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Prism Splitting Light
It blew my ten-year-old mind when my “all knowing” grandmother told me that the Blue Jay we were watching was in fact not blue. She explained that light is composed of many colors, and it is the colors that are reflected, not absorbed that our eyes perceive as the color of an object. This is a necessary reminder that what is perceived might not be what it appears to be. For decades the indoor gardening community has used Lumens as the standard increment for the measurement of light. Lumens were unfortunately a poor choice, here’s why.
Diagram of How a Prism Works
coutesey of freedigitalphotos.net
Lumens are essentially a measure of brightness based on human perception. Precisely, a lumen is equal to the light emitted by one candle falling on one square foot of surface located one foot away. This however presumes a human as the perceiver of the light. Plants “perceive” light differently; from a plant’s perspective, light that is useful for photosynthesis is not necessarily bright. Light, or more specifically, visible light is made up of wavelengths of energy on the electromagnetic spectrum ranging from 380-770 nm (nanometers). Plants utilize wavelengths from 400-700nm for photosynthesis. Brightness does not accurately describe if the light will be more or less useful to a plant.
Light can be characterized in other ways when discussing its benefit to plants. Color temperature is often referred to in the horticultural industry on lamp boxes to describe the color of the light emitted by the lamp. Does 4,000K grow a plant better than 7,500K? Color temperature is listed in Kelvin (K), which is a measurement of temperature. The temperature of what you might ask? It is a description of the relative whiteness of a piece of tungsten steel when heated to that particular temperature in degrees Kelvin. This accurately characterizes the color of the light as we perceive it, but color temperature again fails to address how effective a particular light source will be at providing the energy necessary to drive photosynthesis.
Don’t get frustrated by this inadequate information. There is in fact a measurement that precisely describes how effective a particular light will be for growing plants; PAR (Photosynthetic Active Radiation). PAR spectrum accounts only for light or more precisely photons emitted between 400-700nm. Scientists have concluded that it requires about 9 photons to bind one CO2 molecule in photosynthesis [6CO2 + 6H2O (+ light energy) C6H12O6 + 6O2]. Even though blue photons have more energy it has also been found that there is little difference between the effectiveness of red versus blue photons at driving photosynthesis as long as the photons are within the 400-700nm range. This leads to a direct correlation between the number of photons produced in the PAR spectrum by a given light, and the photosynthetic potential of that light.
Photons are emitted by light sources in very large numbers so we do not talk about billions or quadrillions of photons, instead we refer to them using the multiplier moles (which stand for 6.0221415 x 1023) To make the numbers even more accessible, the number of moles is often divided by 1 million resulting in micro-moles (μmol). Light sources emit photons continuously over time so the number of micro-moles is more accurately described as μmol/per unit of time (most commonly seconds).
When trying to quantify how effective a light source is beyond the total output of μmol/per second, you must consider one last piece of information… the size or area of your garden. Inevitably some of the photons produced will not reach your garden. So the most accurate representation of a light source’s ability to drive photosynthesis will take into account the area being lit and how many photons reach that given area per second; usually a square meters. That representation which actually summates the effectiveness of a light source for photosynthesis is written as μmol/m2/s. This descriptor is actually referred to as Photosynthetic Photon Flux Density or PPFD for short.
So in light of all of the information above, let’s remember that lumens are not a useful descriptor of a light’s ability to drive photosynthesis. I think I will sit back with a drink, and digest all of the information about PAR & PPFD while I watch the not blue Blue Jay outside my window.
After my one millionth discussion about light and constantly explaining why some bulbs cost so much more than others, it seemed like a good idea to conduct a side-by-side garden comparison to demonstrate the difference between plants grown under standard HPS grow lightsand Full Spectrum Metal Halide lights. For our experiment, we grew ‘Pingtong Long’ Eggplants under 150 Watt HPS bulbs and 250 Watt MH bulbs. Sounds to me like everyone is convinced that HPS is the only way to get the big harvest, but I suggest taking a look at the following pictures to decide what looks better to you.
Below you can see the difference between fruits grown under varying color spectrums.
The HPS light produced 5.4 GPW (grams per watt) for a total harvest of 1.8 Lbs of eggplant.
The Full Spectrum MH Light grew a whopping 6.4 GPW for a total yield of 3.6 Lbs of fruit!
The color of light is just as important to consider in the development of your plants as plant nutrientsand air circulation. So the next time you visit your local grow light store and ask for the “cheapest” bulb on the shelf, remember these photos and think about the harvest you want from your garden. Happy Growing!
