Koi pond in greenhouse - bad idea?

Juneau, AK

This was posted in the water garden group, but the only comment suggested I post here:

We are still in the planning stages, but my husband is on board. We have koi, inside. They need more water, and I want more koi. However, SE Alaska has bear, porcupine, blue herons, bald eagles, kingfishers, etc. We are fairly close to the ocean so mink and martin could also be a problem.

I've also wanted a good-sized greenhouse. So here is the plan.

12 x 30 greenhouse with a pond, 8 x 5 then 5 feet deep. Straight sides for space. The pond would actually be partially under grade which would help insulate. Of course, this means the greenhouse would be kept warmer than the outside for several months of the year, and while this is a consideration, it isn't the end of the world.

Has anyone else tried something like this? I'm worried about condensation and mildew from the excess humidity but I'm also excited to be able to have at least some plants year around since our garden season is so short.

Dublin, CA(Zone 9a)

I think if you put in extra ventilation and fans you may be able to avoid the mildew concerns. Two concerns though--first thing would be to understand what your water temperature will be in the summer with the sun beating in through the greenhouse--even if your outside temps don't get too high, you'd be surprised how much the greenhouse can magnify that and with your long, long summer days I could see that the water might get quite warm. No idea how warm it can get before it gets too hot for the koi though. Second problem is during the winter--you'll have to keep the greenhouse warm enough that the koi won't freeze, and that could be expensive.

Columbia, MO(Zone 5b)

I have never had Koi but they can stand freezing temps can't they? Otherwise I would think that you would not be able to have them in outside ponds.

Fulton, MO

This is absolutely do-able.

Having that much water in the greenhouse will keep it nice and humid. The ground will moderate the water temps. You will have considerable condensation, but greenhouses can be built so that drips and condensation is managed.

What sorts of temps does Juneau have? How warm do you want it? What temp requirements do the fish have, and what kinds of plants do you want to grow?


Dublin, CA(Zone 9a)

You can't let it get so cold that the pond actually freezes--AK gets pretty cold in the winter so it's definitely a concern! I think as long as you don't get ice on the pond they're OK though but I don't have any so I'm not positive.

Juneau, AK

Okay... good thoughts going here. Exactly what I needed!

Our winter temps aren't as cold as what you think of as "Alaska". Juneau people whine when it's in the 20's are rarely does it go below zero. We understand it will be quite expensive to keep things above freezing, but fortunately my sweetie loves fresh spinach salads and the costs are not going to break us.

Summers are mild with temps ranging from the 50's - 70's and rarely in the 80's. I figure when it's that warm the doors will be wide open. The location is partly shady as well. When my aquarium got too warm (when it was poorly placed in the sunny part of the house) I added ice to keep it cool. I also do water changes, like 25% of the water on a regular basis. Doing that on a warm day would help too. Besides, my plants like that water :-)

So.... now we are dealing with the issues of condensation. Obviously common lumber for the wood portions will not work. I'm not a fan of pressure treated lumber near my vegetables... should I be looking at eliminating wood product all together? Maybe the "wood" should be Trex or some other "plastic lumber"?

Thank you so much for your input so far... and do keep it coming!

Fulton, MO

Are you set on building from scratch? If not, consider a kit. My kit, for example, has a system of little gutters build into the cames. This gutter system channels water so that I have virtually no drips. The extruded aluminum is not affected by condensation. Otherwise, if you build from scratch, consider a rot-resistant wood like cedar or redwood, then you could apply a penatrating stain to the structure. You might have to reapply periodically. You could build the roof with a steeper pitch, then the water would be more likely to run off, rather than dripping on the plants.

If you use poly, use the kind with the anticondensate coating, and use double layer with a blower. If you use polycarbonate, get at least twinwall, and consider triplewall if you can afford it. Not only will it reduce heating costs, it will reduce condensation as well (consider the exposed aluminum can of cold soda in the summer, compared to the same aluminum can in a foam sleeve).

Other general measures to control humidity will help with condensation: circulate plenty of air, heat and ventilate, don't water to excess.

I don't think Trex will work, it isn't a load-bearing material.


Dublin, CA(Zone 9a)

SB is right, you can't use composite lumber for anything that is providing structural support (if you've ever looked at a Trex deck, you'll notice the beams on the underside are pressure-treated lumber)

Juneau, AK

I don't really have a kit. What I have is the bones from a greenhouse someone was no longer able to keep up with. Probably little of the piles of metal, etc will be used, and my husband understands that going into the project. The actual kits are not very cost effective to ship to my location. In about two weeks, some friends who live in a remote part of Alaska are flying in to spend a week to two weeks building this. I need to have the entire plan in place before they get here, and of course have all materials on hand. After 3 years of dreaming, it's finally going to happen.

Fulton, MO

Akdebs, I'm still not exactly clear...

