High-Pressure Sodium Lamps in Cannabis Cultivation

HPS lamps explained: spectrum, PPFD, light penetration, heat, power consumption, and a comparison with modern LEDs – including the question of whether this former gold standard is still relevant today.

The high-pressure sodium lamp, commonly known as HPS in the grow community, is one of the most definitive lighting systems in indoor cannabis culture.

For decades, it was effectively the benchmark for high-performance indoor lighting. Entire generations of grow rooms were built using 250, 400, 600, or 1000-watt HPS systems, long before modern LED grow lights achieved their current levels of efficiency.

Technically, however, the HPS is not a specialized "cannabis light" but a high-intensity gas discharge lamp. Its strengths lie in high light output, proven technology, and a spectral distribution that plants can utilize. Today, its weaknesses primarily involve energy efficiency, heat management, light distribution, maintenance, and lifespan.

At the same time, modern cannabis research shows that HPS can still deliver very good yields. However, the former notion that sodium vapor is inherently the best light for flowering can no longer be scientifically supported.

What is a high-pressure sodium lamp?

An HPS lamp belongs to the group of HID lamps – High Intensity Discharge.

In this system, light is not generated by a semiconductor as it is in an LED. Instead, an electrical gas discharge occurs within a small arc tube.

In high-pressure sodium lamps, this arc tube contains elements including:

• Sodium
• Mercury or a sodium-mercury amalgam
• Xenon as a starting gas

The electrical discharge excites these substances and produces the lamp’s characteristic intense radiation. HPS lamps also require a suitable ballast and, depending on the design, an igniter.

A classic grow HPS system therefore consists of more than just the bulb itself.

The system typically includes:

Bulb + Socket + Reflector + Ballast + Wiring

This technology differs significantly from modern LED lights, where the light source and electronics are usually designed as a complete fixture.

Why HPS lamps defined indoor growing for so long

As indoor cannabis cultivation grew, HPS offered a highly attractive combination at the time:

• high photon output
• relatively high efficiency for a discharge lamp
• high power availability
• robust technology
• relatively low initial costs
• years of experience in commercial horticulture

HPS was already established in professional greenhouses and could provide supplemental light to relatively large areas.

It was therefore logical to use the same technology for light-intensive indoor crops.

Modern LEDs had to measure up to HPS for years as a result.

What is the spectrum of an HPS lamp?

Typical HPS light is often simply described as red light.

That is inaccurate.

High-pressure sodium lamps usually possess a very warm-looking spectrum with strong portions in:

• Yellow
• Orange
• Red

and, depending on the lamp, also portions of:

• Green
• Far-red

Blue, by contrast, is comparatively weakly represented.

In a controlled cannabis study, the HPS system examined contained only about 4% blue photons, while the LED systems tested delivered between approximately 10% and 20% blue.

Another horticultural measurement characterized an HPS spectrum as roughly 6% blue, 50% green, 34% red, and 7% far-red. However, such values depend heavily on the specific bulb used.

It is therefore more accurate to say:

HPS typically possesses a warm, yellow-orange dominated spectrum with little blue and relevant red/far-red components.

Why HPS was traditionally considered the flowering light

In classic indoor growing, this led to a simple division:

MH for growth
HPS for flowering

Metal halide lamps typically provide more short-wave or blue radiation. HPS provides significantly less blue and larger portions in the yellow-orange-red range.

Since blue light can, among other things, promote more compact morphologies, while low blue portions often allow for stronger stretching, HPS seemed well-suited for the reproductive phase.

Cannabis trials confirm at least part of this observation.

At the same PPFD, plants under HPS were found in several studies to be taller or more stretched than plants under certain LED spectra.

However, this should not lead to the conclusion:

Red light forces cannabis into flowering.

For photoperiodic cannabis, it is primarily the photoperiod or length of the dark phase that is decisive for reproductive development. The spectrum can influence morphology and plant physiology, but it does not replace photoperiodic control.

Is 12/12 necessary because of the HPS?

No.

The well-known 12/12 cycle is not related to the high-pressure sodium lamp.

It is a classic photoperiod strategy for photoperiodic cannabis.

Newer cannabis research even shows that 12 hours of light and 12 hours of darkness do not necessarily represent the yield optimum for every genotype. In one study, some chemovars were able to be developed further reproductively even with 14 hours of light, with flower mass and cannabinoid concentrations changing differently.

HPS and photoperiod are therefore two different levels:

HPS = Light source
12/12 = Photoperiod

Does an HPS have particularly good penetration?

