HPS Distance From Seedlings

Hydroponics seedlings with HPS lights

Among all the options for grow lights, high-pressure sodium (HPS) are commonly used, mainly to encourage flowering and/or setting fruit. Unlike LED grow lights, HPS lights produce more heat, so growers need to know how far to put HPS lights from plants.

The HPS distance from seedlings should generally be between 10 and 14 inches from plants. However, the optimal distance between an HPS light and seedlings will vary, depending on the HPS wattage, from 10 – 25 inches. For 1000-watt HPS lights, a distance of 5 feet might be needed.

Because HPS can produce high heat,, growers need to be careful about HPS distance from seedlings to ensure healthy growth for their plants. In this article, we’ll cover the following:

  • Why the right HPS distance from seedlings matters
  • The suitable HPS distance from seedlings for a 250W, 400W, and 600W
  • The benefits of using HPS lighting

Can I use HPS for seedlings?

First, you might be wondering if you can actually use HPS lights for seedlings–especially given that HPS lights can produce a lot of heat.

The short answer is that, yes, with proper precautions, you can indeed use HPS lights for seedlings.

If you want to geek out a bit more, you can learn even more about how the different colors of light affect plants.

However, you’ll want to keep an eye on the seedlings so that the heat and intensity from the HPS lights don’t burn your seedlings. 

This is especially crucial, given that some seedlings are fast-growing. For example, you might position your HPS lights 14 inches from your seedlings, only to find a few days later that the seedlings got burned as they grew too close to the lights.

Kind of like Icarus flying too close to the sun!

Creating the Right HPS Distance From Seedlings

When growing plants indoors, you need to have “grow light,” which is an artificial light that attempts to provide a light spectrum similar to that of the sun. 

Sometimes growers use HPS lights with Metal Halide (MH) grow lights, which are suited best for vegetative growth. 

When growers combine HPS and MH lights, HPS for flowering, and MH for vegetative growth, the results are often significant. 

If you have to choose between the two, select the type that will be better suited for the plants you intend to grow. 

Even without natural sunlight, plants can grow using High-Intensity Discharge (HID) lights, such as HPS, which has relatively high fluence (max.200 lumens per watt) and high photosynthesis radiations (PARs) efficiency, up to 40%. 

HPS’ lights are powerful, so you must pick the lights’ right distance from seedlings.

Oh, and in case you were wondering: fluence is basically a measure of intensity of light. 

Yep, it’s a thing–and your word for the day. And not to be confused with flatulence.

Too Little Light Makes Plants Work Harder

The light amount your plants receive will determine if they will thrive or otherwise. 

If you put your grow light too far from your plants, you can cause damage to them, or worse, stretching, which is when your plants work more to grow taller so that they can reach the nearest light source.

This is what happens when plants get too little light–they get leggy, as they stretch toward the light to obtain more light for growth.

When plants use too much energy to get sunlight, they won’t have enough power to thrive or produce fruits. Too much light could lower your yields by 30% — what a waste of potential! 

Therefore, make sure you don’t put your HPS lights too far from your plants.

Too Much Light Is Bad for Your Plants

Neither should you let your plants struggle for light, nor should you make them get too close to the light. 

The heat that HPS lights emit can quickly burn your plants’ tops.

If HPS lights are too close to your plant, it can also stunt the plant’s growth.

The temperature at the top of your plant canopy should not go over 82.5°F (28.1°C) without CO2 supplementation. However, with CO2 supplementation, the temperature at the top of your plant canopy should not go over 89.6-95°F (32-35°C). You can use a thermometer to measure the top of the plant canopy’s temperature, with an external probe that you can attach to the plants’ highest spot.

You can also use the back of your hand to check the temperature. Place your hand on a level with your plant’s top using the back of your hand, facing the light for 30 seconds. If the light is too hot for your hand, raise the light until you can comfortably put your hand on the same level as the plants’ tops.

