How to Choose the Right Grow Light for Your Plants

The Problem with Choosing Grow Lights

Many people unintentionally make poor choices when selecting LED grow lights. They often follow popular public opinion, or trust online influencers who typically lack a deep understanding of light science and/or photosynthesis, and simply recommend any light that they have experience with, or more often whichever pays them the highest affiliate fees.

Understanding light is pretty f*cking difficult. People's eyes glaze over pretty quickly as soon as they start digging into product literature and lighting metrics. After about 15 minutes and with everything sounding sort of similar, but kind of different, most people simply walk away more confused then they ever thought possible, and wind up considering a light that they've seen others use, and then choose a model with all the buzz words, and a price they can afford.

The industry as a whole knows that the more confused you are, the easier it is to sell you low quality lights, at high profits. For the most part, the know that if a light appears bright and is labelled as 'full-spectrum', many people will simply put it into their cart and after a few more days of frustration, they'll simply press the buy button and cross their fingers.

This isn't just cheap no name brands on Amazon or Temu, some of the most expensive grow lights that target the average plant parents deliver the least amount of value. I'm not talking about $3 offshore wholesale lights selling for $20, I'm talking about retail priced lights in the $20-$30 range selling for $200 or more. The light output is what you can usually get for between $20-$50. Zero additional benefit from a lighting perspective, but they look nicer and are marketed to appeal to a certain consumer. Unfortunately, a lot of people who buy those lights believe that they got what they paid for, and all they really got was a bag of magic beans, that had no magic at all.

Lighting mistakes often take months to manifest, sometimes longer, so lighting retailers know that by the time a plant has obvious issues, the lighting is often the last thing blamed. "Nothing has changed. It was doing great for months, and all of a sudden..." Think pests, disease, dropped foliage, root rot, etc.

Ignoring the technical complexities of light and the fundamental importance of how it affects photosynthetic response means anyone who doesn't grasp lighting fundamentals will often make purchasing decisions that eventually likely lead to poor plant health, suboptimal growth, and wasted money on ineffective lighting. It's time to illuminate the science behind effective grow lights, ensuring your plants can truly thrive, not just survive , until they don't.

We all have a general idea that light is important for the health and growth of your indoor plants, but what many people don't realize is that it's directly responsible for 50%-75% of growth and vitality. As well, it contributes to just about every biological function a plant engages in.

Everything hinges on the balance between light, water, oxygen, humidity, and nutrients, but light is the fulcrum on which it all rests. In this guide, I’ll walk you through everything you need to know about choosing the right LED grow light, including factors like light coverage, light intensity, beam angle, hanging height, and other specific needs for your plants.

Let's get started!

1. Understanding Plant Light Requirements

Before diving into grow lights, it’s important to understand what your plants need in terms of light. Different plants have different requirements, and knowing a bit about photosynthesis and light will help you make an informed decision.

Light Spectrum (PAR vs. Full Spectrum):

Plants use light in the Photosynthetically Active Radiation (PAR) range (400–700 nm) for photosynthesis. This range includes blue light (10-25% of photosynthesis, great for vegetative growth), red light (60-70%, crucial for flowering, stem elongation, and fruiting), and green light (canopy and foliage penetration). While consumer lights may claim to be "Full Spectrum" (including UV and IR), only PAR light directly supports photosynthesis.

Light Intensity (PPFD):

PPFD, or Photosynthetic Photon Flux Density, is the measure of light (photons i.e. light particles - μmol/m2/s) that actually reach a plant's leaf surface allowing the plant to convert light into energy (carbs) that the plant uses for growth and other biological processes. It’s often the most crucial metric when selecting a grow light as plants need a specific volume of light to grow.

Photoperiod (Light Duration):

The amount of time that plants are exposed to light affects their growth cycles. Short-day, long-day, and day-neutral plants all have different photoperiod needs.

Daily Light Integral (DLI)

A measure of the total amount of photosynthetically active radiation (PAR) that a plant receives over the course of a day. It represents the cumulative number of photosynthetic photons (measured in micromoles - μmol) that hit a square meter of plant surface in a 24-hour period.

2. Determining the Area That Needs Illumination

The size of your growing area will often determine the type, power, and number of grow lights you need. You want to try and ensure that your entire plant area is receiving the same light intensity so that all your plants get effective coverage. Light isn't a constant, so it will never be identical over a growing area, but knowing and preparing for how beam angles affect light falloff helps mitigate this issue.

Measuring Your Space

Start by calculating the dimensions of your growing area. Whether you have a small shelf, a larger area of a room, or even a single plant, knowing the size of the space you need to light up will help you determine the best lighting.

