Mother Nature does it better… true, but sometimes, you have to grow plants indoors.
Without the light of the Sun and the natural cycles of Earth though, you will have to use artificial lights.
But choosing a good light is not enough; far too many plants still suffer and die because we don’t understand that copying the work of Mother Nature is far, far more difficult than we think.
When you grow plants indoors, giving them the right quality of light, of the right intensity, at the right times, for the right cycles, at the right distance and from the right direction is an art you will need to master.
Choosing appropriate lights among the four main types available is also important: fluorescent lights, high intensity discharge lights (HID) and light emitting diodes (LEDs) and light emitting plasma (LEPs).
Thus, we will need to see how light works, how you can give the correct light exposure to your plant and, what kind of light plants need and which are the best lights for you.
Can plants grow as well with artificial lights as with sunlight?
Very interesting question. Let’s say that, to start with, growing plants with artificial lights is comparatively new.
You could not use candles back in the days… Even the classical “Edison type” light bulb was nowhere near adequate.
Technology, however, has made huge strides in this direction, and now we can, in fact, grow plants very successfully with artificial lights. In fact, some growers and gardeners actually prefer them.
On the one hand, there is no way we can match the perfect light quality of the Sun. On the other hand, we can control the growth of our plants very precisely with artificial lights.
If spring is cloudy or too rainy, for example, outdoor crops will suffer as a consequence.
Instead, with artificial lights we can make sure that our plants always have the right quantity and quality of light our crops or decorative plants need.
This comes at a price though, and it is not only a financial one. Using artificial grow lights requires scientific knowledge and technical competence. And the more professional you want to become, the more you will need.
Light and plants
Image Source- http://grownindoors.org
We all know that plants need light for photosynthesis, but there’s far more to it than meets the eye (sorry about the pun…)
You can’t just expect any light to be good for your plants; instead there are five factors that influence how well your plants will grow: light quantity, light quality, photoperiodism photosynthetically active radiation (a.k.a. PAR) and the heat produced by the lights.
Plants need a quantity of light within as certain range; some will like more light (cacti, for example), others will like less (most ferns, periwinkles and all those plants that love a shady position). This happens in Nature, and we can’t change it.
While plants are tolerant to variation, when the light is too much or too little, they will suffer and may even die.
Light quantity is measured in lumens or in lux. These are different measurements; a lumen measures the total flux of light from a given source, the lux measures the quantity of light received on a given surface.
Energy and light quantity
When we talk about sunlight, all we need to measure is the lumens and lux that reach our plants.
It is a different matter though if we don’t have such an immense source of energy as our star to draw on. Instead, we will use electricity that we transform into light.
With our grow lights, we will have to use wattage as a measure. Wattage is the amount of electrical energy the lamp uses.
The higher the wattage the higher the luminosity of the grow light we have. But this is not a straightforward equation.
It really depends on the lumen to watt efficiency rate. Some grow lights, like LED, have a very high efficiency ratio (up to and even beyond 200 lumens for every watt you put in), others, like fluorescent lights only produce between 33 and 100 lumens for every watt.
What does this mean in practical terms? Plants care little about the wattage, what they need is luminosity.
To give the same luminosity to a plant with an LED light you will need a lower wattage than with a fluorescent light.
But we pay bills according to wattage, not lumens… Got the point? Efficient lights will save you money.
How many lumens do plants need?
leaf (and stem) surface which means that they have more chloroplast thus photosynthesize more. This means, of course that they will need more light too.
But that’s not all; some plants need more light (cacti, most succulents, ponytail palm etc.) while others will need little light (plants that like shaded positions like peace lilies, Caladium bicolor and lady palm, or Rhapsis excelsa).
When it comes to using natural light, all you need is to choose a position that best describes general guidelines like “full Sun”, “indirect light”, “direct light”, “part shade”, “full shade” and “diffused light”. In some cases, even the exposition (South, East, West, or North facing etc.) is quite handy.