...so you have part of a kit, right? What parts? Frame?

Shenandoah Valley, VA

Are you doing this because you think it's too cold for the koi? Koi are very hardy. As far as the pond freezing - ours freeze over every winter. As long as there's an air hole, the koi will be fine. I use a floating heater, which you can buy at a pond supply store or a feed store.

It sounds like your winter temps are similar to ours and your summers cooler. The koi should be perfectly find outside.

Juneau, AK

I have a frame that was custom made with 2.5 inch galvanized pipe, bent to the proper angles. Chuck was a metal worker by trade. I still need to dig a hole for the lower part of the pond, put in footings and foundation, then the frame goes up. Some of the plastic panels are reusable but many are not. When he had it up, there was common lumber on either side of the metal frame. I'll see if I can find some photos to post....

Thumbnail by akdebs
Juneau, AK

It's not necessarily that it is too cold for them outside, but they need protection. I have seen the following on my property: black bears, eagles, blue heron, mink, marten... and I'm sure having captive fish would bring more creatures. Besides, the greenhouse is supposed to be the place I can go, even in winter, to enjoy the fish. Right now, they are in a 55 gallon aquarium and it isn't enough for the two of them let alone the two goldfish and plecos. Once I have a large pond, I can get more koi, keep them in the tank until they are large enough, then out to the pond they go. Here's another picture of the greenhouse structure from when we took it apart last spring.

Thumbnail by akdebs
Marysville, WA(Zone 7a)

Would 40 sq. ft. of water surface really create such a problem that good ventilation and air circulation would not solve ? Aeration of the pond water would seem to increase the evaporation/humidity too, but perhaps not to an extreme. As to the freezing of the pond, when I was researching Koi I seem to recall a comment that they are just fine under ice as long as there is sufficient depth, which you would have with a 5 foot deep pond. If excess moisture does in fact become a problem the pond could be covered partly, or in total, with a removable sheet of plastic. All in all it sounds like a great project. Please keep the progress reports coming.

Fulton, MO

Now we're getting somewhere!

Are the panels plastic or are the polycarbonate? Are they only for the roof, or sides too?

Right away, I would suggest that you position the pond on one the end instead of in the middle. This way if you decided you didn't want to heat the entire GH, you could put up a divider and heat only 1/2 or 1/3 of it.

"Would 40 sq. ft. of water surface really create such a problem that good ventilation and air circulation would not solve?" I'm with you, Balvenie, I don't think so. Evaporation will use a considerable amount of heat (latent heat of vaporization and all that) but if you have decent insulation, ventilation, and air circulation, I think it would be totally manageable.

Shenandoah Valley, VA

We have bears, and I know they come in the yard, hawks, herons and even an eagle that hunts here. I've never had anything bother the koi in 10 years.

Except a neighbor's dog wandered into the yard one day and when I shooed him, he ran straight into the pond not knowing it was there. If you could have seen the look on his face. LOL He didn't hurt the fish or the pond luckily.

Fulton, MO

Looking at your photos, it appears that the roof panels go all the way from ridge to eave, is that right? If so, drips may not be so much of a problem. Anticondensate sprays are available which you may be able to use on the panels you already have: http://www.redpathaghort.com/bulletins/accessories1.html.

Juneau, AK

I can rebuild this thing however I want with the exception of the roof angle. That would be really hard to change. I do like the idea of having the roof with eaves and we will probably do that. As to the cover material, the panels I was able to salvage are corrugated plastic. The wavy ones on the sides had too much UV damage to recycle. If I do 4 feet between panels, I will have a 30 foot long greenhouse. If I remember correctly it is 12 feet wide. Everything is still under snow, but we reached mid 60's today. Think Spring!

Bucyrus, OH(Zone 6a)

Do you have permafrost in your area? If so, I would suggest checking with a local basement contractor regarding insulation. I could see a heated pond tunneling its way through the permafrost, or an unheated pond freezing from the bottom up.

If you have permafrost depending on what the contractor says you might consider a layer of moisture barrier, a few layers of insulation, then another moisture barrier followed by sand, then your liner.

As for koi and freezing temperatures, koi are temperate carp. Years of keeping them in Ohio taught me that as long as the bottom of the pond doesn't freeze koi will survive an iced-over pond with no problems. I have stood on the frozen surface of a koi pond and had koi watching me under the ice. :)

Comets and fancy goldfish can also survive outside with no problems. :)

Now tropical and sub-tropical fish like mosquito fish, fighting fish, and killifish need year-round warm water.


Juneau, AK

I'm in Southeast Alaska, and there is no permafrost. No basement contractors either because it is a rain forest! However, I was going to insulate around the bottom of the pond, if nothing else, just to help protect the liner. Thank you for chiming in! The snow is almost gone.

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