# Do plants like cold feet? The Sequel



## MiamiAG (Jan 13, 2004)

Hi all,

Some time ago, I asked the question "Do plants like cold feet?" in order to discuss the practice of heating cables. Heating cables were popularized by Dupla and further advanced by ADA with their heating plate. The thinking was that heating cables created a current of water through the substrate that brought O2-rich water and nutrients down to the substrate solution. 

I'd like to throw out the topic for discussion again as some time has past and we are more knowledgeable now. What do you think?

Do plants like cold feet?


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## Nelumbo74 (May 2, 2008)

They don't technically have cold feet in a tank without heating cables. The temperature in the water column creates convection that in a sense pulls water up through the substrate. That's one of the main reasons that most people don't use heating cables.


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## hoppycalif (Apr 7, 2005)

I'm not sure that the water temperature gradient in the unheated substrate can create any water currents. In a cold house the bottom of the substrate would be slightly above room temperature and the top would be slightly below tank water temperature. That is in the wrong direction to create any convection currents.

But, the substrate is suffused with water, so it will have a heat conductivity close to that of water, which is pretty good. That means there can't be a big temperature gradient in the substrate - heat is just lost through the substrate to the bottom glass of the tank, another fairly good heat conductor. 

My guess is that the plant roots are never cold, with or without the substrate heater.


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## MiamiAG (Jan 13, 2004)

Thanks for your comments!

So, let me try to understand. In an aquarium, the water column and substrate may or may not have a temperature difference. If there is a temperature difference, this may create some movement of water by convection. 

Some questions that I would pose back to you are:

1) Generalizing a bit, can it be said that the substrate would be warmer than the water column? Does this depend on any heat sources underneath the aquarium or, perhaps, the type of substrate being used?

2) If we can agree that the substrate is usually warmer than the water column, is it warm enough to create a convection current strong enough to circulate water through the substrate? If not, wouldn't heating cables increase the temp to a point where the current is strong enough?

3) Lastly, being that I fertilize the water column, this convection current would bring this fertilizer down into the substrate where the roots would be able to take it up. This is good, I suppose. I'm thinking of the old adage, "Better to fertilize where it isn't available to algae." I'm just not sure this is the only benefit of bring water down to the substrate. Is there another way to do this?

Thanks in advance.


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## Bert H (Mar 2, 2004)

Has anyone actually measured what kind of temperature differential there might be? I just can't see there being that big a difference in temps, I assume we're talking in the wintertime here, between the water temp and the bottom of the substrate, which is maybe 3, max 4 inches deep. I'd be interested in knowing if any such measurements exist.

If so, what about the reverse? In the summertime, warm weather climates, what kind of temp differential is there between the top of the tank by the light source, and the substrate?


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## hoppycalif (Apr 7, 2005)

Convection occurs because a substance has a lower density at high temperature than at low temperature. This causes the low density, high temperature substance to be buoyant, to try to float in the high density low temperature substance. So, if you can add heat to the bottom of a mass of water the heated water will rise, creating convection currents.

Without a substrate heater, or a heater under the tank, an aquarium will never have warmer water under the substrate relative to the tank. (An exception would be for those using water chillers to keep the tank water below the room temperature.) Therefore, none of us with conventional tanks will ever have convection water currents in our substrate.

It isn't difficult to calculate the probable temperature gradient in a substrate, given the room temperature and the tank water temperature. I'm not in the mood to do it right now, but I may decide to do that some time.

EDIT: Thinking about this some more: in nature the sun provides almost all of the heat for bodies of water. And, the ground is a heat sink, absorbing heat from the sun or from whatever is sitting on the ground that is heated by the sun. Therefore, I don't think natural bodies of water will experience "substrate" convection currents either.


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## Zapins (Jul 28, 2004)

I found an interesting article about exactly this topic. It was a study done on tomatoes.

I have the PDF file, but I will try type up a brief summary of their results.