But when you use grow lights, you need to be a bit more scientific than that. You will need to measure (though roughly) the lumens per square foot (of green surface) your plants will receive:
This is why you will need to check the lumens your grow light emits; then, you will need to check the surface of foliage you will want to cover with that light.
You don’t need to use precision measuring tools though, a rough measurement is more than fine.
As you know, light is a spectrum of wavelengths; not all these waves are visible to us; we only see light in the visible spectrum but as you know we cannot see infrared light (past the red end of the spectrum) and ultraviolet light (the light past the violet wavelength).
We only see wavelengths between approximately 380 and 740 nanometers.
Light quality means which colors and wavelengths are necessary to plants.
Plants are quite “picky” when it comes to light quality in fact; different plants like different lights. Actually, even the same plant will like different lights according to its life phase…
In fact, when a plant produces foliage, it likes to have blue wavelengths (with a length of 500 to 600 nanometers).
On the other hand, when they produce fruit and flowers they will also want red wavelengths (600 to 700 nanometers, while far red within 700 and 750 nanometers).
This does not mean that a spectrum that contains other colors will harm the plant, but that you need to provide a spectrum that contains the colors your plant needs.
To be correct, plants will absorb light of any color, but not all colors give the same amount of energy to them, with green being the least effective, as leaves tend to be green and reflect it rather than absorb it.
Light quality and the phases of a plant’s life
Light quality also changes in Nature; you don’t get the same quality of light on the Equator and near the Polar Circle, for example, as every traveler can witness. The same applies to different seasons and times of day.
It is impossible, however, to copy the huge range of natural light we get from the Sun in different parts of the planet and on different days, and even times of the day.
Still, we can look at some basic light quality differences.
We said that when plants are growing foliage (the vegetative phase), not only do they like less light, but also lots of light in the blue range.
If you want to grow lettuce, for example, you will want your plants to produce as much foliage as possible. Thus, give them a lot of blue light.
On the other hand, red light encourages flowering and fruiting, something you will want to avoid with leaf vegetables but you will be looking for with tomatoes rather than dahlias…
Thus, especially if you want to grow flowers or veggies professionally, you may want to take a leaf out of professional indoor gardeners and reduce red light with leaf vegetables and plants (some are monocarpic, which means that once they flower, they die), and give plenty to fruiting and flowering plants.
Plants don’t just need light, but also darkness. Their metabolism changes when they are without light (at night in Nature); this, however is a natural process and it is necessary for the health of your plants.
In botanical terms, a short day is one with less than 12 hours of light, while a long day is one with more than 14.
Plants respond very clearly to this cycle of light and darkness, some plants actually flower according to the ratio of hours of light and darkness; this is called photoperiodic flowering and, for examples, chrysanthemums will flower when daylight hours reach 15 a day, spinach at 13, dill at only 11 hours…
Some gardeners and especially growers use photoperiodism to force flowering and even growth; this is quite common with bulbs and other plants, like hemp for example.
Managing light and darkness cycles
Therefore, plants need light but they also need darkness. This changes according to the species but also according to the age, maturity or better life phase of a plant. Usually, plants go through three life phases:
This is not a hard and fast rule; a few plants, for example, don’t go into the dormant phase.
On average, plants will like more hours of light during the reproductive phase, and less during the vegetative phase and even less during dormancy.
Having said this, the majority of plants will need between 12 and 16 hours of light a day.
If you want to be very professional, you will need to adapt the light and darkness cycles according to the species and the life phase.
As a rule of thumb though, spring flowers, spring and winter vegetables etc. will need less light than summer ones. Even plants that need very little light, like carrots, radishes and beets, need a minimum of 3 to 4 hours of full light every day.
Lettuce will need about 10 to 12 every day; spinach prefers between 12 and 14 hours (but it will manage with about 10) for example, so, do check for each crop or species you have.
