The Humidity Myth, Part 2: Your Nepenthes Doesn't Need a Humidity Dome

Jacob here with Sarracenia Northwest. This is part 2 of the Humidity Myth and Nepenthes. In part 1, I explained how Nepenthes adapt to fluctuating humidity and don’t need a rainforest to grow pitchers. I talked about it within the context of a grower who placed her plant under a humidity dome. So in part 2, I’ll go over the risks of actually growing a Nepenthes under a humidity dome. Do the risks outweigh the benefits when compared to windowsill growing?

We’ll start with an explanation about how Nepenthes adapt to fluctuating humidity. And then, I’ll talk about the three most common risks associated with humidity domes. Each section of this video is timestamped, so you can skip to appropriate sections and rewatch them if necessary.

Throughout the day, relative humidity will rise and fall. When the temperature goes up during the day, humidity typically goes down. But when the temperature cools at night, relative humidity goes up. This happens because the air’s capacity to hold

water vapor changes with temperature. When plants adapt to fluctuating humidity, what they’re really doing is regulating moisture loss in their leaves. All terrestrial plants, including Nepenthes, regulate moisture loss in three different ways – transpiration, cuticle, and leaf size.

Transpiration is the most important method that plants use to regulate moisture loss. The basic concept is this: When water vapor escapes from leaves, water is pulled from the soil through its roots. The plant literally functions like a straw. As water vapor escapes from leaves, that action sucks water up from the soil through its roots, and is transported throughout the entire plant. This process of transpiration is how plants maintain leaf moisture throughout the day. It literally governs how a plant grows.

Now, leaves are somewhat water-resistant, but they’re covered with microscopic pores called stomata. You can’t see them with the naked eye, but they’re all over the leaf. The majority of stomata are located on the underside of a leaf. Stomata allow for gas exchange for photosynthesis. Plants take in carbon dioxide for photosynthesis, and release oxygen as a waste product. It’s like breathing. However, plants are breathing in carbon dioxide and releasing oxygen. During this breathing process through their stomata, water vapor naturally escapes. To replace that water vapor, plants rely on water pulled from the soil through the roots.

Transpiration rate varies when relative humidity fluctuates. When humidity is low, transpiration increases because more water vapor escapes. So, more water is pulled from the soil to maintain moisture balance in the leaves. If more water vapor escapes from the leaves than the amount of water the plant is able to pull from the soil, wilting occurs.

If you grow tomatoes during the summer, you might be familiar with this effect. Tomato plants are notorious for being water hogs. On a hot summer day, they can use up a lot of water. Here in Oregon, summers can be very dry, and we sometimes have to water our tomato plants twice a day just to keep them happy. So transpiration is the primary way plants regulate leaf moisture when humidity fluctuates. Just remember this. When water vapor escapes, water is pulled from the soil through its roots. 

The second way plants regulate leaf moisture is by producing a waxy layer on the leaf surface. This waxy layer is called a cuticle, and it’s a natural sealant to protect the plant from UV damage and excess moisture loss. It also makes leaves look shiny and feel firm to the touch. The cuticle occurs on both the top and bottom of a leaf, but it’s thicker on the top where it’s exposed to direct sunlight. So, when plants are exposed to bright sunlight, they will produce a cuticle barrier for UV protection.

Plants will also produce a cuticle when they’re exposed to lower humidity for a long period of time. Think of a cuticle like a layer of varnish on wood. Varnish protects the wood from sunlight and water damage. It also prevents it from drying out and developing cracks. So, a cuticle works in a similar way. It’s one of the ways plants regulate moisture loss.

Plants also regulate moisture loss by growing smaller leaves. With smaller leaves, there’s less surface area for water vapor to escape. In regards to Nepenthes, this translates to smaller pitchers. So, when Nepenthes are growing in fluctuating humidity, it will naturally have smaller leaves and pitchers to reduce moisture loss. How much smaller? About 25%. So relative humidity will affect leaf and pitcher size, while sunlight and temperature will affect the overall production of leaves and pitchers.

So these are the primary ways plants regulate moisture loss – pulling water from the soil through a process called transpiration, producing a thick layer of cuticle, and growing smaller leaves. Now that you know how plants regulate moisture loss, you can rewatch Part 1 with a better understanding of why Nepenthes don’t need a rainforest. These plants are smart and will adapt to fluctuating humidity.

But for the sake of argument, let’s look at how a Nepenthes would grow under a humidity dome. What could a grower expect when growing a plant under a humidity dome? In Part 1, I explained why humidity isn’t a driving force for pitcher development. Instead, humidity affects pitcher size. So, you might expect larger pitchers when using a humidity dome. This happens because high humidity decreases transpiration, so there’s less moisture loss from the leaves. With less moisture loss, a plant can afford to create larger leaves and pitchers without losing too much water.

Very high humidity also stabilizes temperature. Water holds on to heat very well, so when there’s more water vapor in the air, the air can hold on to the heat longer. By holding on to the heat longer, you prevent large temperature swings throughout the day. With more stable conditions under a humidity dome, you would expect a larger and healthier plant. But that’s a best-case scenario. When growers grow their Nepenthes under a humidity dome, they rarely see this happen. Instead, they often see health issues.