Photo courtesy of Freedigitalphotos.net
In my prior blog I discussed the obstacles that indoor gardeners must contend with when using horticultural lighting as the primary source of light for their gardens. With those challenges of indoor garden lighting in mind, lets review what options indoor gardeners have when selecting lighting.
Fluorescent lighting comes in a variety of sizes, shapes, wattages and styles. The old standard 4’ shop light T12 bulbs (40 watt) produce about 2,600 Lumens. The newer 4’ T8 bulbs (40 watts) produce about 3,200 lumens. The preferred horticultural fluorescent lamps are T5 bulbs (54 watt) and produce about 5,000 lumens per 4’ bulb. There are also compact fluorescent lamps (or CFL) that have built-in ballasts in either 125, 200, or 250 watt options that produce about 45 lumens per watt.
| Lamp Type | 4’ T12 | 4’ T8 | 4’ T5 | Compact Fluorescent |
| Lumens Per Watt | 65 | 80 | 92.6 | Roughly 45 |
The pros of fluorescent lighting are that they have relatively low wattage, can be utilized in all sorts of size areas and configurations, and they are more affordable than some of the other options. However, fluorescent lighting does not deliver the intensity of light to provide good penetration through the canopy. There are also issues with fluorescents lacking the intensity necessary to produce fruit on high light plants. Lastly fluorecent lights give off a good bit of heat. They are an excellent choice for mother plants, clones, and young seedlings.
HID or high intensity discharge lighting has been the standard lighting for horticulture for decades. Although they produce an enormous amount of heat, they are able to provide good penetration of light through the canopy to about 3 feet. Utilizing switchable ballasts & different bulbs you can choose a spectrum heavy in blue light (Metal Halide) for vegetative growth, or heavier in red light (High Pressure Sodium) for flowering and fruiting. HID lights are reasonably priced, they are proven as a primary or sole light source, and they able to produce enough intensity of light to allow high light plants to produce excellent crops. For more information about HID lighting, check out this helpful HID lighting guide.
Plasma Lights are a type of electrodeless lamp energized by radio frequency or microwaves. The interest in this type of lighting is driven by two factors: spectrum and the potential for financial savings (based on lower electrical consumption). The spectral output of a plasma light is almost identical to the light spectrum of the sun making it ideal for horticultural applications. Plasma lights are also capable of producing large amounts of light from relatively small amounts of electricity yet they still produce large amounts of heat. Currently there are Plasma Grow Lights available for sale made by Gavita Lighting in Holland but they are still in their respective infancy. Based on my personal testing I will say they hold a LOT of promise.
LEDs or Light Emitting Diodes are starting to draw a lot of attention, not just from indoor gardeners but also from the general public. There are LED bulbs to replace your standard incandescent household light bulbs, LED flashlights, and even LED wallpaper. Why this explosion of LEDs, and can we as gardeners benefit from LED Grow Lights? Stay tuned to future blogs to find out.
cirrhopetalum lovely elizabeth
When you see this orchid flower you may wonder if it is it real. It certainly doesn’t resemble any flowers you see in your day to day life, but there is something intriguing about it.Maybe it is the bold contrast of pink spotting on the cream background or the tapering slender flow of the flowers, possibly the unusually fimbriation on the top segment of the flower, or the gently bobbing hinged lip. Whatever it is about this flower that attracts or repels you, you certainly must agree it grabs your attention. Cirrhopetalum Lovely Elizabeth is a hybrid of Cirr. Elizabeth Anne crossed with Cirr. rothschildianum. It blooms only once a year, but because it makes several growths each year when grown well, it is capable of putting on quite a show!
Required Light: In the greenhouse I use 50% shade cloth and have it in a spot where it gets mostly morning and afternoon sun but not Midday. Under artificial lights like T5 fluorescents I recommend it be 16” underneath at least two 4 foot T5 bulbs.
Temperature: This orchid prefers night-time temperatures to be between 65° F – 75° F. Daytime temperatures can easily reach the mid 90’s without causing a problem.
Humidity: Best kept at 60% RH or higher.
Required Watering: Depending on environmental factors, twice a week or more under normal conditions. More waterings are generally necessary when it is hotter in the growing environment. Clean water is preferred by this orchid genus of orchids so a Reverse Osmosis water filter may be necessary depending on your water quality.
Fertilizer: Should be applied at ¼ strength weekly. I prefer to use the Grow More brand of fertilizers.
Potting: Should be done annually as you see new roots just starting to emerge from the newest pseudoblulbs. I use a mixture of 4 parts coconut husk chips, 2 parts #4 Perlite, and 1 part Hydroton. Also I prefer plastic pots as they will not dry out as fast allowing the roots to stay comfortably moist.
To learn more about orchids and orchid growing contact the American Orchid Society or your local Orchid Society.
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