This does mean, however, that if you want to grow your plants with artificial lights, you will get best results with monocultures or at least with crops of different species that like the same or similar light cycles.
If you are working on a small scale, for example if you just have a few decorative plants in your sitting room, then divide them into two or three groups; put Sun loving plants in one corner and those who love a bit of shade in another place…
Photosynthetically active radiation (PAR)
This is a very technical scientific term; in simple words, it is the measurement of all the “useful” light received by a surface area of the plant in a given period, for example in a day or in an hour.
This takes into account the wavelengths that the plant can absorb, the intensity of the light, the periods of light and darkness etc.
By calculating all these, we can work out the approximate numbers of photons (therefore energy) that an area of a leaf receives over an hour, a day a month and so forth.
You won’t need to calculate this, but just keep in mind that this is the calculation of the energy available to the plant and it is what ultimately determines its growth and wellbeing, and that if one of its component factors (quality of light, quantity, distribution over time etc.) is wrong, the plant will have too much or too little useful light and it will suffer.
Light temperature and heat
This is a very important factor indoors, especially if you want to use grow lights extensively.
Not all types of light produce the same heat; LED lights, for example, produce very little heat while HID lights produce a lot.
This affects how much moisture your plant will need, because heat causes perspiration, but also how far you need to place the lights from the plants and, in some cases, it can even cause your plants to dry up and even die.
Temperature and dehydration
Image Source- https://agreatgarden.com
Plants perspire; this is a natural phenomenon. When the temperature is higher, they perspire more. This tells us an important thing when growing plants with artificial lights: the more they produce heat, the more plants will dehydrate. For this reason:
The Kelvin scale
When you look at an artificial light, in most cases you will notice that it is not actually white. In fact, it may be slightly yellow, even orange-reddish or, on the contrary, it may have a blue coloring.
This is useful to create an atmosphere or ambience indoors, and in fact, the three main ranges have names that reflect this.
Still, there is a scale, named after an Irish President of the Royal Society of Science from 1890 to 1895: William Thompson Fist Baron Kelvin. These colors are measures in Kelvins, a unit of measure that tells you exactly which hue your light has.
These are the three main ranges of light available with lamps and grow lights; you will find the kind of light and even the Kelvin units on the box when you buy them.
You should avoid giving your plants light which is only cool white or warm white as a rule of thumb; use a light in the cold white range together with one in the warm white range to give your plants all the different spectrum wavelengths they need.
Symptoms of wrong light quantity, quality, and direction
Let’s look now at ways of “interpreting what your plants are telling you”…
If plants have too little light, for example, you will get the following symptoms:
If your plants get too much light:
When it comes to the direction of the light source, you need to keep in mind that if you have windows, plants will still receive it, and may tend to grow in that direction.
This is a phenomenon called phototropism: plants grow in the direction of light.
Some professional growers and gardeners like to block off windows to avoid this. You may, on the other hand, just turn your plants every now and then.
Note that the same applies if you place your grow lights to the side of your plants… They will simply bend in that direction.
Finally, you will notice that the light quality is not correct if plants tend to flower too early (too much red) or produce lots of leaves but do not flower and fruit (too much blue).
It’s now time to look at the different artificial grow lights available. There are four main groups of lights used nowadays, each with its own characteristics and each with different types and models of light bulbs (or lamps) within them:
Choosing your grow lights
When choosing your grow lights, you will need to look at these factors; if they give the right quantity of light, the right quality (wavelength range), and how much it will heat up the place.
When using them, you will need to understand photoperiodism, but then, there are other factors, like:
Keeping these things in mind, now, let’s look at the different lights you can find on the market.
I am including fluorescent lights for two reasons: to start with, they were, for a long time, the only reasonable form of artificial lights we could use indoors and in greenhouses etc.
This means that they have played a major role in gardening. Next, you may end up using them even if they should not be your first choice.
In fact, nowadays they are a bit outdated, but if you have some to recycle, for example, you may well use them. For this reason, you may need to know about them.