Plant Spacing and Growth

While you immediate needs are likely forefront in your mind, think about how your plants are arranged and how they’ll grow over time, as well as how your collection will grow (number of plants, not just how tall them may grow). Crowded plants may need more intense lighting to ensure all leaves get the proper amount of light.

3. Calculating the Required Light Output

• PAR and PPFD: PAR (Photosynthetically Active Radiation) measures the spectrum (wavelenght) of light beneficial to plants, while PPFD measures the intensity of that light reaching a leaf's surface. These metrics are crucial for determining if your plants are getting enough light.

4. Selecting the Appropriate LED Grow Light

Quality LED Grow Lights are an excellent choice for indoor plants due to their efficiency and versatility.

• Advantages of LEDs: LEDs are highly energy-efficient, they produce less heat compared to other lighting options, and they can be customized to provide the exact spectrum your plants need.
• Spectrum Options: Full-spectrum LEDs are popular because they mimic natural sunlight, providing all the colours needed for various growth stages.
• Intensity and Coverage: Choose an LED grow light that provides enough intensity (PPFD) to match your plant’s requirements and that can cover the entire growing area evenly.
• Quality and Durability: It’s worth investing in high-quality LEDs. While they may have a higher upfront cost, they last longer and provide more consistent results.

5. Understanding Beam Angle

The beam angle of your LED grow light determines how the emitted light spreads over an area. LED grow lights generally have a beam angle of between 60° and 180° with many using 120°

• Definition: The beam angle is the angle at which light is emitted from the fixture.
• Impact on Light Distribution: A wider beam angle covers a larger area but reduces intensity, while a narrower beam angle focuses the light more intensely over a smaller area.
• The Right Beam Angle: Any beam angle can work, but to ensure equal light coverage, you will usually need to overlap the beams from multiple lights. Where the beams overlap, the PPFD can be added together.

As an example, if part of a light beam delivers 75 μmol/m2/s and another light overlaps that same area with 50 μmol/m2/s then the combined light density for that overlapping area will be 125 μmol/m2/s.

Because light falloff is not linear, positioning and spacing lights is a bit of an art form. Many leading commercial lighting manufacturers provide what is called a PAR Intensity Map (PAR Map for short) that usually represents the light falloff over a specific area (often a 4'x4' area), at a specific height (Often 12") to make this complex visualization easier to understand.

Unfortunately, most of the lower end lighting solutions do not provide this critical visualization tool which leads to consumers placing their lights at all sorts of distances, at odd angles, and using their eyes as some sort of lucky guess at proper placement.

Luckily, the Unlikely Gardener has created a PAR Map Generator which uses some complex physics to approximate how light is spread over a targeted area. It's not perfect, but for most people it will help give them a much clearer idea of how light intensity falls off over a specific area.

6. Determining the Hanging Height of the Light

As you can likely already imagine, the height of your grow light impacts light distribution. As height increases, the beam spreads over a larger area and light intensity drops. I'm sure you've seen this when using a household flashlight. This pesky quirk of physics is called the Inverse Square Law and its a critical thing to understand when it comes to handling light intensity. The positive however, is that it also means when a light gets closer, it increases in intensity.

To better illustrate this important concept, let's imagine that the grow light you just bought delivers 100 μmol/m2/s at a 12" distance from your plant's biggest leaf. It's an amount of light that meets the minimum level for a lot of indoor plants, and you're pretty happy with it. You didn't know about the importance of its hanging height, so you decide position it 24" above your plant so it can illuminate more space, and more of your plants. Unfortunately, that same light will now only deliver about 25 μmol/m2/s to your plants in that same spot. If you chose to move 36" above your plants, they now only get about 12 μmol/m2/s - that's an 88% reduction in light intensity from the stated specs of the light, and result in an amount of light far below what plant needs for basic metabolic functions. We often think this is still fine, because to our eyes the level of brightness is still quite intense. To a plant who only cares about getting its photon fix, it's going into withdrawal.

On the other hand, if that same light was moved 2" closer to a 10" distance from the plant, that initial 100 μmol/m2/s would increase to 144 μmol/m2/s, and if it moved to only 8" above the canopy it would deliver 225 μmol/m2/s. As great as this is it can also mean a plant can get too much light if the initial intensity was already significant to begin with.

While this light increase might seem like you can save on lighting by just moving lights closer (and sometimes you can), you also need to remember that as it gets closer, the area it illuminates also gets smaller. The volume of light a plant needs is the average over the entire plant, not just some of the leaves that are closest to the light source. Keep this in mind, since we are effectively trying to compete with the sun's natural light, which has a much higher ambient (bright indirect) light volume.

