What happens when you dim your grow lights?

These days, there are many brands and models of HID (HPS and CDM / CMH) grow lights that have the facility to be dimmed. It tends to be only the cheap ones (with magnetic ballasts) that do not. Nearly all digital ballasts have a control for dimming, and many also have a “boost” function too.

There’s 3 main reasons why you might want to consider dimming your grow lights. It is no secret that HID grow lights produce a lot of heat. During the warmer months of the year, it can be handy to turn down the lights to reduce the amount of heat produced by them to ensure that your grow space doesn’t become too warm.

The second reason is that you might not need the full area that a grow light can cover. Turning down the grow light means that you can reduce the footprint and save a little money on the electricity bill.

Finally, it may be that your plants have got a bit out of control and grown too tall. If you have run out of headroom to raise the light any more, you might want to turn the light down to avoid causing damage to the tops of your plants.

What actually happens when a grow light is dimmed?

It is fairly obvious that dimming your grow lamp makes it dimmer, produce less heat, use less electricity, and cover less canopy area. But is there anything else that happens? Well, yes. Very much so. The spectrum alters too, and this is very important. Lamps are designed to produce the correct spectrum at their nominal wattage. Turning them down means that they no longer produce the spectrum that they were built to produce.

A very quick summary about light spectrums is as follows:

  • Red light is what makes plants most productive.
  • Blue light is what enhances final quality and keeps plants short
  • Yellow and green light enhance plant health but huge amounts are generally not required for this. These colours also drive some photosynthesis.

Now let’s see what happens to an HPS light when it is dimmed. We used a 1000w 400v grow light for this, but the effect will be similar with pretty much any HPS grow light.

Here is the spectrum at the full 100% power setting:

Next, here is the spectrum when dimmed down to 75%:

Notice how the productive reds between 600nm and 700nm have become reduced. This means that watt for watt, this spectrum will not produce a proportional amount (i.e. 75%) of the yield as it would at 100% This situation continues to decline as we dim it down further to 60%:

Just out of interest, here is the spectrum when it is boosted up to 115%:

So, if your grow space can take the heat (and you can stand the increase in the size of the electricity bill) then the extra reds produced when boosted will be be proportionally greater than 115%. Bear in mind that this will tend to reduce the lifespan of the lamp so you might want to factor that in to the cost of running your light in “boost” mode.

Now let’s take a look at dimming a CDM / CMH Grow Light

Although, the old type of Metal Halide lamp used to be the other type of HID grow light, it has been superseded by a superior version of it called CDM (Ceramic Discharge Metal Halide) which is sometimes called CMH (Ceramic Metal Halide). When they were first released onto the market, it was said that they should not be dimmed, and definitely not boosted. However, some of the ballasts for CDM grow lights have recently been appearing with dimmer switches on them.

We took a 4.2K CDM lamp and powered it up with a ballast with this facility. Here is the spectrum at 100%:

At 90%:

At 80%:

At 70%:

At 60%:

At 50%:

The story here is that as the CDM lamp is dimmed, most of the colours are progressively reduced, except for a large green spike in the middle which gets relatively larger. The reduction in reds is harmful to productivity. Crop quality will probably be reduced too.

The Conclusions

While it is completely possible to dim your HID grow lights, we would say don’t do this if it is at all avoidable because the relative efficiency will take a hit. It would be better and more efficient to use a 600 watt grow light at full power rather than to use a dimmed 1000 watter.

Fill the entire footprint with plants, and, if necessary, train them sideways so that they don’t grow straight upwards and get too close to the lamp. If you are struggling with your grow space temperatures then we would seriously recommend that you consider upgrading your extraction system before turning the lamp down.

As an aside some LED grow lights can be dimmed. However, dimming an LED has little or no effect on the output spectrum.