Fluorescent lights are well known and very common indeed; they can be found in offices, factories and even in schools still.
There are two types of fluorescent lights:
Fluorescent lights were certainly an improvement on “Edison style” light bulbs in terms of light production; they range between 33 and 100 lumens for every watt of electricity they use. Still, this is not much by todays’s standards.
If you use tube fluorescent lights (T lights), they come in three standard sizes, according to the diameter of the tubes: T5 has a have a diameter of 0.621”, T8 of 1.0” and T12 are 1.5” in diameter.
T5 lamps are virtually useless as grow lights; they are far too weak to provide enough light for plants. T8 are “passable” and T12 would be your best choice.
Even so, if you use these lights, make sure you use a refracting surface behind the light (a metal sheet, a mirror or even tinfoil) so that the light is all redirected towards your plants and does not end up on the ceiling or a bare wall.
Having said this, they do have some advantages:
On the other hand, the disadvantages make them suitable as a “back up” option, but not for a modern or professional indoor garden:
Still, they bring back memories to old growers and gardeners, and they may still come in handy for small indoor crops.
High intensity discharge lights (HID)
Despite their daunting name, HID lights are fairly common in most households and they have become common as grow lights too, surpassing fluorescent lights for technology, and functionality.
They come in different shapes and sizes, some bell shaped (or small cylinders) others look like bulge reflectors, ellipsoidal reflectors, sealed beams and similar shapes.
This makes them more eclectic than fluorescent lights when it comes to fitting them into small or oddly shaped spaces.
Unlike fluorescent lights, they use a noble gas and fused quartz or fused alumina to turn the gas into plasma, which, charged electrically, gives off light.
They are very efficient, much more than fluorescent lights, when it comes to how much light they produce for the energy you put in (lumen per watt). This means that they will save you money in the long run and they will provide lots of light.
There are many types of HID lights, but not all of them are suitable as grow lights.
Metal halide (MH) grow lights
These produce light by charging a mix of metal halide and vaporized mercury. They were invented in the 1960s and have become very common.
They are very efficient in terms of quantity of light, as they produce about 110 lumens per watt (depending on the model, of course).
They have a light spectrum which is similar to that of the Sun, and they give off wavelengths on the violet to blue range; this means that they have the quality of light that you will find in outdoors in springtime.
Gardeners like them because plants look good under these lights.
Ceramic metal halides (CMH or also CDM) lights
They are a recent development of metal halide lights. Unlike MH lights, they use a pulse starter to produce light and the light bulb is made of polycrystalline alumina, a type of ceramic (also known as PCA).
This has the effect of countering loss of sodium, which results in more stable light than HM lights; in fact, when HM light lose sodium, there is a phenomenon known as color shift, which is a change of color which may result in less efficient lighting for your crops.
High pressure sodium (HPS) lights
These lights excite sodium to produce light. The color range they produce is more on the red to yellow, which means that they are better suited for the later life phases of plants (the reproductive phase, when plants flower and fruit). Therefore, these lights are indicated for fruit and flower crops.
In fact, these lights are even used by professional gardeners and growers in greenhouses.
Why would you use these lights in a greenhouse, which is already well lit though?
Because they are very strong on the red side of the spectrum, which means that they encourage and even anticipate flowering and fruiting.
In terms of energy efficiency, they produce between 60 and 140 lumens per watt, according to the model.
Unfortunately, if you use these lights you will need to be careful with pests; yes, because they produce an infrared light which can attract them.
Other HID lights
There are then other types of HID lights you can use for gardening; these can be hybrid solutions, like dual arc lights (a combination of HM and HPS), conversion bulbs and switchable ballasts.
These are quite useful because you can switch the light from blue range to red range as the plants change life phase from vegetative (leaf producing) to reproductive (flower and fruit).
