Grow Light Basics, Explained: LEDs vs. Fluorescents, Watts, Lumens & More

All Your Burning Questions About Light Bulbs, Answered

Grow Light with yellow background

Lumens? Micromoles? And who is Kelvin? Start researching grow lights for your indoor garden needs, and you’ll find there’s no limit to the new vocabulary and specifications that get thrown around. What’s a gardener to make of all this terminology? Let’s take a quick run through some bulb basics.

Battle of the Bulbs: Fluorescent vs. LED

Fluorescent bulbs were once the “go-to” grow light, but over the past decade, as LED bulbs have gotten cheaper and more widely available, it's become a bit harder to decide which one is best for YOU! But don't worry — your friends at Gardener's Supply are here to break down all the pros and cons of fluorescents and LEDs, so it's easier for you to have that "lightbulb moment" and find the perfect grow light for your situation.

Fluorescent Bulb Pros & Cons

Fluorescent bulbs are an excellent choice for brand new gardeners or gardeners looking for just a few weeks of grow light use per year (seed starting, for example). They’re super affordable, widely available, and have moderate lifespans (typically around 10,000 hours). Fluorescents come in an assortment of shapes from traditional “bulb-shaped” compact fluorescents (CFLs) to tubes that can be installed in a standard shop light fixture.

Important to note: fluorescent bulbs contain mercury, a toxic element. Meaning, they cannot be thrown out with your typical household trash or recycled. Contact your town or county waste management program for special disposal instructions.

Fluorescent Bulbs
Pros Cons
  • Relatively low cost
  • Widely available
  • Moderate lifespan
  • Require special disposal due to mercury content

LED Bulb Pros & Cons

LEDs (light emitting diode) bulbs have exploded in popularity over the last several years. They typically cost a bit more upfront, but growers may find that their longevity and energy efficiency are well worth it. With a lifespan typically 3-5 times as long as traditional fluorescent bulbs, it may be wise to calculate how much it would cost to buy one LED vs. two fluorescents (at least). And, especially if you're growing seedlings or "long-day" plants (more on that later), LEDs can be much cheaper to operate, using about half the energy that fluorescents do. Specific “high output” LEDs deliver brighter light for each unit of energy they consume.

LED Bulbs
Pros Cons
  • Extremely energy efficient
  • Very long lifespan
  • High output bulbs available
  • Higher initial cost

Fluorescent vs. LED Pros & Cons

  Fluorescent Bulbs LED Bulbs
Cost ✔✔
Lifespan ✔✔✔✔
Energy Efficiency ✔✔

Lighting Specs & Terminology

While cost, energy-efficiency, and lifespan are super important factors to consider, if you want to choose the best grow light for your plants, we should talk about all those words and numbers on the lightbulb package. Yes, they are jargon-y. Yes, they are overwhelming. Yes, they sometimes look like an alien wrote them. But understanding lighting specifications is key to knowing what benefits each bulb will provide for your precious plants.

Lumens Are For Humans (Not Plants)

The intensity or brightness of light can be measured in lots of different ways — this is why you’ll see several different numbers reported on a light bulb package. You’ve probably heard of “lumens,” which is a measure of brightness to the human eye — but this doesn’t really take into consideration the specific wavelengths that plants need to grow.

Light is electromagnetic energy measured across a spectrum of wavelength sizes, from long-wave radio all the way up to gamma rays. Plants rely mostly on a specific portion of this spectrum — a section called Photosynthetically Active Radiation (PAR). This is the light we (as gardeners and plant-y people) should be most interested in.

Electromagnetic spectrum

PAR can be measured in micromoles of photons, or more specifically, micromoles per square meter per second. Photosynthetic photon flux density (you may see “PPFD” written on the bulb packaging) is a measure of how much plant-usable light reaches a surface like a plant leaf.