The so-called penetration of the HPS is one of the most persistent terms in the grow community.

In this context, several things are often conflated.

An HPS lamp is a relatively concentrated light source. Therefore, very high PPFD values can be generated directly under a powerful lamp.

Subjectively, this can create the impression that the light "penetrates deeper" through the plant.

However, the actual amount of light within a canopy depends on, among other things:

• Distance to the lamp
• Reflector
• Fixture geometry
• Plant spacing
• Leaf angle
• Canopy density
• Spectrum
• Reflections in the room

A modern LED bar light, for instance, distributes the same number of photons over a much larger area. As a result, the peak intensity directly under the light source may be lower, but the uniformity across the entire canopy can be better.

Newer research on cannabis lighting explicitly describes the shift to LED as also providing more homogeneous lighting of the plant canopy.

Therefore, the statement:

HPS generally has better penetration than LED

is too generalized.

Green light and canopy penetration

An additional factor is the relatively high green-yellow proportion of many HPS spectra.

Green photons are less strongly absorbed by upper leaf areas than a portion of the red and blue light, allowing them to penetrate deeper into leaves or plant canopies.

However, this does not mean that only HPS possesses this advantage.

Modern full-spectrum LEDs can also generate relevant amounts of green light.

The question of canopy penetration should therefore be considered through:

Spectrum + Light distribution + Plant architecture

rather than through the technology alone.

What does PPFD mean for an HPS lamp?

For plants, wattage alone is not very meaningful.

A 600-watt HPS lamp only tells you something about the electrical power of the system at first.

More interesting is the PPFD – Photosynthetic Photon Flux Density that actually reaches the plant canopy.

It describes the number of photosynthetically active photons hitting a square meter per second:

µmol/m²/s

This makes it much easier to compare different lighting technologies.

A comparison should therefore not be:

600 Watt HPS versus 600 Watt LED

but rather:

What PPFD and light distribution do both systems produce at what power consumption?

DLI – Light throughout the day

In addition to PPFD, the Daily Light Integral – DLI is interesting.

The DLI takes the following into account:

PPFD × Photoperiod

A plant under 600 µmol/m²/s for 18 hours therefore receives significantly more daily photons than the same plant at 600 µmol/m²/s for 12 hours.

This is particularly important when comparing the vegetative and flowering phases.

A high PPFD alone does not describe the total daily amount of light.

More light can increase cannabis yields

Cannabis can utilize relatively high light intensities.

A controlled study with a THC-dominant cultivar examined average canopy PPFD levels of:

• 600
• 800
• 1000 µmol/m²/s

The highest light level produced about 1.6 times more dry inflorescence mass than the lowest. The cannabinoid concentration itself did not change significantly.

A more recent study with CBD-rich hemp also found increasing floral biomass and cannabinoid yield with increasing LED light intensity between 200 and 600 µmol/m²/s.

This shows:

For yield, the actual amount of photons is often more important than the historical question of “HPS or LED?”

HPS versus LED – what do cannabis studies show?

The comparison is more complex than marketing from both sides might suggest.

A cannabis study compared HPS with two LED spectra at approximately the same light intensity.

The HPS-grown plants were:

• taller
• had more dry floral mass

However, the THC concentration of the flowers was lower under HPS than under the two LED treatments.

In terms of total cannabinoid yield per plant, there were no significant differences between the lighting systems.

This is a good example of why:

flower weight,
cannabinoid concentration
and
cannabinoid yield

are not the same thing.

A second cannabis study shows a different picture

In another study, PPFD was strictly controlled between the lighting treatments.

The tested HPS lamp had the lowest blue content and produced slightly higher floral yields per area than some of the tested LED spectra.

But:

The HPS light achieved only about 1.7 µmol/J, while the most efficient LED tested produced about 2.5 µmol/J.

The result was remarkable:

The LED produced slightly less per area, but due to its higher electrical efficiency, it achieved about 27% more yield per dollar of electricity spent within this experimental model.

The decisive metric was therefore not just the spectrum.

It was the photon efficiency of the entire fixture.

What does µmol/J mean?

The µmol/J metric describes how many photosynthetically active photons a light produces per joule of electrical energy used.

The higher this value, the more efficiently electricity is converted into usable plant light.

This makes it much more helpful for comparing modern grow lights than:

• Watts
• Lux
• subjective brightness

A high-quality double-ended HPS typically lies in the range of 1.7–1.8 µmol/J.

Modern LED technology can be significantly higher.

This is exactly where the decisive technological advantage of LED arose.

Yes – but this point also requires precision.

