Okay, so your Venus flytrap or pitcher plant is having problems. You’ve looked at the growing mix in its pot, making sure that the sphagnum moss and silica sand are in the right proportions and not contaminated with green moss or anything else that might cause its untimely death. You’ve done the same thing with the water, making sure that you’re using either distilled or rain water if your local municipal water is too mineral laden. (You also already know that boiling tap water won’t get rid of dissolved minerals unless you’re running a still.) Even so, the plant is either declining, either growing long and lanky or pale, or it’s dying. You’ve checked the pH of the soil mix, you’ve checked on drainage if the plant doesn’t like sitting in water, and you’ve checked over and over for possible fertilizer contamination. So what else?
Well, have you looked at the light? I didn’t think so.
It’s funny that the most important aspect that distinguishes plants from animals or fungi, the ability to take water, carbon dioxide, and light and turn them into stored energy via photosynthesis, is also the aspect that gets neglected the most because it doesn’t occur to anyone that it may be a problem. Too little light, and the plant can’t produce enough sugars for growth. Too much light, and its leaves burn. Most indoor house plants originate from deep forests and jungles, mostly because they survive and thrive in the marginal light found in most houses and offices. However, even they have problems with too little light, which helps explain why the chrysanthemum or Spathophyllum in the back corner of the office hasn’t bloomed once in the five years since it was given as a “Get Well Soon” present.
Unfortunately, with the possible exception of the Australian lance-leafed sundew Drosera adelae, there’s really no such thing as a truly indoor carnivorous plant. Each and every variety requires a tremendous amount of light, in order to expedite growth, digestion, and even coloration. Give a Venus flytrap an insufficient amount of light, and it can’t digest trapped prey, causing the prey to rot and kill the leaf or sometimes the entire plant. Sarracenia pitcher plants, when deprived of light, can’t produce the nectar and coloration necessary to attract wasps and other common prey. Cut the light off enough, and a typical Sarracenia produces purely photosynthetic leaves, called phyllodia, in lieu of traps because getting enough light for survival is more important than snagging the nitrogen and potassium necessary for growth or reproduction. Sundews are even nice enough to let growers know if they’re getting enough light: the mucilage used to snare prey requires a lot of sugars, and that sugar production requires a lot of light. The best gauge of a happy sundew is a sundew covered with plenty of “dew” on its tentacles.
If it’s this easy to make sure that a carnivore gets enough light, then why do so many carnivores die from a lack thereof? I’ve run into so many people that tell me that their plants got “plenty of light” before it died, and they forget one very important fact: their eyes are lying to them.
The human eye is an exquisite sensory organ, the culmination of approximately 600 million years of evolution. It can register a wide range of color wavelengths, it can be used for closeup and long-distance viewing with a minimum amount of adjustment (just try to use a magnifying glass as a telescope in a matter of seconds), and it can adjust to both bright light and near-darkness with equal alacrity. That’s, of course, the problem: because the eye’s evolution was dictated by its need for such rapid adjustment, what seems to be adequate illumination for navigation or even reading isn’t the same as what’s necessary to keep a plant alive.
To demonstrate the deceptive nature of eyeballing light levels, it’s necessary to buy or borrow a standard light meter. This can be a professional photographer’s model, or it can be a horticultural light meter. The horticultural supplier Worm’s Way sells a variety of light meters of varying accuracy, including a solar-powered gauge that retails for about $30 US. I use this model for checking on light levels in my greenhouse, as well as for light levels in windows. It’s not perfect, but it gets the job done.
The first thing to do, after getting the light meter, is to get an idea of its upper range. In most circumstances, the best thing to do is stand outside on a good sunny day and note the reading. Since clouds, haze, dust storms, and obstacles such as trees and buildings interfere with that light, having an idea of the perfect conditions give a baseline for your further tests. By way of example, most carnivores thrive in open spaces such as bogs in areas where scrub and trees are burned off in regular grass fires, and they generally either disappear or go dormant if the scrub becomes too thick to allow sufficient light to hit the bog.
Now that you know what your gauge gives you when exposed to unfiltered and unobstructed daylight, let’s look at the areas where your plants are growing. One of the reasons why standard plate glass is used for windows, other than its cheapness compared to plastic, is because it allows approximately 90 percent of light hitting its surface to pass through without being absorbed or deflected. The polycarbonate used for most greenhouses these days transmits about 80 percent of that light, and it yellows and darkens over the years as it’s exposed to high levels of sunlight, so it’s not as efficient as glass. It gets used, though, because it’s a lot lighter than glass, it’s much less dangerous to move and position, and it’s more likely to absorb or deflect the impact of wayward rocks, baseballs, hailstones, and small firearms without shattering or exploding.
This is also considering that the glass or polycarbonate is clean, new, and unsullied. Experienced greenhouse managers know that too much light can be just as bad as not enough, and Texas gardeners understand that a judicious amount of shade can sometimes be the only thing that gets a collection of plants through the summer. This includes shade cloth being strung over the greenhouse, applying whitewash or thinned latex paint that washes off throughout the growing season, or even planting annual vining plants such as morning glories or moonflowers to provide shade until the first serious frost. Any shading that helps cut the heat in a greenhouse is also going to cut the amount of visible light available to plants inside, so the varieties of carnivore that don’t go dormant in the height of summer, such as most butterworts and bladderworts, may have to be grown indoors if the outside temperatures get above 96° F (34° C) for too much of the growing season.
