Luminary Prospects
Water Gardening Under Artificial Light
By Sean Stevens

While some of us may be lucky enough to live in a zone 10 or 11 climate and enjoy the lush growth or our aquatic plants all year long out of doors, most of us are limited to shorter growing seasons. Aquatic gardening has been seasonal at best allowing a short season of June through October for most. This season can be even shorter for those living in a zone 5 or cooler climate.

While I have been able to grow tropical and hardy specimens in my zone 8b climate from March through December, there has always been a lull in the season where plants must be allowed to go dormant or in the case of tropicals be brought in to the warm confines of the indoors. It was due to this seasonal lull that I decided to conduct experiments on growing aquatic plants under lights indoors. We can all agree that keeping plants actively growing would be preferable to allowing them to go dormant since there is a long period of re-growth to get plants back to blooming size.

The basis of this article is to show what lighting is best for growing any plant but at the end of this I would like to focus on aquatic plants, especially waterlilies. While natural light in is always going to be favorable to artificial light I will show you that you can successfully over-winter your aquatic plants as well as very successfully propagate waterlilies not only from seed but also tuber starts as well as viviparous leaf starts.

Proper lighting is the most important factor for high quality plant growth. When depending only on indoor lighting, making the correct choices will be very rewarding. The following will help you make the right decisions so you can get the most from your investment in indoor/outdoor gardening. It has long been known that the green parts of plants, when exposed to light under suitable conditions of temperature and water supply, use Carbon Dioxide from the atmosphere and release Oxygen to it. These gaseous exchanges are the opposite of those which occur in respiration and are the external manifestation of the process of photosynthesis. The intensity, quality, and daily duration of illumination all have influence on the amount of photosynthesis accomplished per day. The minimum light intensity at which a measurable rate of photosynthesis occurs varies according to the species. Under natural conditions, maximum rates of photosynthesis are attained in single leaves of many species at 25-35% of full sunlight intensity and in some shade species at even lower intensities.

For equal intensities, more photosynthesis appears to occur in the orange-short red and blue parts of the spectrum than in the green and yellow. In general, the longer the daily period of illumination, the more photosynthesis will be accomplished by a plant in the course of the day.

Understanding these factors I will go into lighting options for growing aquatics indoors. Metal Halide lamps, Mercury Vapor lamps and High Pressure Sodium lamps all belong to the H.I.D. (High Intensity Discharge) family of lights. Metal Halide and High Pressure Sodium lamps are used for garden lighting because of their high light output per watt, and the spectral distribution of their light.  Mercury Vapor lamps are NOT used for horticultural purposes due to their poor spectral distribution (useless for plant growth).


Metal Halide / Daylight / MS Lamps

These lamps emphasize the blue/green spectrum to promote vegetative growth. If you have little or no sunlight available these are the systems to use. MS lamps contain a 30% boost over standard Metal Halide lamps in the red/orange spectrum and more lumens per watt than standard Metal Halide lamps. The Super Son Agro (Daylight) bulb is very high in blue spectrum and also high in red/yellow. This makes it an excellent light source for propagation and for supplementing HPS lamps.

Metal Halide bulbs produce an abundance of light in the blue spectrum. This color of light promotes plant growth and is excellent for green leafy growth and keeping plants compact. It is the best type of light to be used as a primary light source (if no or little natural sunlight is available). The average lifespan is about 14,000 cumulative hours. The bulb will light up beyond this time but due to the gradual decline of light, it is not worth your while to wait for the bulb to finally burn out. If you compare their lumen (brightness) per unit of energy consumed, Metal Halides produce up to 125 lumens per watt compared to 39 lumens per watt with fluorescent lights and 18 lumens per watt for standard incandescent bulbs. Metal Halide light systems are the number one choice for gardens. Plants such as the aquatics seem to flourish under halide light, and burst with buds before flowering. Halide is a wonderful general purpose garden light, and if your garden is to only have one light source, halide will be your best choice.


High-Pressure Sodium/ Son Agro Lamps

These lamps are primarily red/yellow in spectrum. These are the best lamps available for use in conjunction with natural sunlight. High-Pressure Sodium lamps promote budding and flowering in plants. The Son Agro lamps emit 30% more blue spectrum than standard High-Pressure Sodium lamps, thereby reducing inter-nodular elongation of plants. Son Agro lamps are the choice for most professional growers because they are very efficient and contain a balanced spectrum for all stages of growth. This band of light triggers hormones in plants to increase flowering/budding in plants. They are the best lights available for secondary or supplemental lighting (used in conjunction with natural sunlight). This is ideal for greenhouse growing applications.

