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How much light should my nepenthes get ?

The set up:
I have two terrarium on my windowsill that get direct sunlight ( 2 hours in total ) and indirect sunlight for an additional 4 hours). The rest of the day and night I have the CPF tubes running. My question is: How much light does nepenthes need ? Does it need only 16 hours of light or 12 hours of light?
Direct sun can cook your plants in your terra. From my experience, 16hrs of indirect sunlight or/and your tubes is perfect. Your nepenthes will tell you if they are happy, listen to them :)
Nepenthes like as much light as you can offer, but they get fussy over rapid changes in conditions. As Maiden says, direct sun can cook Neps in a terrarium. Honestly, direct sun can cook Neps even not in a terrarium - proceed with caution. If you want to give them more sunlight, consider taking them out of the terrarium. If you're already growing them in sun, chances are that the plants themselves are tough enough to handle it; just watch out when the weather gets warmer that you don't bake the whole terrarium with the greenhouse effect. As for timing, 12 or 16 hours are both fine - choose what works for your schedule. Once you become more familiar with your plants, you can use small variations in photoperiod and supplementary sunlight to encourage flowering. Nepenthes Around the House is a must-read.

Nepenthes are pretty tolerant, adaptive plants in general so long as you don't surprise them. The most helpful tip I've found regarding Nep cultivation is this; you can get one aspect of care totally wrong as long as you get all the others right. Nepenthes like bright light, moderate or higher humidity, soft water, moderate temperatures and fresh air at the root zone. Sunlight in a terrarium will change your light levels, obviously, but it'll also raise the temperature, as well as humidity. Well, humidity is good, right? Yes, sort of, most of the time. In this case, however, humidity trapped in a terrarium can make it harder for your plant to breathe. Mugginess prompts Nepenthes to make larger, flimsier leaves that are better at exchanging air, but can't hold onto moisture as well, which causes the plants' water requirements to rise. Identify which conditions are easy for you to provide and those that are difficult and plan accordingly.

If you like having your plants in a sunny windowsill, you can make the most of it by finding a way to provide them plenty of fresh water and an airy media. I have similar constraints - it's easy for me to provide lots of light, but most of my collection is too big for conventional terraria, which makes them drink a lot. I leave my plants sitting in water because I can't hand-water often enough to keep up with their needs. Sitting in water could smother the roots, so I use net pots, rough aggregate media (lava rock, orchid bark, ceramic clinkers) and live sphagnum to let air in. The extra air circulation could cause the media to dry rapidly in a draft, so I overpot to maximize the media volume to surface area ratio. Plain drip trays made the organic parts of my media break down faster than I'd prefer, so I've moved towards using fountain pumps and air stones to keep the water moving and fresh.

So, it's about as easy as a Rubik's cube. All the care requirements are interconnected and if you change one, the plant will respond and alter it's strategy accordingly, which may create entirely new problems. It's not a total loss though - the lesson to take away is to not act unilaterally. So long as your plants aren't obviously dying right now, you should think about the change you want to make, do it in little increments, and watch how the plants respond. I like to use each new leaf as an indicator of what my plants think about what I'm doing for them. Plants are pretty smart; each leaf is a little power plant that's tuned for precisely the place and time in which it grows. Changes in stem size, the speed at which the pitcher matures, and the texture/feel of the petiole give a lot of hints about environmental conditions, water exchange, nutrient uptake, and root zone aeration. A plant that is in distress due to dryness will have limp, lukewarm leaves; a plant that is slowed by dryness but healthy will instead have leaves that are thicker and rubbery, possibly fuzzy, and will be cool to the touch from transpiration. It takes time to learn all the signs, and they vary to some extent from species to species. If you want to get a jump start on your Nepenthes fluency, try reading up on habitats and the different Nepenthes subgroups.