Every electrical grow light ultimately contributes to the heat load of the room.

So an LED is not a cold light.

The key difference lies in how much electrical power is needed for the same amount of photons and how the heat is dissipated.

HPS lamps have very hot burners and emit a comparatively large amount of long-wave heat radiation toward the plants.

LED systems dissipate a larger portion of their waste heat through:

• heatsinks
• convection
• ambient air

For comparable PPFD, leaves under HPS can therefore become warmer.

An energy-balance study under typical indoor conditions found approximately 1.3 °C higher leaf temperatures under HPS than under LED, whereby water status, air movement, and transpiration can have an even greater influence on leaf temperature.

Why this is important for climate

Under HPS, special attention must therefore be paid to the following factors:

• Distance to the lamp
• Air temperature
• Leaf surface temperature
• Exhaust air
• Circulating air
• Humidity
• VPD
• Irrigation

A room with a 600-watt HPS usually requires different climate management than a more efficient LED system, which generates the same PPFD with significantly less electrical power.

This also explains why a direct watt-for-watt comparison makes little sense.

Heat can sometimes also be useful

The heat dissipation of HPS is not exclusively a disadvantage under all conditions.

In a:

• cold basement
• unheated room
• conservatory
• cool greenhouse

additional radiant heat can sometimes be desirable.

Horticultural research shows that plants under HPS can have warmer leaves due to higher long-wave heat radiation.

The supposed efficiency disadvantage can thereby partially replace heating energy in a cold production system.

In an already warm indoor room, however, the opposite happens:

The heat must be removed again using exhaust air or air conditioning.

HPS and light burn

Due to the combination of high spot PPFD and heat radiation, insufficient lamp distances can become problematic.

Possible consequences are:

• very high leaf surface temperatures
• bleaching
• dry leaf edges
• foxtailing
• reduced photosynthesis
• light burn

A blanket distance table is not very helpful here.

Crucial factors are:

• Lamp power
• Reflector
• actual PPFD
• Temperature
• Air movement
• Genetics

A PAR meter therefore provides more information than the pure centimeter measurement between the light source and the tip.

Why MH and HPS were combined in the past

Before full-spectrum LEDs were widespread, indoor growing often involved switching between two HID systems:

Metal Halide – MH

stronger blue content and cooler-looking light.

High Pressure Sodium – HPS

less blue, significantly warmer spectral distribution.

The classic practice was therefore:

MH in the vegetative phase → HPS in flowering

In professional horticulture as well, different discharge lamps or mixed systems were used to better adapt the spectrum to cultivation goals.

Today, a single LED light can combine such spectral ranges much more flexibly.

Does cannabis need a special flowering spectrum?

Not in the sense that only red light could produce flowers.

Cannabis uses photons across a broad range of photosynthetically active radiation.

The spectrum can influence:

• Plant height
• Internodes
• Leaf morphology
• Photosynthesis
• Biomass distribution
• possibly individual secondary metabolites

However, the results are highly genotype- and experiment-dependent.

Cannabis studies show, for example, different reactions of morphology and cannabinoid profile to different blue-to-red ratios.

The simple rule:

Red = Flower

is therefore more of a growing tradition than complete plant physiology.

HPS lamps age

An important disadvantage compared to many modern LED systems is lamp degradation.

HPS bulbs lose light output with increasing operating time.

Michigan State University describes replacement after approximately 12,000 operating hours as a typical guideline for HPS, when performance has dropped to about 85–90% of the original output.

At the same time, the bulb continues to consume approximately the same electrical power. As a result, efficiency decreases with age.

Also, dirty:

• reflectors
• lamps
• protective glass

can further reduce the actual photon output reaching the plants.

Why Lux is not an ideal maintenance measurement

For plant lighting, lux is only suitable to a limited extent, because the value is weighted according to the sensitivity of the human eye.

HPS in particular has large emissions in wavelength ranges to which the human eye reacts strongly.

Consequently, an HPS lamp can appear extremely bright in lux, without this value being directly comparable to plant-relevant PPFD.

For grow light, it is therefore better to use:

• PPF
• PPFD
• DLI
• µmol/J

are more suitable.

You can find more on this under Lux vs. PPFD.

Ballast and Ignition

A classic HPS lamp requires a compatible ballast.

It limits the current and, in conjunction with the ignition technology, establishes the electrical conditions necessary for startup.

HID systems also require a certain warm-up time.

Following a power interruption, hot HPS lamps cannot always reignite immediately. A "restrike" can only occur after the burner has cooled down sufficiently.