SO — Whenever dealing with lighting conditions of plants, understand that while lumens are great for home lighting, they might not accurately represent the real light intensity suitable for photosynthesis. If you’re looking for optimum lighting for your plants (represented by PAR), then compare PPFD values between bulbs; the larger the number of micromoles, the higher the bulb intensity.

Comparing LED Bulb Intensity

  LED High output LED
Lumens 2640 Im 3960 Im
PPFD 110.8 μmol/(m2/s) 114 μmol/(m2/s)

Color Temperature

Color temperature is a way to describe a light’s appearance. It’s measured in Kelvin (named after a nineteenth century physicist and simply abbreviated as “K”) on a scale of 1,000 K (warmer red hues) to 10,000 K (cooler blues).

As gardeners, why do we care about color temperature? Certain light colors have been shown to trigger different plant growth characteristics:

  • Blue light encourages vegetative growth. Think strong stems, lush leaves, and dense roots.
  • Red light promotes flowering and fruit.

Selecting red or blue bulbs give gardeners more control in selecting for different growth characteristics. Commercial growers looking to harvest a specialty crop on schedule may swap to red light bulbs to prompt flowering. However, for all-purpose growing of seedlings, houseplants, and herbs, “full spectrum” bulbs are the optimal choice. Full spectrum bulbs will generally be between 5000 and 6500 K, and will mimic bright, natural sunlight in appearance.

Color Temperature scale   

What Are Watts?

Wattage is a measurement of power consumption, but not necessarily light output — so, it has less to do with the light needs of a plant or the brightness of a bulb, and more to do with your power source and how much you’d like to spend powering your grow light. A high-efficiency bulb will put out more light per watt than a standard bulb.

There are an increasing number of financial incentives available for those who choose energy efficient appliances, windows, electronics — and now grow lights! Efficiency Vermont, for example, offers Vermont residents up to $100 in rebates for residents who swap high wattage grow light fixtures for energy efficient LEDs. Check out this map assembled by North Carolina State University’s Clean Energy Technology Center to see what policies and incentives are offered in your state.

Get the Right Timing

Have you heard of “short-day” and “long-day” plants? This refers to the number of hours of light a plant needs per 24-hour day, or photoperiod. The amount of darkness can be nearly as important to plants as the amount of light. In nature, seasonal changes in the photoperiod are a trigger for some plants that it's time to start flowering or fruiting.

Short-day plants, like poinsettia, Christmas cactus, and chrysanthemum, require long stretches of uninterrupted darkness as part of their growth cycle. They will only flower if they receive less than 12 hours of light per day. Inversely, long-day plants need a minimum of 12 hours of bright light per day. Many of our garden veggies, like lettuce and spinach, and summer-blooming flowers, like sunflowers and coneflowers, are long-day plants. Ensure your indoor plants are receiving the correct amount of light:

  • Seedlings: 14-16 hours per day
  • Herbs and leafy greens: 12 hours per day
  • Houseplants: 6-12 hours per day (depending on the type of plant)

Almost no plant appreciates 24 hours of constant light. Set your grow lights on a programmable timer to guarantee your plants are getting the optimal balance of light and dark.

Set Your Seeds Up For Success: How to Garden With Grow Lights

Putting It All Together

Gardeners have many options for setting up their light bulbs. Retrofitting an existing shop light fixture with an individual grow bulb is a great low-cost option for those interested in a DIY approach. If you're using a fluorescent bulb, be sure to match up the size of the fixture with the correct bulb size; for grow lights, these are generally T5 or T8.

Looking for an all-in-one growing solution? Grow light stands offer gardeners a whole host of benefits including built-in trays to catch water spills, adjustable bulbs that can be easily raised and lowered as your seedlings grow, and modular design capacity, so you can expand your indoor garden over time. Plus, most of these models are just plain stylish! No need to hide your seed starting operation away in the basement; these are attractive enough to be front and center in your home.

Feeling… illuminated? We hope so! Check out our full line of grow light systems and get your indoor garden going.

Related Article: How to Choose a Grow Light


Last updated: 11/15/2023