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Lighting an Indoor Garden

Posted by Gordon Redman on 1/1/2016

Most plants will grow despite what I say here and I'm sure that at least one of my views will be news to you. You may not agree with me because of what you may have heard, read or experienced. What can I say? My views here represent many years of helping gardeners be better growers. I'm not going to tell you that they are better gardeners than you, but most of them are successful growers time after time. To me, consistency in great harvests is the most important thing for gardeners. When you're not fighting failure you inherently have the time, money and fortitude to try different techniques. I like time, money and fortitude; it’s that combination that made me a better gardener. I hope that your continued success will give you command of time, money and fortitude. So here we go. I will expound on some of my basic lighting views that I assure you will not hurt your plants. I think will give gardeners who are new to indoor growing a good head start. For others, it may be time to think fresh.

Lighting the Vegetative Stage of Plant Growth

When it comes to lighting of the vegetative stage of plant growth I recommend Metal Halide lighting. However, the new T5 fluorescent lamps are quite nice if you can get them down close to the plants. The reason behind this view is the blue spectrum that Metal Halide lamps inherently emit from the burning of the metal halides inside the lamps electrode. It is my belief that good vegetative plant growth starts with a light spectrum of 3000-4000 degrees Kevin. A spectrum below 3000 degrees Kevin may signal the plant to manufacture hormones for bloom growth and stretch toward the light source but may also prematurely prepare for winter’s die-out. You don't want this happening in the vegetation stage of plant growth.

Metal Halide (MH) lamps can be purchased with Kelvin ratings of 3000K to 14,000K. For gardening we only need to pay attention to MH lamps with Kelvin ratings of 3000K to 6000K. When comparing MH 6000K lamps with MH 4000K lamps, many gardeners have reported better vegetation growth with 6000K lamps; but then again just as many gardeners don’t like the 6000K lamps. Before you try the 6000 Kelvin lamps know that there is a significant extra cost with the 6000K lamps. It is up to you and your pocket book to decide that one. Mixing the MH 6000Kwith 2000K HPS (High Pressure Sodium) or another MH 3000K lamp or 3000K T-5’s might very well be a good application choice. Many gardeners have successfully used the MH 3000 Kelvin lamps for vegetative growth and used the same 3000K lamps for bloom. While HPS lamps rated at 2000K are used by some gardeners for vegetative growth, I would never use them without the use of helper lamps to boost the Kelvin temperature rating to at least 3000K. Usually these helper lamps are T-5 fluorescent with a color output of 5000-6500 Kelvin with the actinic lamps really showing great promise.

HPS lamps are rated at 2000 Kelvin. It is my view that there simply isn’t enough blue light emitted from these lamps for a proper and healthy vegetative plant growth and UV is very low. Even the HPS lamps that advertise 30% more blue light than standard lamps don’t have enough blue light. The standard HPS lamp only has about 2% of its spectrum in the blue range. HPS lamp manufacturers that advertise 30% more blue in their lamps really don't have much to brag about. In other words, they raised the output of blue from 2% to 2.6%.

New tests have shown that combining one blue actinic T-5 lamp for every three 3000 Kelvin T-5 tubes can be a great help in producing healthy strong vegetative plants. Actinic T-5 tubes produce Ultra Violet (UV) light. I believe it is this frequency that promotes the plants positive health response. I also believe this is the frequency that LED light manufacturers are trying to emulate with their red and blue lamps. Caution to those who use Actinic T-5 tubes as the UV will give you a sunburn just like the Sun so use sunglasses when you’re around them. An actinic lamp's Kelvin color temperature is about 28,000 K. Wow! Not a misprint, they’re really blue.

Those who use HPS in vegetation growth will be very happy to know that there have been good reports of plant growth using actinic tubes in conjunction with HPS lamps. The ratio is seems to be a minimum of 6 actinic 4 foot tubes for every 600 watt lamp. More would be better but information available to me at this time suggests that no more than 8 actinic 4 foot tubes for every 600 or 1000 watt lamp is necessary.

How many lamps per square foot is always a question worth investigating. Don’t get me wrong here; I am a firm believer of "the more lamps the better”. However, my views here are intended to serve the minimum amount needed to get the job done. Having said that, the standard rule of thumb is a 400 watt MH lamp for every 4-foot by 4-foot area is pretty close to being correct. Place Mylar on all surfaces in the growing area for best results. I believe it to be more reflective than white paint or white plastic. This will help to illuminate the plant on all of its surfaces and get you closer to outdoor lighting conditions and its subsequent growth characteristics. I would always recommend two 400 watt lamps or two 600 watt lamps to any one 1000 watt lamp. You simply give the garden a better coverage of light and a more even plant growth throughout.

Reflectors should be chosen on the basis of climatic conditions that are currently occurring or may occur in the future and, any climatic conditions that you wish to induce to help your microclimate deliver a proper photosynthesis. As an example, those gardeners who wish to artificially enhance the growing area’s atmosphere with supplemental carbon dioxide should make use of enclosed air-cooled reflectors. They will reduce the heat from the lamps, take the burden off of air conditioners and allow the atmosphere to hold a consistently high level of carbon dioxide without exhaust fans constantly blowing all of your CO2 out of the room. For those growers who will never modify their micro climates atmosphere, just about any kind of reflector will work.