• Adjusting Light Height Based on PPFD Requirements: To ensure plants get the correct PPFD, adjust the height of the LED light accordingly. For seedlings and immature plants 50-200 μmol/m²/s is a good volume; for vegetative growth across most plants 200-300 μmol/m²/s should be your target measurement. High light plants like Alocasia and Monstera will want 300-500 PPFD, and other sun-loving plants like succulents and cacti will require PPFD levels of 600-1200 μmol/m²/s. Flowering plants are often in the 600-800 μmol/m2/s range, although there are a few notable exceptions.
• Measuring and Adjusting: A PAR meter, as already noted, can help you measure PPFD at the plant canopy and adjust the light height to maintain the appropriate levels throughout growth stages.

7. Matching Light to Plant Needs

Different plants and growth stages require different light intensities. As mentioned above, different plants at different stages of their growth cycles. Here is a recap of these general averages.

Growth Stages and PPFD Requirements:

Seedling Stage: Provide a lower light intensity of 50-200 μmol/m²/s.

Vegetative Stage: Moderate intensity of 200-300 μmol/m²/s is ideal, with a blue spectrum focus for optimal growth.

High Light Plants: Aim for 300-500 μmol/m²/s, often with more red spectrum if they may have flowering abilities or you want to encourage it.

Flowering Plants: Somewhere in the 600-800 μmol/m2/s range is a good place with increased red spectrum.

High-Light Plants: Plants like succulents and cacti thrive with 600-1200 μmol/m²/s to replicate their natural sunny environments. This can often be up to 1,800-2,000 to more closely match direct sun exposure.

Daily Light Integral (DLI):

The DLI is the total amount of light a plant gets over a 24 hour period. Many people believe that your lighting needs to match up with the natural light cycle of a typical day, roughly 12 hours. This is not 100% accurate. Research has shown that most vegetative plants don't really require any down time. This isn't set in stone, there is lots of research still to be done, but many vegetative plants are absolutely fine getting light for 24 hours at a time with no breaks at all.

What this means however is that if your plant needs 100 μmol/m2/s for a 12 hour period in the real world, indoors you could give it 75 μmol/m2/s for 16 hours, and the plant's needs will be met.

This means that in many cases you can get away with less powerful lights by simply running them for longer periods of time. Personally, I don't regularly run any of my plants under lights for longer than 16 hours at a stretch, but I have run lower intensity lights on higher intensity plants like alocasia when I don't have more powerful lights. I have also done this in my greenhouse when I want more overall light for my tomatoes or peppers and its mid April and the days are still dark and grey. I'll light up my entire neighbourhood with my bright "blurple" lights so I get early blooming and fruiting.

An exception this is 'light hack' is that for new seedling, and transplants, make sure they are getting at least 8 hours of darkness. Their ability to photosynthesize at a optimal level is compromised and they need the break to recuperate. I've broken this rule too, and all worked out fine, but most research shows that at the early stages, especially with flowering and fruiting, it can make a difference for sure.

For shits and giggles I've also created a DLI Calculator so you can determine what your DLI is based on your light output.

Average Daily Light Integral (DLI) for Indoor Plants

Understanding the Daily Light Integral (DLI) is essential for optimizing indoor plant growth and maximizing photosynthesis. Below is a table summarizing the average DLI ranges suitable for most indoor plants.

8. Energy Efficiency and Heat Output

LED grow lights are known for their energy efficiency and minimal heat output. While this is 100% accurate it does not mean that grow lights don't get hot, just the light itself isn't, the heat sink for the light can easily get to 100° or 200° however depending on the thermal transfer rate, so keep this in mind with high output and commercial LED lights, and especially with grow bulbs like Sansi. To get an idea of how much it will cost you to run your grow lights, take a look at our Energy Calculator for a reasonably accurate idea of ongoing costs.

• Importance of Efficiency: LEDs help reduce energy costs and are more environmentally friendly than other grow light options.
• Heat Management: While LEDs produce less heat, it’s still important to ensure proper airflow in your growing space to prevent overheating. Look for LEDs with integrated heat sinks for better thermal management.

9. Additional Features to Consider

Modern LED grow lights come with many additional features that make indoor gardening easier.

• Dimmable Lights: Dimmable LEDs allow you to adjust light intensity without altering the height, which is particularly useful for achieving the desired PPFD levels, especially with high output lights like Barrina's BU200 or Sansi's 200W/400W panel lights.
• Timers and Automation: Programmable timers help maintain a consistent light cycle, essential for plant health.
• Modularity and Expandability: Choose an LED system that allows you to add more lights as your garden grows, giving you flexibility to expand.

