Why do aquarium plants grow? Some may find this a seemingly easy question. Often the best questions are deceptively simple. Growth of macrophytes is defined by the same definitions in terrestrial plants and algae: The progressive irreversible development of an organism (Taiz and Zeiger, 1998). Lambers, Chapin and Pons (1998) define growth as increment in plant mass, volume, length and/or area. If a plant takes up water, it may gain mass, it may increase in length, volume, area with no net gain in carbon. So does this definition by Lambers, Chapin and Pons (1998) adequate for aquatic macrophytres and aquarium plants in general? Perhaps not by many people. Another set of good simple questions are : what type of growth is being discussed? For example Floral growth? In general, aquatic macrophyte growth in the narrow field of aquatic horticulture tends to be almost entirely vegetative for hobbyists and a few species are grown from seeds on rare occasion. Basic water plants can be helpful.

How is live aquarium plant growth measured? Generally aquatic plant growth can be measured quite a number of ways. Dry weight biomass is a very common and simple method. Wet weights are sometimes used (but think about water that sticks to the plant by adhesion) of a plant, would this be a good assumption, or a bad assumption? If the wet/dry weight ratio is stable, then these wet weights can be utilized, there are methods to address such issues. Cell number is another approach at the microscopic scale. Kinematics is a rather interesting method to describe and measure growth (see Silk, 1994). This involves the movement of fluids such as ocean waves produce specific forms, as plant cells are primarily fluids this method has been used to describe plant meristematic growth. An introductory hobbyist guide to Aquatic Plant Food is one of the best resources available.

What about hobbyists? Often the amature hobbyists has little access to such methods, but more basic approaches may be employed even if they have some assumptions that might not be ideal in a more rigorous approach. Simply measuring shoot length over time or new bud formation over time can be quite successful. Wet weights can be used as long as the treatment for each measurement is similar. A salad spinner to remove the water consistently is useful and relatively inexpensive. Drying the plants for 30 seconds of spinning in the salad spinner is fairly consistent and some researchers have used this method with fairly good results.

This requires accurate scales as do dry weights. One of the biggest problems for the hobbyist is the control tank. Most often aquarist do not set aside a control tank, they have little way to know if their test treatment has any real affect compared to a non treated tank. Additionally, the aquarist that might have two similar aquariums often has trouble keeping them in similar shape even without any such control tests. Split tank test work well as the water chemistry is similar between each side, but often the only parameter that is easy to keep isolated is light.

To learn more about aquarium plant nutrition, check out the Perpetual Preservation System to learn more about plant growth nutrition needs.