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## cs_gardener (Apr 28, 2006)

hoppycalif said:


> Convection occurs because a substance has a lower density at high temperature than at low temperature. This causes the low density, high temperature substance to be buoyant, to try to float in the high density low temperature substance. So, if you can add heat to the bottom of a mass of water the heated water will rise, creating convection currents.
> 
> Without a substrate heater, or a heater under the tank, an aquarium will never have warmer water under the substrate relative to the tank. (An exception would be for those using water chillers to keep the tank water below the room temperature.) Therefore, none of us with conventional tanks will ever have convection water currents in our substrate.


This makes sense to me. At one time during the winter I didn't have sufficient water movement in my 29 gallon tank because the filter needed cleaning. I reached into the tank for maintenance of some sort and the upper part of the tank was right at the correct temp while the bottom 6" of the water column was definitely cooler if not outright cold. I cleaned the filter right away and the temperature became consistent throughout the tank again.

I've never measured the actual temp of the substrate in my tanks, but there has never been a noticeable difference from the water temp at the bottom of the tank when I'm digging around with my fingers trying to plant or uproot something.

For me, as long as I have sufficient current in the tank to keep the water well mixed with no stratification I don't worry about the temperature of the substrate. My plants are growing well and I don't think heating the substrate is worth the hassle or expense since I'm happy with the results I'm getting.


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## MiamiAG (Jan 13, 2004)

In my experience, the typical aquarium has very little temperature stratification. This is because of the water circulation that we maintain in our aquariums. 

Also, the substrate solution (the water that is semi-trapped within the substrate pores) does have a slightly elevated temperature over the water column, especially if you have a lot of equipment underneath the aquarium. However, this difference is negligable and is certainly not enough to create any sort of convection current. Therefore, without artificial heating of the substrate, we will not have a forced circulation of water column water and substrate solution. Instead, this process will occur naturally and at a much slower pace.

The question, I think, then becomes one of benefit? In other words, do we care that we don't have a modest convection current circulating water column water into the substrate? Do we care whether the substrate solution is warmer than the water column?


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## bsmith (Dec 13, 2006)

The only thing I can contribute to this thread is that if you are in a creek/stream/pond/lake the mud on the bottom of it is usually quite abit cooler then the water. I attribute this to less sun getting to it. 

I have no idea if it would be beneficial ot not though.


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## jazzlvr123 (Apr 29, 2007)

go to the barr report he has a whole article up on his site entitled "why heating cables don't work" its a very interesting read


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## Gilles (Oct 12, 2007)

Well here are my 0.02$.

I have used heating cables for over 2 years now, and since i moved to my new house i finally removed them since my tank had to be emptied anyway. Not only does it use 25 watts of power (which was on 24/7) it also was in the way of my plant roots. The plants rooted to the cables, meaning when i did a trim, the cables went up also.

I fixed this by tying the heating cables to so called "root-cloth" it is a woven plastic which is used in the Netherlands in the garden to prevent weed from growing through your garden. This also gave the possibility to "lead" the heating cables exactly the way i wanted them to be.

Anyway, since i moved (1 week ago ) i removed the heating cables so i can't really tell if it is going to make a difference in root development untill some time has passed.


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## MiamiAG (Jan 13, 2004)

Well, I guess I have no choice but to write a rebuttal to Tom's post.

To avoid copyright issues, I'll TRY to summarize some of the points and then give my thoughts.

The paper that was referenced does not seem to support any position on heating or pore size of the substrate. The paper referenced is here.

To me, what this paper shows is that for the aquatic macrophyte Littorela uniflora, light, root morphology, root density, and sediment reducing capacity all played a role in the oxic zones around the plant's roots. In other words, this plant, like most other aquatic macrophytes, transports oxygen to the substrate via it's roots. This system was impacted by the above factors.

I personally think this papers supports some of the assertions we are making in this post. Please note that this post is not a "Heating cables are great" discussion. The intent is to think through, as Tom suggests, what it is that creates the best environment for our plants.

