Difference Between HPS And MH Grow Light Bulbs
The biggest challenge of indoor
growing is getting the light just right. And there are more options than ever,
now that LED fixtures have joined fluorescent, plasma and high-intensity
discharge (HID) lights on the list of possible choices as primary light sources.
have their advantages, but most gardeners continue to rely on the two most
common types of HID grow lights, high-pressure sodium (HPS) and metal halide
(MH). They’re less-expensive than plasma and LED, and produce better quality
and yield than fluorescent lighting.
you decide on HPS or MH lighting (or alternate between the two, as many growers
do), it will take quite a bit of experimentation to strike the right balance
between cost-effectiveness, ease of operation and yield. But making that choice
is the most important step you’ll take; to do that, you need to know the
similarities and the difference between HPS and MH grow light bulbs.
What Are High-Intensity Discharge
HID grow lights, whether they’re HPS
or MH, are basically very large light bulbs filled with gas. The gas is
illuminated by an electric current running through it, in much the same way that
the bright lights used in stadiums or modern street lights operate. As you’d
guess, the gas used in a high-pressure sodium bulb is sodium, while metal
halide bulbs are filled with gas made from compounds of metals and either iodine
That’s important to know for three
reasons. First, the type of gas determines the type of light emitted, and we’ll
discuss that more in a bit. Second, the generation and regulation of the
electricity that runs through HID grow lights requires the use of a ballast (a
type of transformer), so that needs to be figured into setup costs and
logistics. Finally, both HPS and MH grow lights run very hot, so it’s best that
the hood or reflector which will be used to efficiently direct the light is
either water-cooled or air-cooled. All of those elements – bulbs, ballasts and
reflectors – are integral to a HUD grow light system.
with those extra costs, HPS and MH lights are much less expensive to purchase
than options like LED and plasma. They do, however, require more a lot more power
so they’re more expensive to run. HPS and MH bulbs can be chosen in a wide
range of sizes; between 400 and 600 watts is usually considered ideal for
indoor personal or hobby growing. And these lights are strong. HPS bulbs will
often need to be hung at least a foot or two away from plants so that the tips
of the plants aren’t burned, they’ll require a lot of attention to cooling
and/or air conditioning, and they may not be suitable for very small, enclosed
spaces. MH gives off a little less heat than HPS, but they’re quite similar in
requiring cooling and ventilation.
Now that you know what they have in
common, let’s look at the difference between HPS and MH grow light bulbs.
HPS Grow Lights
The sodium gas that is in HPS grow
lights means that the bulbs emit light in the orange to red spectrum, similar
to outdoor light during the late summer and early fall harvest season. That
makes HPS ideal for plants’ reproductive (flowering) and fruiting stages, since
the orange/red light acts as a hormonal trigger and leads to more budding and flowering.
There’s a downside, though. The strength of the light and the lack of blues in
the HPS spectrum mean that plants won’t flourish to best advantage during the
vegetative state and may end up being too “leggy,” growing more quickly than
they should and leaving more space between branches. Full-season growth and
production using only HPS grow lights is normally good, but not optimal.
If you believe manufacturer specifications,
the average lifespan of an HPS grow light is just under 20,000 hours, about
twice as long as that of an MH bulb; most experts recommend replacing them once
every two years or so. However, the real truth is that HPS bulbs start losing
their oomph after a few growing cycles, even though they may look like they’re
radiating the same amount of light. Many growers will tell you that the “real”
lifespan of these lights is closer to 3,000 hours, or 3-4 growing cycles.
MH Grow Lights
The light produced by the metal
halides in MH bulbs is focused in the blue/violet spectrum, rather than the
orange/red of HPS bulbs. The blue light is similar to that of springtime
outdoors, and is ideal for the starting and growth stages of plants because it
encourages photosynthesis. Plants grown in MH light are more likely to develop
stronger roots and stems, will have more compact growth and will be more
disease-resistant. If only MH light is used throughout the growing season,
though, expect yields to be drastically reduced. One other slight benefit to
metal halide light: it looks more natural to the naked eye because there’s no
orange “glow,” so you’ll get a better view of the true color of your plants.
The specifications claim that MH
bulbs will last about 10,000 hours and should be replaced every 12 months. Once
again, though, these lights will lose a lot of their effectiveness over time
and it’s a good idea to replace them more frequently. It’s also good practice
to run grow lights longer during the vegetation stage, meaning that if you’re
only using MH lights while the plants are growing and then switching to HPS,
the MH bulbs will be burning for more hours per day.
Using HPS and MH Grow Lights Together
For many growers the best way to
grow indoors with HID grow light bulbs is to take advantage of the strengths of
each type, relying on MH bulbs for the growth stage and then switching to HPS
(or adding it as a secondary light source) when it’s time for the plants to
flower. There are several ways to do that without having to set up two
completely different systems; among them are reflectors which have both HPS and
MH lights inside, convertible light fixtures which will accommodate either type
of HID bulb, and so-called HID conversion bulbs which allow you to run HPS
bulbs from an MH ballast and vice versa.