This clearly distinguishes them from LEDs, which can be turned:

• on
• off
• dimmed

practically instantaneously.

Safety with HPS systems

An HPS lamp operates with:

• high electrical power
• high ignition voltages
• very high surface temperatures

Therefore, the following should be designed for the appropriate power rating:

• lamps
• sockets
• ballasts
• cabling
• timers
• plug connections

Improvised wiring, overloaded power strips, or unsuitable sockets are particularly unwise solutions for high-performance HID systems.

Reflector and light distribution

In an HPS system, the reflector is a central component of the lighting setup.

Since the lamp emits light in many directions, a portion of it must be directed back onto the plant area.

The shape of the reflector therefore influences:

• hotspots
• edge lighting
• uniformity
• usable area

A poor HPS setup can produce extremely high PPFD in the center while simultaneously delivering significantly too little light at the edges.

A modern, large-area LED bar design can reduce these discrepancies.

This makes it clear that:

Light quality is not just spectrum – it is also light distribution.

What is a Cooltube?

A cooltube, or air-cooled hood, was developed to dissipate some of the heat from an HPS system directly at the light source.

Air is drawn past the hot lamp and transported directly out of the grow area.

This can:

• lower room temperature
• allow for closer lamp distances
• make climate control easier

At the same time, the glass has a disadvantage:

It reduces some of the usable light output.

Scientific HPS/LED trials also account for this effect; a glass pane in front of an HPS lamp can measurably reduce photosynthetic photon flux.

Is an HPS system still useful today?

Technically:

Yes.

Economically and practically:

It depends on the setup.

An existing HPS system can still be interesting if:

• it is already paid for
• the room is cool
• heat can be utilized
• there is sufficient exhaust air
• power consumption is less critical
• growing is only done seasonally

A functioning HPS system does not suddenly become useless just because better LEDs exist.

When LEDs are usually more sensible today

For completely new purchases, there are now many reasons for choosing modern LED systems:

• higher photon efficiency
• lower power consumption for the same PPFD
• less directional radiant heat on the canopy
• better surface area distribution possible
• easy dimming
• no regular bulb changes
• more flexible spectra

The cannabis study by Westmoreland et al. demonstrates perfectly why efficiency is so important: while HPS achieved slightly higher biomass per area in their trial due to its low blue light content, the more efficient LED produced significantly more flower mass relative to electricity costs.

HPS or LED – what is more important?

The more sensible question today is not:

Which technology is fundamentally better?

But rather:

How many photons reach the canopy uniformly – and how much energy does that cost?

Important metrics are:

• PPFD
• DLI
• µmol/J
• uniformity
• power draw
• heat load

Followed by:

• spectrum
• purchase price
• lifespan
• climate situation

This makes a comparison much more robust than "600 Watt HPS versus 600 Watt LED".

HPS and power consumption

A classic example:

A 600-watt HPS requires not only the nominal lamp but also a ballast with its own losses.

The actual power consumption of the system can therefore be slightly above the value stated on the lamp.

If an LED produces the same usable amount of photons with less electrical power, two efficiency advantages arise:

1. lower direct electricity costs for lighting
2. less heat that may need to be actively removed from the room

Especially with long lighting periods, this difference adds up significantly.

HPS and cannabis quality

Another myth is:

HPS produces fundamentally better flowers.

There is no general scientific basis for this either.

A controlled study found:

• higher flower dry matter under HPS
• higher THC concentrations under the two tested LEDs
• no significant difference in total cannabinoid yield.

Other studies, in turn, show that low blue light content can promote flower yield without significantly changing THC or CBD concentrations.

The result therefore depends on:

• genetics
• PPFD
• spectrum
• light distribution
• climate

No single lighting technology possesses a universal quality bonus.

HPS and terpenes

There is no simple rule for terpenes either.

The light spectrum can influence the composition of secondary metabolites, but the reaction is highly dependent on the genotype.

Therefore, a specific HPS lamp does not guarantee:

• more terpenes
• more resin
• stronger aroma
• better sensory quality

Just as little does a full-spectrum LED automatically guarantee a superior profile.

Genetics remain the starting point; light is one of several environmental factors that influence how this potential is expressed.

Disposal of sodium vapor lamps

HPS lamps do not belong in household waste.

Gas-discharge lamps can contain mercury and, in Germany, are subject to the Electrical and Electronic Equipment Act.

The Federal Environment Agency points out that gas-discharge lamps must be collected separately and can be handed in, for example, free of charge:

• at municipal recycling centers
• at appropriate collection points
• at retailers obligated to accept them

The lamp should be transported in a break-proof manner if possible.