Under a humidity dome, a common symptom you might see are elongated leaves that are poorly colored and dull. If we look at the grower’s Nepenthes saguinea more carefully, we can see evidence of this happening. The newer growth is dull and unnaturally elongated. Poor lighting can cause elongated leaves, but the leaves would be dark green if that were the case. Because the leaves are uniformly light green something else is going on.

When humidity is too high for too long, a plant will have a hard time exchanging gases for photosynthesis. Essentially, it’ll have trouble breathing. Think about entering a steam room and how difficult it can be breathing with all that steam. The same is true with a plant in very high humidity. To improve breathing, a plant will grow elongated leaves to increase surface area. With more surface area, gas exchange or breathing, if you will, improves. However, because leaves are stretched out, they will be thinner than normal and prone to leaf damage, such as tearing and spotting.

The leaves on this Nepenthes sanguinea don’t look so bad, but they appear longer than normal. Even the pitcher looks elongated when compared to a plant grown in fluctuating humidity. The leaves are also light green. When a plant struggles to breathe because the humidity is too high for too long, it will shut down part of its photosynthesis pathway. Leaves will then turn pale green, and eventually yellow.

The leaves also appear dull, which means this plant has less cuticle than normal. Cuticle is the waxy sealant that reduces moisture loss or transpiration. If humidity is constantly high, there is less moisture loss, so there’s less need for a cuticle barrier. With less cuticle, leaves appear dull, much like wood without varnish. Leaves would also feel flimsy and be prone to sunburn.

Now, the grower said she purchased the plant a few days before she took this photo. The appearance of this plant is consistent to other plants we’ve seen grown under a humidity dome. So, I’m going to assume the plant was originally grown in very humid conditions before she got this plant.  

Here’s also an extreme example with several carnivorous plants grown in a plastic cube. It doesn’t have a Nepenthes, but the plants in it are showing the classic health issues we see with humidity domes. Their leaves are paper thin and light green. The leaf of this flytrap is so thin that the veins to transport water are showing. You can also see it happening to the sundew. When you compare them to properly grown plants, you might have a better appreciation for fluctuating humidity.

Under a humidity dome, root rot can also happen. I don’t see symptoms related to root rot in the grower’s plant, but it is a common risk associated with humidity domes. When humidity is constantly high, transpiration decreases, so a plant won’t pull up as much water from the soil. This means the soil will remain wet longer. With Nepenthes, this is very bad because waterlogged soil will promote pathogenic bacteria, causing root rot. When roots die from root rot, plants can’t sustain proper growth. One of the early signs of root rot is older leaves turning yellow and dying off quickly. In late stage root rot, the growing tip will turn black. When that happens, there’s not much you can do to salvage the plant. We’ve seen this happen many times to Nepenthes when the soil is waterlogged.

Fungal and mold issues are also common with plants grown under humidity domes. I’ll admit that mold and fungus aren’t limited to humidity domes. They can happen to plants grown in a greenhouse. But many growers who maintain plants in high humidity will often complain about mold along a pitcher or leaf. Most times these issues are cosmetic and don’t pose a serious health problem to the plant. However, plants can become susceptible to more serious fungal attacks if the cuticle layer is thin from plants growing in constant high humidity.

 So, these are the common issues growers encounter when using a humidity dome. In a closed system, such as a humidity dome, humidity can be unnaturally high. This will cause disruption with transpiration and gas exchange, which can lead to weak leaves, root rot, and fungus. There are other risk factors associated with humidity domes, but the three I just covered are the most common ones we’ve seen.

Now there are Nepenthes aficionados who grow their plants in closed systems with constant high humidity, and their plants are jaw-droppingly gorgeous. However, these growers have rare specimens that cost several hundred or thousands of dollars. So, they’re willing to invest an equal amount money on equipment to create a tightly controlled environment to protect their investment and maximize growth.

 For the average grower who picks up a Nepenthes at a garden show for 20 bucks or so, that method of growing is more like a science project than casual indoor gardening. As long as you follow the recommendations for lighting, temperature, and watering, Nepenthes will acclimate to fluctuating humidity and produce pitchers.

Sure, your plant might have smaller pitchers than one grown in a tightly controlled environment, but if you’re the type who thinks you’re a bad grower because your Nepenthes produce 6-inch pitchers rather than 8-inch pitchers, find a new hobby. But if you simply enjoy the company of plants in your home because they make you smile, you won’t need a fancy setup. The Nepenthes you might find at your local nursery or garden show will grow well in a sunny windowsill without a humidity dome or expensive equipment. The majority of Nepenthes available to the average grower can easily be grown in a sunny windowsill or under lights. And this is how we recommend growing Nepenthes. We’ve been giving out this recommendation since we started the nursery in 1995, and we stand by this recommendation to this day.

 If you’re looking for Nepenthes that are already acclimated for windowsill growing, check out our inventory at growcarnivorousplants.com. We produce our Nepenthes with windowsill growing in mind, and we have many plants that are great for first-time growers. Our inventory changes monthly and seasonally, so sign up for our newsletter or subscribe to this channel for notifications of inventory changes. Thank you so much for watching. I look forward to seeing you in another video.

• The original question and response have been edited for publication.
• To submit a carnivorous plant question, visit Ask the Growers.