Pros and cons of HID lights
These are very popular lights, and, before the advent of LED lights, they were everybody’s favorite grow lights. In fact, they do have many advantages:
Still, growers and farmers have had to put up with some disadvantages for a long time:
Light emitting diodes (LED)
LEDs have revolutionized the entertainment industry but also indoor gardening. They have soon become many gardeners’ favorites, because they are cheap, they use little electricity but also because the light quality is really good for growing plants.
Light emitting diodes use semiconductors that, when they are crossed by electricity, produce light. The concept is very simple and functional.
LED lights have soon become very popular with gardeners, also because they allow good control on the quality of light you want.
You can have LED lights of many colors, and, when it comes to gardening, the combination of blue and red has given that violet haze that has now become synonymous with professional grow lights and indoor gardening.
They have many advantages in fact:
How about the disadvantages?
Light emitting plasma (LEP)
Though these have existed for a few decades, they have only recently become popular as grow lights.
Some suggest that they are “the ones to watch”, as they are becoming all the rage. However, I have my doubts about this.
These lights are different from conventional bulbs because the have no filament or electrodes.
Instead, the electricity runs from one end to the other of the bulb through plasma, and this produces light.
They do have some good advantages:
However, they also have some major disadvantages:
These disadvantages may explain why I have my doubts about this new trend. Unless some technical developments make them cold and not bulky, it is hard to see them catching up as very popular grow lights.
Which ones are best artificial grow lights for you?
We met the lights in their chronological order, starting from the oldest to the youngest. Maybe your choice should take into consideration the likely future of these lights. Let me explain…
Fluorescent lights ares till used only because they have been around for a long time, some gardeners are just accustomed to them, some others are just using them because they already have them, and need to get something out of them.
HID lights have certainly made a big difference to grow lights, and they have dominated the market for a long time. Given the fact that the light quality, efficiency and lifespan of these lights is really good, they will certainly keep being used in years to come.
LEP lights appear to me one of those crazes that lovers of innovation adopt unwisely; too hot, too expensive, too bulky and even too dangerous to have a real future. Still, excellent light quality.
LED lights, on the other hand, have an advantage over all other lights: they are cold.
The fact that the light quality is also good and adaptable, that they are easy to use and can be adapted to all shapes, directions and sizes, in my opinion, makes LED lights the best choice even for amateur gardeners.
They may cost a bit to buy, but once you have them, you will have that peace of mind that comes with safety and no collateral problems (overheating plants especially).
Add to this the low running costs and very long lifespan, and I think you can agree with me that there must be a reason why so many gardeners have no doubts in choosing them as their favorite grow lights.
The light at the end of the tunnel
When it comes to replacing the Sun, there is a lot of work to do; you will need to understand all the aspects of light (quantity, quality, photoperiodism, the heat it produces, photosynthetic active radiation etc.) and now you do!
But even this is not enough; you need to match the light you give them to their needs, and these change according to the species, the life phase and even other factors (like ventilation, position etc.)
Growing plants with artificial lights is, therefore, very scientific: you need some basics of botany to manage it successfully, especially at professional level.
What’s more, it also requires a lot of technical competence; there are so many grow lights available that the choosing the best ones for you must be done carefully.
Finally, however, never forget that the real “x factor” is your care, your will to learn and to adapt.
Only if when you manage to understand what exactly your plants want and offer your skills and knowledge as a service to them will you be able to make them really happy and get the results you hope for…
In the end though, is this not what it means to be a gardener?
Updated on by Amber Noyes
Amber Noyes born and raised in a suburb Nebraska town, San Mateo. She holds a master’s degree in horticulture from University of California as well as an BS in Biology City College of San Francisco. With experience working on an organic farm, water conservation research, farmers markets, and potted plants she understands what makes plants thrive and how can we better understand the connection between microclimate and plant health. When she’s not on the land, Amber loves informing people of new ideas/things related to gardening, especially Indoor gardening, houseplants and Growing plants in a small space.