In this respect, the disposal of an old HPS lamp also differs significantly from that of a classic incandescent bulb.

Typical myths about HPS

HPS is automatically the best light for flowering

No. Cannabis studies show both strengths of HPS and advantages of modern LEDs. Decisive factors are PPFD, spectrum, genetics, and fixture efficiency.

HPS light penetrates deeper than LED light

Not as a universal technology trait. Light distribution, spectrum, and plant architecture determine how much radiation reaches deeper canopy areas.

More watts mean more yield

Only if this actually results in more usable photons and the plant can utilize them.

Red light triggers flowering

For photoperiodic cannabis, the length of the light or dark phase is the decisive factor.

LEDs do not produce heat

False. LEDs also convert a portion of electrical energy into heat. However, they are typically more photon-efficient and transfer less long-wave heat radiation directly to the plants.

HPS bulbs stay equally bright until failure

No. Light output decreases over the operating life.

An HPS lamp automatically produces more THC

No. In a direct cannabis study, the THC concentration under HPS was actually lower than under the two LED spectra tested.

FAQ – Frequently asked questions about HPS

What does NDL stand for?

NDL stands for sodium vapor lamp. In the context of cultivation, this usually refers to a high-pressure sodium vapor lamp, or HPS.

What does HPS mean?

HPS stands for High Pressure Sodium.

Is an NDL a HID lamp?

Yes. High-pressure sodium lamps belong to the category of High-Intensity Discharge lamps.

Does an NDL require a ballast?

Yes. A classic HPS requires a compatible ballast and the corresponding ignition technology.

What spectrum does an NDL produce?

Typical HPS systems have a warm, yellow-orange dominated spectrum with relatively little blue, as well as relevant red and some far-red components.

Is NDL only suitable for the flowering stage?

No. Cannabis can also grow vegetatively under HPS. However, due to the low blue light content, the morphology may end up more stretched than under bluer spectra.

Why was MH used for growth in the past?

Metal Halide typically has a higher blue content, which can promote a more compact plant morphology.

Is NDL better than LED?

Not fundamentally. Modern LEDs usually have a clear advantage in photon efficiency and can distribute light more evenly. HPS can still achieve high yields, however.

Does HPS result in higher yields?

In some individual trials, HPS produced slightly more biomass than LED spectra with higher blue content at the same PPFD. However, in terms of electrical energy used, more efficient LEDs were more economical.

Does HPS result in more THC?

No. A direct comparative study found lower THC concentrations under HPS than under the LEDs tested. Conversely, the total cannabinoid yield did not differ significantly.

Does an NDL get hotter than an LED?

HPS generates more directed long-wave heat radiation towards the canopy. At a comparable PPFD, leaves can therefore become slightly warmer. The actual difference depends heavily on air circulation and transpiration.

What does PPFD mean?

PPFD describes the density of photosynthetically active photons hitting a surface per second.

What does µmol/J mean?

This key figure describes how many plant-relevant photons a light fixture generates per joule of electricity consumed.

Does an HPS need to be replaced regularly?

Light output decreases as operating hours increase. In commercial horticulture, it is therefore recommended to replace old HPS bulbs before they fail completely.

Can an NDL be disposed of in household waste?

No. In Germany, gas discharge lamps must be collected separately as waste electrical equipment; they may contain mercury.

Conclusion

The sodium vapor lamp is one of the most important technologies in the history of indoor cannabis cultivation. For decades, HPS was the benchmark for powerful artificial lighting—and for good reason. The technology can generate high PPFD, reliably guide cannabis through a complete cultivation cycle, and deliver very high bud yields under suitable conditions.

Many of the classic arguments for the NDL must be viewed more precisely today, however. Its spectrum is not simply "red," but is heavily characterized by yellow-orange and is comparatively poor in blue. A special "penetration effect" is not a universal property of the technology. And even regarding cannabinoids, controlled comparisons show no fundamental quality advantage over LED.

The biggest difference today lies in efficiency. Modern LED systems can generate the same amount of plant light with less electrical energy and distribute heat differently. As a result, depending on the setup, both power and climate control requirements decrease. HPS, on the other hand, can even use the additional heat radiation to its advantage in cold rooms.

Therefore, in 2026, the NDL is neither "bad" nor technically insignificant. An existing, well-functioning system can continue to serve its purpose. However, for a complete new design, energy efficiency, light distribution, dimming, and maintenance requirements now mostly point much more strongly toward modern LED technology.

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