Okay, you say, you’ll just grow your plants indoors next to windows. It’s time to pull out the light meter again, and measure light both throughout the day and at varying distances from the window. Sure, the light coming through the east window is enough for reading or watching television, but remember that almost all carnivores need the equivalent of full daylight for at least six hours per day, every day. Whip out the light meter and measure the difference between the light available right against the window and the light available at the couch or chair in which you do most of your activities in that particular room. Unless you’re the sort to light everything with multitudes of halogen bulbs, I’d bet that what’s suitable for general indoor human activities is a slow death sentence for your plants.
When we get into indoor lighting, then the light meter gets its greatest workout. Most of the indoor growing guides that recommend that the lights remain only a few centimeters above the growing plants aren’t saying this for grins and giggles, as the light intensity drastically decreases upon distance from the light source. This is why almost nobody recommends using incandescent “grow bulbs” any more, because they throw off so much heat along with their pathetic amounts of visible light that any plant underneath them bakes or parboils at the distance where they receive enough light to make a difference. (To a much lesser extent, this can happen with fluorescent bulbs and tubes, as the ballast throws off enough light that some can be a fire hazard in certain circumstances.) Serious indoor growers use halogen or sodium lamps to supply their plants with suitable light, but those usually come with metal cases with ventilation ducting to vent heat away from the plants. As of this writing, many of those high-intensity halogen or sodium bulbs are being replaced with LED arrays that supply the light without the heat, and for a significant energy savings as well.
When testing indoor lighting, it’s not just enough to test the light intensity at varying distances from the light source, but at varying times. Besides the lack of heat and the lowered electricity costs inherent in using LED lights, LED lighting’s greatest advantage is that the individual light-emitting diodes still produce the same light intensity over years of use. This isn’t true with fluorescent lighting. While standard fluorescent tubes and compact fluorescent bulbs are great for plant lighting, being cheap, energy efficient, and easy to replace, most have an effective lifespan of about eight months to a year. When I say “effective lifespan”, I mean that while the tube or bulb continues to emit light, it’s too low for any plant’s benefit. Unfortunately, as I noted about our eyes’ ability to lie to us, we don’t see the gradual decline, but the light meter never lies.
To give an example, I have to share a story about my good friend Cheryl LeBeau, who lives in Connecticut. She recently started raising American anoles (Anolis carolinensis), which live and thrive in roughly the same conditions as most North American carnivorous plants. One night, she called me asking for serious advice, as her two anoles were dying and a third lizard in the enclosure was not far behind them. I asked about heat and humidity, and she had exquisite control of both. I asked about food and water, and discovered that they were well-fed and well-hydrated, and vitamin and mineral supplements in their water wasn’t making a difference. Cheryl was fond of these lizards, and didn’t want them to die, so I kept asking further questions about their husbandry. Finally, after having eliminated everything else, I asked “What sort of light do you have?”
She related that she was using a fluorescent fixture with a bulb that emitted additional ultraviolet light for reptile health.
“When was the last time you changed the bulb?”
“Oh, about a year ago.”
“That’s your problem.”
“But how could it be the light? The bulb is still working.”
See, Cheryl’s eyes were lying to her, and they were telling her that the light off that old tube was still adequate even as the lizards were telling her it wasn’t. Two days later, she was able to purchase a new tube for her enclosure’s light fixture, and the lizards jumped back to good health so rapidly that she’s still amazed at the difference. She didn’t mean to let things get so far, and she was working with the best tools and information available to her about her lighting. Unfortunately, she wasn’t informed that she needed to change those tubes on an annual basis, and her lizards almost died because nobody thought to inform her until she asked me.
This, incidentally, is why I just replaced all of the lights in my indoor propagation tanks, which use both sunlight from a west window and 20-watt fluorescent tubes. These don’t have to be expensive UVB and UVA tubes designed for reptiles or for saltwater fishtanks: standard tubes sold at hardware and home improvement stores as “plant lights” get the job done, and they can be mixed with the typical cool white tubes to save money. With compact fluorescents, try not to use any bulbs rated higher than 23 watts if they’re going to be used in enclosed areas such as a terrarium because of potential heat buildup. (A handy money-saving tip: considering that full-spectrum CF bulbs are still a bit expensive, they can still be used for standard illumination, such as in hallways or on front porches, long after they’re no longer suitable for plant growth.) Other than that, just measure the light produced by a new bulb or tube with your handy light meter, and check the light every month. When the light produced drops below 50 percent of its original output, switch out bulbs or tubes and notice how rapidly the plants bounce back.
Now, it’s not absolutely necessary to buy a light meter. I’m sure that your eyes are up for noting increases and decreases in ambient light. Considering the cost of your carnivorous plants, though, do you feel particularly lucky?