Reflectors that allow the grower to be exposed to a hot lamp are risky at best. HID lamps get extremely hot. If a hot lamp touches your cheek or other unprotected skin, it will leave a nasty burn. I do not recommend open reflectors with exposed lamps when children are around. Little faces can be scared for life. Also, a lamp’s glass envelope can break when accidentally hit with a hard surface or implode with a quick shot of water. Reflectors with glass safety shields are good for gardeners. Ask your indoor garden store for the safest reflector you can afford. Everything starts and finishes with safety. Let’s face it; living without good health just isn’t living.

Lighting the Bloom Stage of Plant Growth

When it comes to the blooming stage of plant growth, I really would like to divide blooming into two stages. The first stage of bloom starts with a transition from the normal 18 hours of light a day of vegetation to 12 hours of light a day in bloom. This lighting change triggers specific hormones to start making buds and blooms. The two main environmental conditions that inherently start the manufacturing of hormones that induce the bloom cycle are:

  • The changing of light/photo ON periods (18 hours ON, 6 hours OFF to 12 hours ON, 12 hours OFF)
  • The physical shifting of the light spectrum from 4000K-6000K to 2000K-3000K

It is my opinion the transition of the light spectrum from a vegetative growth to a bloom growth is best accomplished on a relatively gradual basis.

The initial lamp or combination of lamps used in the first stage of bloom should have an average rating of 3000 Kelvin. This will ensure that the plant will have enough time to manufacture essential bloom hormones and fully fuel photosynthesis to keep the plant healthier and more compact in stature.

What kind of lamps should be used for the first stage? Let’s explore our options.

  1. Metal Halide lamps can be purchased with a 2700K-3000K rating.
  2. Add a higher Kelvin degree MH lamp to a HPS lamp so that an average of 3000 Kelvin is achieved for the transition from vegetation into the first stage bloom.
  3. Use of fluorescent tubes in combination with HPS lamps. I have always used this rule: 100-200 watts of 4000 - 28,000 Kelvin fluorescent light to every 1000 watts of HPS light will get you close to the 3000 Kelvin goal.

The Second Stage of Bloom

After the plant develops buds and the gardener wishes the plants to be firmly placed into a harvest growth, switching to a lamp or lamps with a spectrum rating of 2000 Kelvin will push the plants into their last one to two weeks of life or the second stage of bloom. This final stage of bloom sets up plant physiology to better translocate essential oils and sugars to the consumable parts of the plant or to the blooms of an extra special blue ribbon show plant.

What kind of lamps should be used the second stage of bloom? Let’s explore our options.

  1. Simply turning OFF the actinic lamps in the final or second stage of bloom, the HPS lamp’s spectrum will end the plants cycle nicely.
  2. Better yet, let the plant know that you would like to wind things up, specific red T-5 tubes are available to drive the plant to finish its cycle and be all it can be. Pentron red T-5 tubes aren’t just redder in color than your normal 4000K T-5. Nope! These bad boys are really red; like a red Crayola crayon. A Pentron red tube is about 1000 Kelvin in color.

At the time of this writing, feedback from growers say that Pentron red tubes will really work wonders to help end the plant’s cycle. To initiate the second stage of bloom, their story is to simply replace the Actinic blue T-5’s or other higher Kelvin fluorescent tubes that they are already using alongside their HPS lamps with the Pentron red tubes. It gives an eerie red glow amongst the green plants.

Those gardeners who only utilize the vegetative growth cycle for very short periods might ask if the aforementioned two stages of bloom is right for them. I would I have to say that any growth cycle can be split up to accommodate the extra amount of blue light applied to the garden in the first stage of bloom. As a matter of fact, I believe it would be this gardener who would benefit most from it.

All lamps have a Kelvin rating should the manufacturer decide to disclose it. This rating indicates the color of light that the lamp emits. The lower the Kelvin rating the more orange-red light it emits. HPS lamps are rated at 2000K. As the temperature rating of a lamp in Kelvin (K) goes up, so does the amount of blue light the lamp emits. So, consequentially a Metal Halide 3000K lamp is not as orange-red as a HPS lamp and a 4000K lamp is approaching white in color. 5000K - 6000K lamps have obvious blue tinges to them.

I have often times explained Kelvin temperature ratings with a camping analogy. As the fire dies down the coals take on an orange-red color (2000K). To get the fire going again, you find a piece of sturdy cardboard and start to fan the coals. The coals get hotter from the wind and they start to change color until they are white hot (6000K). The bluer the color of the light, the higher the Kelvin temperature rating will be.

Supplemental Lighting

I believe supplemental lighting is absolutely necessary. Light coming from above will give you blossoms and blooms mostly at the top of the plant. Why? Because the plant will choose a bud site determined by the amount of electromagnetic waves (light) hitting the leaves. The less light, the less buds. Without light hitting the plant from the sides, it leaves a lot of the plant without blooms! Outdoor plants are blessed with the Sun’s strong rays. So strong are these rays that they can bounce from the ground to the side of the plant and be strong enough to coax the plant to make a bud.

T-5 fluorescent tubes are great for side light because they can be purchased separately and be daisy chained together to form an array. This allows the gardener to purchase one every payday and at the end of the year have all the light they need. Place supplemental light on the sides and in between the plants. Some great growers have even placed them close to the floor for light penetration into smaller plants.

Always use Mylar to reflect misguided light back onto the plant. I have always found it better than white paint or plastic. After all, you can see your image in it. Can you see your image in white paint? Think about it. Use Mylar on the ceiling, walls, floor, top of pot and side of pot. The object here is to get the light onto the plant and not sucked up into paint, plastic or concrete. Waste not; want not.

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