How To Grow Cilantro (Coriandrum sativum) Indoors Under T5 Fluorescent Growlights

Despite being fairly hardy, many gardeners find cilantro to be a tricky beast. It bolts when it gets too warm and it loses its flavor and aroma when its over fed! Perhaps the solution is to grow it indoors under grow lights in a controlled environment?

I really don't care if you love or loathe the taste of cilantro. (Hey, if you want to grow cilantro then I'm guessing you don't think it tastes like soap or rotting vegetables—but, what do I know, perhaps you're planning a particularly cunning prank on a parsley-loving friend of yours?

No salsa is complete with cilantro!

Anyway, I live in a part of the world where it's quite tricky to get hold of—whether I ask for cilantro, coriander or even its lesser-known nomenclature—indian parsley. A few oriental supermarkets sporadically offer some wilting, semi-fetid bunches of it in their refridgerators and, as they know it's scarce, they price it ridiculously.

But enough of my supermarket grumbles—I'm a grower for goodness sake! And, as such, I want to cultivate my own year-round supply of this delicious herb so I can infuse my thai, mexican and indian dishes with its beautifully pungent aroma. So how to do it?

Okay so here are my quick tips for growing cilantro indoors under grow lights. The key is creating constant cool, moist, bright conditions so that the plants produce leaves rather than flowers and seeds. Stress (which cilantro is highly susceptible to) will cause cilantro to bolt and flower prematurely.

1. Sow in flats, not cell trays
2. Moisten roots every day - don't let them dry out!
3. Don't over-fertilize (treat them mean to get the flavor and aroma!)
4. Keep temperatures cool (under 72F)
5. Use a timer to set your grow light to come on for 18 (or 15 at the very least!) If you don't have a timer then just have the light on 24/7. 
6. Keep light six inches above propagator lid at first. Once seedlings develop open the vents a little. When they are an inch or more tall you should be able to safely remove the propagator lid. Plants can grow until they are almost touching a T5 light but will thank you for maintaining an inch's distance!


Sow seeds in flats, not cell trays. Cilantro hates to be transplanted and will quickly bolt and go to seed so it's going to stay in this flat until harvest. If you're growing cilantro for its leaves, this is not what you want to happen. Punch several holes in the bottom of the flat for drainage and sit it on a plastic tray. Fill with a quality potting mix. I added a little coco coir (around 30 percent) to my mix to make a nice, light, well draining mix—just what cilantro enjoys. I also added some Sub Culture M by General Hydroponics and wetted the mix thoroughly in the flat with some room temperature water infused with RapidStart (also by General Hydroponics).

Sowing cilantro seeds in flats is highly recommended as long as you irrigate regularly

You can see the seed density clearly in the photo above. This worked really well. Just cover seeds lightly with a bit of extra potting mix. As a general rule of thumb, you should never sow any seed more than its own depth otherwise it has to spend too much energy making its way to the surface!

You can watch a video of me sowing my cilantro seeds here:

Sowing Cilantro Indoors

I show you my propagation grow tent and get a flat of cilantro started in just a few minutes.

Why Grow Cilantro Indoors Under Grow Lights?

Cilantro is a fairly hardy beast. In fact, if temperatures get too warm, it bolts and goes to seed. I like to grow cilantro indoors under grow lights because I can better control the environment and enjoy a year-round supply. Ideally, keep temperatures below 73F. If you want to grow cilantro outdoors and you live in a warm climate, try planting it in the fall.

Awesome grow light! Sun Systems Sun Blaze 42 - 2 x 4 ft T5 High Output Fluorescent lamps

Indoors I used a Sun Systems Sun Blaze T5 grow lamp fitted with 6500K (daylight) lamps. These lamps emit a beautiful white light rich in the blue part of the spectrum which growing plants love. 

I mixed in some coco coir to my regular soil-based potting mix to give it a nice airy feel - good moisture retention but excellent drainage too. 

Always Use a Propagator!

Don't place your T5 fluorescent grow light too close to the top of the propagator lid otherwise you may heat it up too much. At this stage you will want the vents closed to seal in moisture to aid germination and early growth. Aim for temperatures in the low 70s to germinate your cilantro seeds. Be patient! Cilantro seeds can take up to a week or more to germinate.

 After watering in with some RapidStart infused water, I popped the propagator lid on and placed under the grow light.

The propagator lid helps to seal in relative humidity to create the ideal seed germination environment.

Really Important! Careful with the Fertilizer!

Before you know it (well, okay, two or three weeks) you'll have a cilantro afro on your hands! The trick in these first few weeks is to apply a little water often. Keep the potting mix moist at all times and never let it dry out. I connected my T5 fluorescent grow light to a timer so that it came on for 18 hours a day. I also used a grow tent called a Clonebox and a 6" Hurricane extraction fan in its roof to keep the air fresh and the growing environment relatively cool.

All this is taking place inside a HOMEbox Clonebox—note the fan in the roof that helps to ventilate the grow space.

As you can see a didn't bother ducting the exhausted air from the fan in the roof to the outdoors because it's fairly cool and airy in my cellar anyway. However, if heat becomes an issue you should duct the expelled air out of the room. Use a Min / Max thermometer with a remote probe (they only cost around $15) to monitor the environment inside your grow tent. The Clonebox has bug-screen protected air vents which help to protect your plants from bugs—really important when growing indoors in the absence of natural predators!

(By the way, if you're looking for an awesome grow tent like this and can't find the Clonebox, try the CloneLab instead—I understand that it's far more widely available in the United States than the Clonebox.)

