Why can't I grow red plants?

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George Farmer has some advice for a PFK reader who's struggling to grow red stem plants in his aquarium.
 

Q. I have a 180 l/40 gal planted tank that's been running well for more than two years. It’s fairly high-tech with suspended metal halide lighting, CO2 injection; nutrient-rich soil substrate, large external filter and daily fertilisers that include nitrates and phosphates.

I’m growing some pretty healthy plants without any sign of algae and am really getting into aquascaping.

However, I cannot seem to grow many red stem plants of the sort I see in various aquascaping contest photographs. I am keen to grow Rotala rotundifolia and Ludwigia arcuata but they just stay green!

Paul Remnant, email

A. You’re not alone! Some plants are harder to grow red than others and there are many theories but no definitive answers. There are a few techniques to try but they will all impact on the whole system, so be very careful.

The first plan is to increase your lighting levels. A lot of red plant species produce their pigment as lighting intensity increases as a form of protection — similar to us developing a suntan.

How old is your metal halide bulb? Consider changing it and/or lowering the unit nearer the aquarium. Be aware, though, that this will boost the nutrient and CO2 demand for all of your other plants and growth rates will increase.

A more controversial method is to limit nitrogen. Try stopping dosing nitrates and see if this encourages red growth. However, there’s a fine line between limiting nitrogen and starving the plants, so again be careful.

Finally, perhaps the easiest option is to choose plants that remain red in almost all conditions.

Good examples are Alternanthera and Tropica produces a 'mini' version that’s particularly popular as it has smaller leaves.

For bolder forms try lilies such as Nymphaea stellata or even some of the red forms of Echinodorus (Amazon swords).

I have also seen Rotala rotundifolia available in different forms, so perhaps try a different supplier, for example from the Far East. Ludwigia arcuata requires very high light to get the beautiful red colour you’re after.

George Farmer

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