Not only is this a great flowering light, it has two features that make it a more economical choice. Their average lifespan is twice that of metal halides, but after 18,000 hours of use, they will start to draw more electricity than their rated watts while gradually producing less light. HPS bulbs are very efficient. They produce up to 140 lumens per watt. Their disadvantage is they are deficient in the blue spectrum. If a gardener were to start a young plant under a HPS bulb, they would see impressive vertical growth. In fact, probably too impressive. Most plants would grow up thin and lanky and in no time you would have to prune your plant back before it grows into the light fixture. The exception to this is using a HPS light in a greenhouse. Sunlight is high in the blue spectrum which would offset any stretching caused by HPS bulbs.  Sodium light simply does not have what it takes to produce vigorous vegetative growth, but can be used as a fantastic secondary light source to compliment a halide light system.

Conversion Bulbs - There are two types:

A) Sodium bulbs which run on Halide ballasts - available for most systems.
B) Halide bulbs which run on Sodium ballasts - available for 400 or 1000 watt systems only.

A conversion bulb lets you tailor the light source to the growth stage of the plant merely by changing lamps.

High Pressure Sodium Conversion bulbs are specially designed to run off a Metal Halide ballast but they put out more lumens, more red spectrum light, and they run off less electricity. Start your crop under the MH bulb and then switch to the HPS conversion bulb when it’s time for flowering. Bulbs that convert MH into HPS come in 175, 250, 400, and 1000 watt sizes. Metal Halide Conversion bulbs allow you to take the opposite approach: grow your plants under the conversion bulb during the vegetative stage, then switch to your regular high pressure sodium bulb for flowering. HPS to MH conversions are only offered in the 250, 400, and 1000 watt sizes. If you only want to deal with one bulb through all stages of growth, another option is to use an enhanced or “corrected” bulb. High Pressure Sodium Son Agro bulbs are engineered to provide 30% more blue spectrum light than a standard HPS bulb. These are available in 160, 270, or 430 watts. Agrosun Halide bulbs, exclusively from Hydrofarm, are engineered to provide more red spectrum light than a standard halide bulb. The Agrosun Classic provides 38% more red light, and the Agrosun Gold provides 49% more red light. These are available in 175, 250, 400, or 1000 watts.


Fluorescent Lamps

These lights are perfect for starts and seedlings. They are generally a poor light source for production growth and flowering because of their low lumen output, although they may be used in some instances for small scale systems with low growing plants. Plant growth will not be as rapid as under HID lighting due to lower light levels. They are also popular for growing low-light plants. Fluorescent lights are low intensity and need to be placed within 8” (up to 15” for shade loving plants) of the plants to be effective. They are a poor light source for flowering and budding primarily because of their low lumen output.

Fluorescent lights of the proper type can be fantastic for baby seedlings and cuttings, or for low-light tolerant aquatics such as submerged aquarium plants. Unfortunately for aquatic plants, the common “plant lights” sold in supermarkets and hardware stores are very inefficient. Both incandescent bulbs (one’s that screw into a common lamp socket) and fluorescent tubes act pretty much like putting a regular bulb in a sock; the amount of light output is decreased, but the light is shaded for a pleasing, yet deceiving visual affect. Another perspective would be that Mother Nature does not provide soft, purplish light. Why should you?

Fluorescent lights are excellent for starting seeds or rooting clones. Fixtures and bulbs are inexpensive, and the low heat output lets you put them just inches away from your delicate plants. The disadvantage of fluorescent bulbs is the low light intensity—they can grow a plant that is 8-10” tall but then the light simply can’t penetrate any further. If the plant grows taller and you keep raising the bulb, lower sections of the plant will not receive adequate light. Using a “full spectrum” fluorescent bulb will give your plants all the necessary wavelengths of light.

However, fluorescent tubes like the Verilux Tru-Bloom offer unquestioned benefits for healthy plant growth. The benefits come from the natural spectrum of light that Verilux tubes radiate, which approximates the sun better than any other fluorescent tube available. Since they draw only about 40 watts per tube and put off very little heat, they are an economical source of light for small or starting plants. Seedlings and cuttings begun under Verilux tubes get the best start on life possible—at the least cost to the grower. They will get your plants going in the right direction, but remember that is what they are best suited for; after your plants begin to grow out of infancy, they need real light to keep them at their peak potential. That’s where high intensity discharge (HID) lights come in.