10. Budget Considerations

When selecting grow lights, consider both the initial investment and long-term costs.

• Initial Investment vs. Long-Term Costs: Quality LED grow lights may have a higher upfront cost, but they tend to be more durable, energy-efficient, and provide better results over the long run. Often, a small cost difference can mean up to 10x the light output which means spending less money for more light overall.
• Warranty and Support: Opt for products that come with a solid warranty and good customer service to ensure you’re covered in case of any issues. While shopping via Amazon is often the preferred method for many, the major consumer light manufactures like Barrina, Sansi, ViparSpectra, SpiderFarmer, etc. Often have more selection, better prices, and available discounts not available on Amazon. Plant Hoarders Anonymous members can often use PHA discount codes to reduce prices a bit more. It's usually only about 5%, but I'm currently talking with Sansi to increase their discounts for PHA Members.

11. Matching Light Types to Available Space & Purpose

Now that we hopefully all understand the technical side of grow lights, let's consider the types and styles of what are available to choose from, and when they should be considered.

• Bar/Tube Style Grow Lights (Good for Narrow Shelves/Cabinets)
  • Barrina dominates this style of grow light. These types of grow lights are available in 12", 24" 36" and 48" lengths with T5 delivering lower light intensity, and the T8 deliver higher output. They are not typically waterproof, or even water resistant, but are often used in enclosed cabinets, and for shelving.
• Grow Bulbs (Good for plants a max of 18" diameter)
  • A type of grow light that is best used on a per-plant basis. This is the mental equal to a typical incandescent bulb. While a variety of companies make grow bulbs, the undisputed leader across the industry is Sansi who provide the most advanced grow bulbs available starting at 10w and their newest as of November 2024, a 40w dimmable remote controlled bulb, complete with a timer.
• Panel Grow Lights (Good for shelving, and larger grow areas)
  • When people think panel lights for growing plants, they often imagine a commercial cannabis operation, but grow light panels are not much different that the previously mentioned tube/bar style lights. A panel light simply makes use of a larger array of LEDs in a single fixture. Most of the aforementioned grow light manufacturers make what I refer to as a "prosumer" level lighting. These lights are all made for hanging over a larger grow space, often for use in a grow-tent that uses reflective mylar to increase ambient light levels. Sansi makes a small 70w panel light which is effective for a 30"x30" growing area at a 18" height, and Barrina offers a few between 200w and 400w with the 200w unit delivering a max of 500 μmol/m2/s at a 20" height over a 4'x4' growing area. Other vendors, like ViparSpectra make lights specifically targeting shelving with dimensions of 2'x4' and hit a max PPFD of about 1,300 μmol/m2/s at a 12" hanging height.
• Clip-Ons, Halos, Puck Lights (Good for singular plants with lower light needs)
  • There are various lights littering Amazon that fit into this category. Some actually fit into the above categories in terms of style, but what defines these low level lights is that they all generally only have a max wattage of 15-20w and as such their output is really low. These are the lights that often come as part of a plant stand. They often include useless features to make them seem more like higher end lighting solutions, but if they do it's more hat than cattle. These are lights that I'd normally suggest people avoid. The least lighting bang for your buck overall and why most no-name lighting companies sell them for dirt cheap in Amazon and Temu, etc. They are generally crap. There are a couple of exceptions however and those are from Sansi. Sansi use their chip on ceramic technology and advanced lens design of their grow bulbs to concentrate lower volumes of light into a more concentrated area. While they still aren't great for a lot of situations, I'm using half a dozen of their puck and clip-on lights for a few of my singular plants that are spread around my home. These are generally lower light tolerant plants already (African Violet, Snake, Pothos) that I have as accents in various locations. I tend to try and rotate these plants out into better lighting ever couple of weeks so they get better light overall, but as a stop-gap measure they work well. They also make excellent reading lights if you do a lot of reading like I do.

Grow Light Conclusion

Selecting the best LED grow light for your specific situation is an important decision that directly impacts the growth and vitality of your indoor plants. By understanding lighting in general, your plants' light requirements, the growing area, and selecting a quality LED light that fits these needs, you’ll be setting your plants up for success. Remember, start with a setup that meets your current needs but allows for future expansion. With this knowledge, you can confidently make better informed lighting decisions and foster a thriving indoor space that brings joy and satisfaction, while at the same time, spending less and getting more light for every dollar you invest. Lighting is an investment that will save time and money across almost every aspect of plant care.

Happy growing!