The Myths of Red Plants

From time to time I'll see a post on various forums either asking how to make plants turn red or a hobbyist will lament that a very red plant he recently acquired is turning green and thus questioning his own growing methods. Many will offer advice, advice often repeated that appears sagely. Invariably the hobbyist will be told to lower his nitrates and phosphates to the point of bottoming out, increase his iron and increase the amount of light and CO2. All of this is pure bologna. Except the bit about light and CO2 but we'll come back to that. The phenomena of plants turning red is called bronzing. This is the results of two fascinating mechanisms, the anthocyanin pathway which ties directly into photoinhibition. I'll let you take a few moments to read those articles first. For those who prefer to press on, the anthocyanin pathway is how a plant has pigment from its stems to its leaves to its flowers to its fruits and how those pigments are formed and transported to the plant's various tissues. These pigments are hydrocarbons called flavonoids. Photoinhibition is how a plant protects itself from excess light to avoid damage to its cellular structures by reversibly down-regulating photosynthesis. This can happen by, depending on the species, moving itself though in a limited fashion or more commonly by bronzing thus reflecting away the excess light, usually in the blue to UV spectrum. Bronzing is the result of using flavonoids to photoinhibit in the face of overly intense light.

So knowing that is the typical mechanism of bronzing, why would anyone do anything else other than just using stronger light or moving the plant much closer to the light source if it really is that simple? This is likely the result of correlation not implying causation.

For the moment, let's assume that it is well ascertained that merely having strong lights or growing a plant close to intense lights will make it turn red. We also know, still making assumptions for this argument, that increased light increases the demand for nutrients and CO₂. So in trying to get our plant to turn red by subjecting it to the stress of intense light, we've increased its metabolic demand. This follows logically but why on Earth in the face of this understanding would you want to decrease the especially important nutrients of phosphorous and nitrogen at such a critical moment? This would surely harm the plant and invite algae at the same time because of the imbalance.

It doesn't follow, does it? But perhaps there is a way to shoehorn such a belief in. Though I haven't polled or interviewed anyone, I have seen enough hobbyists come and go that I think I might know why. I believe the thinking goes, almost subconsciously, like this, "Since I'm putting the plant under the stress of increased light to make it turn red or stay red then perhaps using other stressors like reducing two of the three most important macronutrients will help as well?" Certainly stress upon stress will cause other physiological changes but all of them bad for both the plant and the tank.

As for the advice of adding extra iron, that is probably the result of affirming the consequent. Iron is red when it oxidizes, plants turn red to protect themselves from intense light; Therefore, red plants need more iron. Reduced to essential terms like that, such a syllogism sounds almost childlike, comical even. Especially since you just learned that flavonoids are hydrocarbons and other plant pigments are reduced carbon compounds. They contain no iron. Iron is important for photosynthesis because of its catalytic abilities but it does not contribute to a plant's pigmentation.

If you are absolutely bent on red plants then try plants that naturally tend towards redness like genus Ludwigia. There are also the old red standbys Alternanthera reineckii and Cryptocoryne wendtii 'Red' or the other variatus 'Bronze'. Blyxa aubertii is famous for bronzing up with little more than planting it in a bright unshaded spot. There are quite a few others and they don't require any different care than most other easy plants. They don't often reach the level of poke your eyes out red that many hobbyists lust for when left on their own, but they do add natural color, interest and accentuation without aggravating your neurosis.

There you have it. If you want really red plants, subject them to intense light but be prepared to meet there other needs as the light increases their nutritional requirements. This can be a dangerous balancing act inviting bad plant health or algae infestation. You have to really ask yourself, "Do I really need striking red plants and the trouble that might come with them or should I just enjoy healthy green growing plants?" Ultimately, it's up to you.