Incandescent Lighting

These lights are also good for starts and seedlings and provide an inexpensive alternative to HID lights, because they do not require a ballast. These lights are only good for individual plants or small groups of plants because of their low lumen output and limited range.

How long should lights run? This depends on the type of plant. Most plants and vegetables need about 10 to 12 hours of light to promote growth. Plants that produce fruits or flowers will show improvement with up to 16 hours a day of supplemental light. The amount of time your light should be on depends in the intensity requirements of the plants and what type of plants you are growing. Plants can generally be grouped into three categories.

Day neutral plants are plants not affected by the length of light or dark periods. Most vegetables are day neutral. Day neutral plants flower or bear fruit due to the age of the plant or temperature. Day neutral plants respond well with 18 hours of light, assuming your plants get no sun at all. If you are using your grow light for supplemental light, you may get the same results by turning it on 4 hours before dawn and another 4 hours after dusk. This method will reduce electrical cost by 50%. Tropical Waterlilies are Day Neutral plants and will benefit from 18 hours a day artificial lighting whether they are in the active growing stage or in the blooming stage.

Long day plants are plants that generally flower in late spring or summer when the daylight hours are long. Plants in this group will flower when the amount of light is increased from 12 to 18 hours per day. If the amount of light is kept at 12 hours the plants will continue to grow and put more energy into foliage rather than flowers. Hardy waterlilies are in this group and should prosper with 12 to 18 hours a day of artificial light.

Short day plants are plants that generally flower in autumn or early winter when daylight hours are short. Plants in this group will flower when the amount of light is decreased from 18 to 12 hours per day. If the amount of light is kept at 18 hours the plants will continue to grow and put more energy into foliage rather than flowers. Aquatic plants in this category are the rushes.

Bulb Life Expectancy
Replace bulbs early. Replace halide and sodium bulbs after 60% of the bulbs rated life. Bulb life is listed below. High Intensity bulbs should always be replaced before they burn out. This will greatly reduce the risk of a violent burn out.

-1000 sodium 24,000 hours 14,400 hours
-400 sodium 24,000 hours 14,400 hours
-1000 halide 9000 hours 5400 hours
-400 halide 15,000 hours 9000 hours

Hanging Height
Due to the heat that is emitted from these types of fixtures, you should hang them according to size. Smaller wattage systems (100 and 250) should be hung about 2 to 3 feet from the top of the plants. Medium wattage systems (400 and 600) should be hung around 4 feet from the top of the plants. High wattage systems (1000 and up) should be placed at least 4 to 6 feet from the plant tops.

HID Light Output         Primary Growing Area    Supplemental Growing Area
100 watts                                 2’ x 2’                           3’ x 3’
250 watts                                 3’ x 3’                           4’ x 4’
400 watts                                 4’ x 4’                           6’ x 6’
600 watts                                 6’ x 6’                           8’ x 8’
1000 watts                               8’ x 8’                           12’ x 12’

● One 1000 watt HPS bulb puts out as much light as 111 incandescent 100 watt bulbs.  
● One 400 watt MH bulb puts out as much light as 20 fluorescent 40 watt tubes. 
● The bulbs in HID lighting are usually called “lamps”.
● A Metal Halide Lamp will only burn in a Metal Halide System with a Metal Halide ballast.
● A High Pressure Sodium Lamp will only burn in a High Pressure Sodium System with a High Pressure Sodium Ballast.
● A 400 Watt bulb needs to go into a 400 Watt system.
● All the wattages need to be the same.
● Look for a manufacturer’s tag on the ballast or for a stamp on the bulb to identify the type and wattage.
● Sometimes you can get the wrong bulb to burn for a while, but this is a really bad idea.
● The bulb will maybe burn for 90 days, maybe not, sometimes a little longer.

