Root zone vs. air temp

hydro_scottyJuly 7, 2010

Hey all -

I am growing lettuce indoors (temporary until I get the greenhouse going) and wanted to see if I could reduce my costs a little. Does anyone here think that I could let my ambient temperatures rise (84F or higher) if I chill the nutrient tank to a constant 65 - 68ish F? I am using NFT and it is killing me to run AC all the time to keep the environment cool. All inputs welcome.


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Well, possibly. I do know that heat can cause lettuce to become bitter in taste. I would take that into consideration. If you don't mind the taste then your idea sounds good to me though!

    Bookmark   July 7, 2010 at 7:50PM
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Hi hydro_scotty,
Actually a simple but also tricky to answer question because it is about playing economy versus safety and successful growing in your case.

It simply is a fact that lower nutrient temperatures (artifical cooling) allow to grow such cool temperature crops as lettuce in much warmer climate or environment. Because in fact these rather famous 24 C (75 F) are originally issued from experimental research work by Thompson, H. C. et al. (1998). As they firstly reported that root (nutrient) temperatures influence lettuce growth in raft culture considerably. They in fact demonstrated the importance of optimizing root- but also air temperatures in lettuce culture and production.

Dr. Howard Resh has also reported, that with tropical climate and daytime temperatures that exceed 95° F (35° C), they were able to decrease bolting and pythium infections, when cooling nutrient temperature even lower, to 65 F (18 C). But as far as I know this (18°C) is a single report and until confirmed with other research, shall be considered as rather selective observation and optional.

Then again here in Thailand, a few farms grow lettuce (commercially not scientifically) in classic NFT systems in similar conditions (30-35°C) in the northern regions, but with much higher nutrient temperatures (28°-30° C) in NFT and in fact successfully. No sopisticated filtering (as in 5 micron spore filtering), and no artificial cooling is used. Why can they do that? Well because they grow just a few heat- and bolting resistant varieties, prevent nutrient temperatures from rising above 30° C with underground reservoirs, etc. And yet, they grow successfully for years with conditions that are far from ideal as we understand it.

My own (actually involuntary ) experiments with NFT during last hot season just a few month ago, have confirmed that lettuce can indeed grow in much higher air temperatures as ideal, - and even with warmer nutrient temperatures I would ever have imagined. Yes, sooner or later, several varieties were bolting one after the other due to excessive heat. And yes, pythium was coldcocking eventually. But actually only later- a bit before I pulled the plugs.

The other lesson I've learned with the same experiment, - was that heat, bolting and bitterness (in taste) aren't necessarily mutually inclusive. I can't actually tell why, but none of the varieties I tested even with heavy bolting, did taste bitter. We were eating plucked leaves from bolted lettuces until the plants were finally cut off and composted. It goes without saying that never the less, bolting or bolted lettuce have strictly no commercial value.

Now, where is the limit, or how high can I actually let nutrient temperatures raise "safely"? Do I simply play safe or do I take some risks for the sake of energy saving? This is not only an individual decision, the actual risk factor depends on too many variables and hence has to be tested under specific (in fact existing) conditions and circumstances. Practically and in real life circumstances of course, - not necessarily "scientifically".

One thing that I know for sure and would adopt for future ventures from now:
I wouldn't stubbornly try to keep and parrot the status quo of 24°C, but just live and let live with some 2-3°C more.

    Bookmark   July 8, 2010 at 12:48AM
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Interesting info on Thompson's research Lucas. I have a question... could high Nitrogen levels in the nutrient solution cause the lettuce to become bitter? Generally speaking my grow tent had been at a constant 80 - 85 F temp and the romaine was very, very bitter. Tasted like I was eating pure nutrients from the tank in lettuce form.

    Bookmark   July 8, 2010 at 3:04AM
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Hi ethnobotany!
I guess that some sources that claim that nitrogen levels are responsible for pronounced bitter taste in Lettuce are either based on a myth or circumstantial only. Means that nitrogen levels (if ever) only matter if other factors also match. Bitterness is primarily dependent on factors like: Genetics, heat, age (advanced bolting). In fact some varieties are already somewhat (more) bitter by trade than others - and this bitterness gets more pronounced with reproduction stage and appearance of a stalk. The bitterness is obviously an allelopathical defence mechanism, to give (increased) protection against natural enemies as soon as the "reproduction organs", as in stalk and seed support develop. Even though, the plant could afford to loose a few leafs in an early stage, - a missing or damaged stalk would be fatal. Thus production of allelopathic substances get increased whit reproduction cycle.

Part of the solution seems indeed to chose the right varieties that are A. heat and bolt resistant and B. do not tend to develop any or no strong bitterness. In your case I'd try JERICHO Romaine, a variety that was bred for Israel's desert heat and is supposed to resist heat, bolting and obviously is unlikely to develop such pronounced bitterness.

As for the Nitrogen content (I doubt you can taste it) I am using a lettuce formula that uses a proportion of around 1:1.15 N versus K, hence lower N as K content. Very low P (around 25 ppm) and equally low Mg. Low Fe as well, only 1.2 ppm compared to usual 3-6.

Hope this helps, - as they say... ;-)

    Bookmark   July 8, 2010 at 4:52AM
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