From time to time, we would receive a concerned 'Oh no! What did I do wrong?' email , with a picture of a plant looking like the one above (a bouquet of Lobelia cardinalis after 4 months)
Actually, nothing is wrong.
Aquatic plants, unlike certain woody terrestrial species, are not perennial. They are are more like herbs, which require regular re-propagation. For aquatic plants, this means regularly cutting and replanting the healthy tops.
Above: cutting and replanting healthy tops is necessary for all aquatic plants, especially stem plants. The 5 steps would be:
1. Cut the top 4- 5 inches of healthy tops.
2. Pull out and discard all old stems and roots.
3. Enrich substrate.
4. Replant healthy tops.
5. Water Change the 2Hr Way to remove stirred up detritus.
In the 5 steps above, step 3 is often neglected. Depleted substrate results in thinner and weaker plants over time. You can enrich substrate by mixing in new aquasoil, or use something like APT Jazz.
Here, it is important to know that merely adding Iron or Trace minerals is not helpful. The element that naturally depletes over time, and which has the most impact, is nitrogen. Nitrates do not naturally stay bound in substrate (they are water soluble and quickly enter the water, which leaves the substrate depleted). So using a root fertilizer that retains nitrogen in the substrate is key (and the reason why we invented APT Jazz)
Step 5 is also often neglected, which creates alot of algae problems. After replanting, always make sure to siphon away floating detritus. The tank water should be crystal clear after a proper Water Change the 2Hr Way.
With regularly enriched substrate and regular replanting, you can keep Lobelia cardinalis looking like the bouquet above 'forever'. For lower maintenance, rhizomes (vs stem plants) like Java Fern (below) are good options.
Old leaves can be simply trimmed and discarded. There is no need to regularly cut-and-replant. Java Fern would grow dense, and after many months it would be necessary to thin the bush, but it does not require cutting and replanting the same way that stem plants do.
You mean aquatic plants are not like trees?
Aquatic plants are more like herbs, where their natural lifecycle involves natural deterioration of lower stems and roots.
I can just replant the top...without roots?
New roots will sprout quickly and naturally.
When should I cut and replant?
Always cut and replant before the tips reach the water surface. Whenever lower shaded stems start to deteriorate, it is time to replant.
But I see people trimming, instead of cutting and replanting?
When bottom stems are healthy, you can trim once, twice, even 3 times. But after 3 trims, it is generally time to replant, even for the healthiest stem plants.
Isn't that alot of work?
Experienced aquarists would keep a mix of low maintenance (non-stem) plants and high-maintenance plants (stems are carpets). A tank that is mostly filled with stem-plants does require alot of maintenance!