Estimating Distance

Choosing the perfect distance for your seedlings is not always easy because there’s no one size fits all. However, here are some general tips for growing healthy and edible plants:

  • If you’re growing your plants without lenses on the surface, your lampshade should be 1 to 2 inches (2.54 to 5.08 cm) from the leaves. You want to make sure that the light is not too intense. Otherwise, your plants may burn as they grow toward the light.
  • If you’re growing your plants with lenses of the surface, you should put your light a little further away.

Now, if you’re using LED grow lights, you can check out this article for calculating the distance from LED grow lights to plants.

These are different HPS light bulbs for growing plants indoors:

  • 250W. Many growers use these bulbs for smaller projects. These bulbs are cheap to set up and are easier to keep cool.
  • 400W. These bulbs are the most common. Although these bulbs emit a lot of heat, they also provide more PAR light to your plants than a 250W light bulb.
  • 600W. These bulbs are the most efficient because they don’t waste a lot of heat, and they can produce around 150 lumens per watt. In other words, these bulbs convert a massive amount of electricity into usable light.
  • 1000W. These bulbs are the most powerful (and hottest). These bulbs have to be placed at least 5 feet above the plants.

250W HPS Distance From Seedlings

For 250W HPS, the distance of the lights from seedlings has to be between 10 inches (25.4 cm) and 14 inches (35.56 cm) from your plants. Place your hand where the seedlings are for about 30 seconds. If the light is too hot for you, that means it’s also too hot for your plants.

That said, you need to increase the height of your HPS light. If your light bulb is new, you’ll want to put it slightly further away from your plants to prevent the light bulb from overheating your plants.

How far should a 400w HPS be from seedlings?

For 400W HPS, the best distance from seedlings is between 12 inches or 30.48 cm (closest) and 19 inches or 48.26 cm (furthest).

400W HPS grow lights are the most commonly used HPS grow bulbs. 

These lights can cover around 16 square feet and produce high yields. Typically, a 400W HID can give plenty of light to illuminate a 4×4 ft area fairly well.

600W HPS Distance From Seedlings

Most growers will be happy growing their plants using a 600W grow light or two 600W lights. You use HPS for growing larger plants where you can have at least 14 to 25 inches (35.56 cm to 63.5 cm) in height in the grow space. For the best results, use a taller space because HPS lights have to be far from plants due to their light strength.

Benefits and Disadvantages of HPS Lights

HPS consumes a lot of energy, with an average energy consumption of 60.8. HPS lights are popular when growing flowering plants because they can be a cheaper alternative to LEDs, considering the higher initial cost of LEDs. Yields tend to get bigger, too, with HIDs. 

HPS lights are sometimes called the “Golden Standard” of grow light for growing cannabis. Growers have used HPS for decades to get excellent yields. Furthermore, HPS lights are great for developing a few plants or dozens of plants.

Due to HPS’ light strength, growers will require a tall grow space. HID grow lights are also not easy to find locally, so growers usually need to purchase them online. Again, heat can be an issue with HPS as HPS lights produce a lot of heat when growers use 250W, 400W, 600W, or 1000W HID lights.

Final Thoughts

Choosing the right distance between HPS lights and seedlings isn’t always easy because there’s no one size fits all. Different factors can determine the placement of HPS lights from your plants, such as your light bulb’s condition, whether or not you’re using optics for the plants, or the types of plants you’re growing. It’s always vital to test the light first.

HPS lights are robust and suitable for many plants. 400W is the most common, 600W is the most efficient, and 1000W is the most powerful (and hottest). With the right amount of light, your plants can thrive.

About the author

Greg Volente

Greg Volente holds a Naturalist Certificate from the Morton Arboretum, worked for The Nature Conservancy leading environmental education programs and doing natural areas restoration, and worked in the soil science research & testing lab at Michigan State University. Besides gardening, he's an avid wildflower enthusiast, and loves botanizing, hiking, and backpacking.