How to Grow Herbs with Grow Lights

By Patrick Dinnen

It’s impossible to beat a sunny, warm outdoor spot for growing healthy, tasty herbs. However, unless you live in the tropics, outdoor herbs aren’t a year-round proposition.

Successfully grow herbs indoors is a useful skill for all of us who live with a gardening off-season. Plus, there’s something extra special about the aroma of fresh-picked thyme or basil in the dead of winter.

Indoors, Light Is Make-or-Break

Many writers recommend growing your herbs on a sunny, south-facing windowsill. I can only assume those people live in Florida or some other southern spot. I’m in Toronto and even our strong summertime sun won’t support a windowsill herb garden. I’ve always ended up with leggy, sad-looking herbs when I tried.

That’s where grow lights come in. High quality grow lights can give sun-like light. And they’ll do it without breaking the bank on the purchase or your power bill.

Grow Lights 101 in 2021

A few years ago there was a maze of potential lighting technologies to work through: fluorescent, CFL, HID, Metal Halide, Plasma and on and on. All with their own pros and cons and usually a hefty energy usage.

Fortunately, things are a lot simpler than they used to be. As a home grower, there are really only two light types in the running. Fluorescent tubes are still great for initial cost and reasonable running costs while covering a large area. Look for HO T5 (High Output T5) tubes—they’re the standard for modern fluorescent grow lights. 

LED grow lights are rapidly taking over the world. They are still a little more expensive to buy than fluorescents—though they’re dropping in price all the time. LEDs can’t be beaten for delivering specific wavelengths of light that plants respond to. And the efficiency of LEDs is state of the art—so the initial purchase price can pay back in lower energy use.

Personally, I use fluorescent lights for seedlings and microgreens—where bathing a large area with a moderate light is important. For everything else I use LEDs in various forms.

How Much Light is Right?

Trying to answer this question, you’ll face a barrage of jargon. Lumens, lux, foot-candles. Seriously, we’re using a candle-based metric for a 21st-century technology? These measurements are common for lighting but they’re not ideal. These measurements are for how humans perceive light and not how light affects plants.

The proper measurements for plant light are:

• DLI (Daily Light Integral)—think of this as the daily appetite a plant has for light.
• PPFD (Photosynthetic Photon Flux Density)—this is the amount of plant-useful light falling on a given area.

Measuring these accurately takes a quantum meter. It sounds like something from science fiction, but it’s the go-to tool for horticulturalists. Unfortunately, a reliable quantum meter will cost $500 and up—understandably most hobby gardeners don’t have one.

The good news is that you don’t need to measure the light yourself. You just need a reliable source of info on what grow lights give out and what different plants need. I’ll toot my own horn and suggest you check out brightgrow.co—where I’m building such a source of info. 

Temperature Matters

Gardening outdoors, we take it for granted that sunlight and warmth go hand in hand. Working with grow lights, you need to try to break that assumption. Modern grow lights give out very little heat—so it’s possible to deliver intense light, but still have a cool grow space.

Some herbs like it warm. Basil for example likes temperatures in the range of 75–85°F (23–29°C). Other herbs are much happier if it’s cool. Chives thrive at 50–75°F (10–23°C).  

Of course, you could change the temperature of your home to please your herbs. Generally, it’s simpler to pick your herbs to match your temperatures. 

I learned this lesson when I tried to grow some dill and basil in my cool basement. The dill loved the cool temps—it shot up and crowded out everything else. The basil, on the other hand, never really got going. It wanted much more warmth than my basement could give.

If you have a warmer space, try:

• Thyme
• Basil
• Oregano

If you’re growing in a room that’s on the cool side look at:

• Mint
• Cilantro
• Chives
• Parsley
• Dill

Selecting Your Herbs and Their Ideal Grow Light

Of course, different herbs have different light requirements. Illumination that will have mint growing like a weed will barely support thyme. 

For grow lights, distance and intensity are tightly linked. As light travels further from the lamp, it spreads out and becomes less intense. So, picking the right light and the right distance is important. 

Generally speaking, grow lights need to be much closer than you might think. Fluorescent and lower power LEDs might need to be within a few inches (or centimeters) of a plant’s leaves to deliver sufficient photons.

Figuring out the right setup can take a lot of trial and error. Or some helpful friends…

I’m building a resource in the edible plant directory—it’s full of grow light “recipes” for all kinds of herbs and vegetables.

If you like having a book to reach for, I highly recommend Gardening Under Lights by Leslie F. Halleck. It’s scientifically grounded but written for everyone. (You can get an ebook, but you would miss out on all the diagrams.)

A Simple Setup

You can get LED grow light bulbs that fit a standard light fixture. Screw-in E26 if you’re in Canada or the US or bayonet fitting in the UK. 

I am a big fan of these bulbs. They’re affordable, efficient and flexible—all good qualities if you’re getting started with grow lights.

For US and Canadian readers, I highly recommend the GE PAR38 Grow Light Bulb. I’ve tested it extensively and it beats all the competitors I’ve come across.

An honourable second place goes to the Sansi 36W grow bulb. It doesn’t deliver quite as much plant-useful light, but it is available outside North America.

As I mentioned, light-to-leaf distance is important. So, an adjustable setup is a must-have. Simply add a clamp fixture or hanging pendant cord and you have a highly adjustable and affordable setup. 

It’s a good idea to add a timer to your grow light too—so you can set the light and forget it. 

As a rule-of-thumb, 12 hours per day of light is a good place to start—though some plants prefer days that are shorter or longer. Basil will be happy under lights on for up to 18 hours a day while cilantro starts to flag as day length passes 10–11 hours. Experience and research can help find what’s best for your herbs.

Get Started

You can dive in with a small grow light setup for under $50 and see how you like it. It can take a little effort to get the hang of indoor gardening—growing accustomed to the subtle differences from conventional gardening. Once you do, though, it can be a satisfying addition to your love of gardening. It’s nice not to have to worry about pests or weather for a change. 

Enjoy!

Patrick Dinnen is an amateur gardener who loves exploring what grow lights can do for edible plants.