Ten days later and we have some cilantro seedlings!

Once the plants got to this stage I felt confident enough to remove the propagator lid all together. However, I then went on to make a big mistake. I thought I'd give my cilantro a boost and add a little liquid fertilizer to my watering mix. By now they were needing watering every other day and some of the leaves looked a little pale to me, especially those up high—closest to the grow lights. 

After two and a half weeks I have a nice flush of cilantro leaves. 

The plants quickly responded by "greening up" to a nice looking, lush color as you can see. The disappointment came when I harvested some leaves around a week later and found that they were distinctly lacking in taste and aroma! I learnt the hard way! You must treat cilantro a little mean to keep it keen! Be super sparing with the fertilizer!

Be careful with the liquid feed. Sure, it makes for nice lush looking cilantro leaves but too much feed will detract from the taste and aroma of your cilantro. 

The foliage above may look lush and healthy but we're not growing this crop just for visual appeal! So—keep the potting mix light and, if you must apply a little liquid nutritional boost, do so only at the beginning. 

Oh dear! I forgot to water one day and came back to this wilted mess!

Once Established, Water Regularly Otherwise This ^^ Happens!

Oops! When I said cilantro likes to be treated a little mean, I didn't mean this! Once the bed was established, it required daily watering with up to a third of a gallon of water! As a general rule, I look to see about twenty percent of the water I apply come out through the drainage holes. Be sure to thoroughly and evenly soak the media. I use a rose watering can as it has a long spout and allows me to really get into the far corners of the flat.

Kind of looks like my metaphorical cat sat on it.... but alas, I have no feline friend to blame.

Don't worry, even though it looks as flat as a pancake here, it recovered after being watered in under thirty minutes. Allowing the growing media to become to dry like this increases the root zone temperature and will cause premature flowering and seeding. So it's really important to irrigate regularly. Given my forgetfulness (and busyness!) I think next time I will opt for an automatic watering system like drip irrigation or even flood and drain.

Around a month after sowing my seeds, I reverted to feeding with just pure water. The remaining nutrients were quickly uptaken by the now, well-established, cilantro and leaves quickly started to pale off.

A quick water and the cilantro recovered speedily. Note how it is paler now due to me stopping the liquid feed. Just plane water from now on to sharpen up that cilantro taste and aroma!

As you can see the cilantro can grow pretty close to the Sun Blaze T5 fluorescent light without any problems.

The Sun Blaze 42 High Output T5 Fluorescent grow light has two x 4 foot T5 6500K lamps (pre-installed with the fixture, nice!) - it runs directly on 120v power with an integrated e-ballast, so basically you can just plug it in and grow! I love them, especially for low level, leafy crops.

Steady on their little cilantro buddy! You are touching the lamp!

Although a T5 fluorescent grow light is incredibly user-friendly and kind to plants, leaves will still burn if they venture too close and touch the lamps.

These leaves ventured a little too close to the T5 lamps. No big deal.

Sure, the paler cilantro leaves do not look quite as pretty - but the taste is 100 times better now that the nutrients have been depleted from the growing media.

Cilantro grows different shapes leaves (they look a little like carrot or fennel) when it is about to bolt and flower

If you see leaves like this develop on your cilantro (they look a bit more frilly, like carrot or fennel) then you know that your cilantro is bolting and about to flower. These are cilantro's pre-flower leaves—a sure sign that temperatures have gotten a little too warm.

After 45 days the cilantro is ready to be harvested. Two weeks of plain water, combined with my light potting mix paled off the leaves and the pungent taste and aroma was back! 

Cilantro ready to be harvested. Begging, in fact. Salsa time!

I know it doesn't look as pretty with all the pale foliage (especially at the bottom) but the taste of 'mean-treated' cilantro is amazing!

For a continuous supply of cilantro, I recommend starting off a flat every three weeks, depending on your consumption levels!

Growing Cilantro: General Notes

What sort of container should I grow cilantro In?
Pots, raised beds, flats you name it. Just don't transplant it! That means you must start cilantro seeds in their lifelong homes! Cell trays therefore are a no-no.

How tall will cilantro grow?
This depends a little on variety and growing conditions but expect it to reach 1.5 to 2 ft.

How much light does cilantro like?
Full sun but if it gets too warm, go for partial shade instead. High output T5 fluorescent lamps are ideal for growing cilantro indoors.

How much water does cilantro need?
Once established, water cilantro a little every day. Keep it moist (don't let it dry out or it will bolt) but be careful not to leave sodden. If you are growing in flats make sure they have drainage holes.

What sort of pests may be attracted to cilantro?
Aphids, various mites, thrips and whitefly. Check under the leaves and inspect regularly.

How many seeds should I sow?
For a quick harvest, sow as I've shown above. If you want to grow cilantro outdoors during the cool season, aim to space your plants at least around 9 inches apart to give them a full opporunity to develop.

What pH does cilantro enjoy?
Cilantro is fairly tolerant of different types of soil but aim for around 6.5 and 7.5. If growing in soiless media (i.e. hydroponics) keep your nutrient solution mild and around pH 6.0.

How long do cilantro seeds take to germinate?
Usually about a week but I've known some stubborn little blighters can take up to three!

What sort of soil or potting mix should I use to grow cilantro?
Anything you like, just make sure it is well-draining. You can also grow cilantro in soiless potting mixes including perlite, coco coir, vemiculite or rockwool.

How long does cilantro take to grow from seed to harvest?
I start picking after four weeks (try not to pick before) and harvest over a few weeks.