, and then just dead plant material/blooms that sloughs off plants as they grow. The only outputs that you mention is maybe cleaning your basket... slight plant thinning from the bog.. and maybe overflow after heavy rains. Right?

My 1000-1200 g pond is running a sand and gravel filter set up. Basically the water is pumped from the pond and pushed up through the 55G barrel that is layered with 1.5" rock, then 3/4" rock, then pea gravel .. and then at the top is chicken grit. At this point it returns to the pond. I don't have a huge fish load. I don't have any plants. And with all that said... the filter begins to get back pressure from getting dirty after a week or two. I see plenty of small debris that has blown into the pond from the garden dancing around the bottom of the pond that will eventually get pulled into the filter. I just don't get how growing plants would pull that much physical input from the system?

When I drain the bottom of the filter I will see the chunkier debris.. and then when I run the air into the bottom to blow out the rocks... it get all the fines out. The below link shows a clean out and the "fines" that I am talking about.

 
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@Nevermore44 I'm not sure if you mean me or @addy1 but I can answer for my bog set up.

I think you're asking if the plants will deal with organic solids and the answer is no. However, in my case the water first flows through a simplified "down flow bog" or a negative edge skimmer. The gravel and rock trap the majority of the solid organic debris before it ever gets to the pump. So the water that gets pumped to my bog is basically clear. The fines that you refer to settle in the bottom of the pump vault. If the water gets low enough in the vault we can tell because the water will be a bit murky coming from the waterfall. But in 8 years, the amount of fine debris in the bottom of the vault has stayed relatively stable. I'm assuming it continuously breaks down. Our bog water is always crystal clear. We have a clean out vault in the bog - never needed to use it, but we can see to the bottom and there's no murky, no muck, no dirt.
 

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in my case I pull water from about a foot off the bottom. I have minimal debris on the bottom.
My bog water is crystal clear also. And the pond is crystal clear.
 

addy1

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Do either of you have to net your ponds? Ie: once a year?
Bird only net on 24/7. It does not stop leaves. But the way my pond is built and pure luck I don't get a lot of leaves in the pond.
 
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