Using the DSM method basically allows the plants to get ample Co2 direct from the air which boosts growth speed and eliminates the issues of algae. It also saves water and electricity during the grow out period, since you don't need to run a filter and do water changes.
Most of these carpet plants grow emersed above water in their natural habitats anyways, so that's their default growth environment.
You can choose to continue growing them out in the tank using DSM or just flood it... just that you will usually tend to get better and faster growth during DSM, compared to when the tank is flooded, especially if the tank is a low tech setup without Co2 injection.
HC can grow in non-Co2 injected tanks, but the growth rate will tend to be super slow, due to the limitation of Co2. More often than not when the new growth doesn't grow fast enough to replace the old melting growth, it'll look like it's gradually deteriorating over time. Algae also tends to be an issue due to the slower HC growth rate and melting parts.
I've kept some HC in my outdoor tank with only sunlight before and while it does grow a little bit, it's so slow that either algae or other more hardy lower demand plants quickly overtake it, so i end up just removing the HC. I haven't found temperature to be a significant factor in its growth rate, though i guess extremes should still be avoided.
When a tank is flooded after DSM is complete, it's common for some older parts of the HC to melt abit due to the transition process (sometimes mold or fungus may also be attracted to the melting leaves, which will usually disappear on their own or get eaten by dwarf shrimps anyways)... but by right, the HC should have already rooted in and formed a dense carpet by then (which is the objective of doing DSM) so the existing plant density should be able to recover quickly and continue growing.
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