How much light do you need?  20-40 watts per sq. ft. is a general guideline. The more efficient the light source, the less watts per sq. ft. needed. For example using 1-1000 watt metal halide light, in a 50 sq. ft. area would give you 20 watts per sq. ft. and a total of 120,000 lumens. Compare this to 3-400 watts metal halide lamps in the same space which will give you 24 watts per sq. ft. and a total of 120,000 lumens. You will get more light from a single 1000 watt light while using less wattage per sq. ft. in addition to savings on timers, and outlets. As a rule, 20 watts per sq. ft. using 1000 watt metal halides is the absolute minimum required. Since a given amount of light can only do so much, the same production can be obtained in a smaller space with less plants because the light is concentrated, and the plants can work more efficiently. Using more light also helps additional CO2 uptake.  The more light on the plants, the faster they grow.

NOTE: To increase the effectiveness of HID lamps, and to make it possible for plants to receive maximum light especially from the side, line the walls with Mylar plastic sheeting. This is fast, easy to install, and is the most economical way to take advantage of it’s reflective properties.

The Cost to Run a Lighting System
To get the operating cost per hour for a light, take the lights combined wattage, and divide it by 1000 to get the kilowatts used. Then multiply that number by the amount your electric company charges per kilowatt hour. HID lights will use the number of watts it emits per hour, i.e., 600w system will use 600 watts per hour (regardless of spectrum).

(light wattage output / 1000) x electricity cost per kilowatt hour = Operating cost per hour
operating cost per hour x hours used per month = Operating cost per month

What happens if the lights burn 24 hours a day? The plants will grow. After about 16 or 18 hours in most plants, though, growth levels off. So if you turn off the lights for a few hours, you can save some electricity and not really affect growth. Plants often use dark time to build up florins which are hormones plants use to bloom. This is why the light is often cut back during the Bloom stage.

Through my experiments however, utilizing 24 hour lighting on not only seedlings but tuber divisions and viviparous starts is beneficial for as long as a few months. It seems that after this period the plants become exhausted and need the nightly rest of up to six hours.

How the Sunlight Effects Plant Growth

200 - 280 nm UVC ultraviolet range which is extremely harmful to plants because it is highly toxic.
280 - 315 nm Includes harmful UVB ultraviolet light which causes plants colors to fade.
315 - 380 nm Range of UVA ultraviolet light which is neither harmful nor beneficial to plant growth.
380 - 400 nm Start of visible light spectrum. Process of chlorophyll absorption begins. UV protected plastics ideally block out any light below this range.
400 - 520 nm This range includes violet, blue, and green bands. Peak absorption by chlorophyll occurs, and there is a strong influence on photosynthesis.
520 - 610 nm This range includes the green, yellow, and orange bands and has less absorption by pigments.
610 - 720 nm This is the red band. Large amount of absorption by chlorophyll occurs, and there is a significant influence on photosynthesis.
720 - 1000 nm There is little absorption by chlorophyll here. Flowering and germination is influenced. At the high end of the band is infrared, which is heat.
1000+ nm Totally infrared range. All energy absorbed at this point is converted to heat.


My System


The system I use indoors consists of a 400 watt Metal Halide lighting system. The bulb I use is the 400 watt Sunmaster Warm Deluxe. The primary benefits of the Sunmaster Warm Deluxe Metal Halide are balanced spectral output, single source provides nutritious light for all phases of growth, enhanced orange-red component promotes flowering, stem elongation and germination, Rich blue content for healthy vegetative growth, highest PAR rating of all HID sources, long life, economical.

SUNMASTER Warm Deluxe Grow Lamps emit balanced light similar to a 3000° Kelvin source, making them ideal for all phases of plant growth while ensuring more natural growth patterns that occur in most species. Its enhanced orange-red component promotes flowering, stem elongation, and germination while a rich blue content assures healthy vegetative growth.

SUNMASTER Warm Deluxe Lamps surpass all other HID sources in PAR (Photosynthetically Active Radiation) watts - the most objective measurement of total light energy available for photosynthesis.

The bulb of the lamp is kept within one foot of the water surface in my setup. This provides the best luminary output for the waterlilies and provides better illumination through the depth of the propagating tank. This propagating tank is 18 inches deep. If the lamp were higher than 12 inches from the water surface, the luminary penetration would be greatly reduced.


This lamp runs for 12 hours each day providing sufficient light to propagate and grow tropical waterlilies throughout the year. When allowed to grow large enough in this tank, I frequently have tropical waterlilies that will bloom just the same as if they were grown outdoors in the prime of the season.


One final note on this setup. This light running each day for 12 – 16 hours a day does not cost me more than $7.00 per month in electricity.


Profile – Sean Stevens


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