Tom states that makers of heating cables and substrates that focus on pore size (ADA's Power Sand) state that they will bring circulation into the substrate. This circulation brings in O2 and this will enhance root development. I disagree with this statement.

These products are trying to turn your substrate into a long-term nutrient storehouse for your plants. This is done by providing the right environment for the substrate ecosystem to develop. It is the substrate ecosystem that has a direct impact on the root/plant developement. Bringing O2 to the substrate is not the main objective.

The question that Tom should be asking (or, maybe he did and I just didn't get it) is whether these products assist in creating a better substrate. The answer is not clear but we can make some educated guesses.

Focusing on heating cables, I believe that there has been some discussion by experts in the field that looked at fluid dynamics and modeling to determine whether the cables would create the convection current we spoke of. The answer was "no." So, if we assume this is correct, then we can scratch heating cables as viable products, right?

Well, not necessarily. I would suggest to you that if you live in an environment where you use a heater in your aquarium, heating cables may bring around some benefit. There have been studies done (I don't have the reference handy) indicating that temperature also plays a role in the development of fine root hairs and overall nutrient uptake by aquatic macrophytes. If we could have the cables keep the temp of the substrate at a temp where root development and uptake are optimized, your plants should theoretically do better. How much better? I don't know. Is it enough to justify the added initial and maintenance cost of the cables? Probably not.

Cables came about because of failures Dupla experienced in their early days with one of their large aquariums in Germany. The found that the substrate was the limiting factor in plant growth after a while because it became anoxic thereby cause root rot. Their large aquarium was not sufficiently planted so that the plants themselves could not pump enough O2 into the substrate to keep it from dieing. So, they came up with cables under the theory that they would bring O2 rich water into the substrate therefore keeping it alive.

To be continued...


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## MiamiAG (Jan 13, 2004)

Continuing from above...

In smaller aquariums that are properly planted, I don't feel that heating cables add enough of a benefit to be worthwhile and justify their cost. This is especially true in tropical environment where the aquarium temp never goes below 80+F.

Looking at products that add to the substrate such as PowerSand, I am of the opposite opinion. The benefit you should see over time justifies the expense. Let's get into why.

Using PowerSand as an example, it uses pumice stone to give the lower substrate more space for colonization by the microflora and fauna. It provides a home to those things that make the substrate ecosystem. Again, this is important for many reasons including recycling of nutrients.

Tom is right in saying that pumice stone can be "squished" by the weight of the water column over time. Apparantly, ADA agrees and offers three different size of pumice stone in order to compensate for the depth of the aquarium. This tends to prevent the squishing effect.

Moreover, if you look at PowerSand, you will find that the pumice is mostly uniform in size. This is to avoid the settling that Tom refers to. Of course, he is right if PowerSand used different sized pumice stones in every package. However, it doesn't.

Lastly, lets focus on the substrate pore size (area between substrate grains) aspect to PowerSand and it's mate, AquaSoil. Substrate pore size is important because you do want to have some exchange between the water column and the substrate solution. The chemical makeup of the two will be different and it's important that it is. However, without some exchange, the substrate would go anoxic and we would end up with the same problem Dupla had. Pore size contributes to this slow exchange. By having a larger pore size in the lower layers of the substrate with a smaller pore size in the higher levels, ADA's substrate succeeds in slowing down the exchange and allowing the substrate solution to become favorable for the development of the substrate ecosystem. The lower section does not become a plenum, as suggested by Tom, as there will be some oxygen there albeit in lower quantities than the water column.

Lastly, please understand I am using ADA products as examples because they have been well thought out. It is perfectly OK for you to make your own version. Heck, I did. I think I made a post on it somewhere in here. However, if you don't want to bother, just save up for it and buy it or something similar.

Conclusion:

Well, after writing a wall of text, what can I suggest to you?

Heating cables? Probably not but now you know why.

Substrate amendment like PowerSand? Probably, it will help your aquarium in the long run and now you know why.


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