Do You Need a Grow Light? How to Choose One That Really Feeds Your Plants

Many home growers don't need a grow light at all. The ones who do often buy by the wrong number. Here is how to tell whether your sun is enough, which numbers on the box describe light a plant can actually use, and how to match a lamp to what you grow.

On many grow light boxes, the biggest number is the wattage. It looks like a measure of how much light you are buying, but it isn't. It tells you how much electricity the lamp uses, and two lamps with the same wattage can give your plants very different amounts of light. That one misunderstanding is behind a lot of disappointment, like lettuce that stretches and turns pale under a lamp that looked powerful on the box.

The good news is that you don't need to become a lighting engineer. You need three numbers, one simple calculation and a clear idea of what you want to grow. But before any of that comes the question most guides skip: do you need a lamp at all?

Start with the light that costs nothing

How much sun is enough depends on what you grow:

• Leafy greens are the most forgiving. Lettuce, spinach, kale, chard and arugula can grow with as little as three hours of direct sun a day, and they do better with more.
• Fruiting crops are much hungrier. Tomatoes, peppers, cucumbers and melons struggle with less than six hours of direct sun and do best with eight to ten.

So if you have a sunny balcony, patio or yard, you can grow almost anything without a lamp, and even a spot in partial shade still suits greens.

Winter is where this changes. Days get short and the sun sits low, so even a good spot can fall short. In an Indiana greenhouse, for example, the daily light from the sun drops to around 5 mol/m²/day in winter. As you'll see below, that is well under what most vegetables need, and that is when a lamp earns its place.

Indoors, it is harder to judge, because your eyes are the wrong tool for the job. They adjust to the light around them, so a room can look bright to you while it gets far less light than outside. Light also fades quickly as you move away from the glass.

Your plants notice long before you do: their stems stretch toward the window, and new leaves come in small and pale. If that is happening on your windowsill, the window isn't enough on its own.

Know what you need before you shop

The right lamp depends on your situation, so answer four questions first. Once you have the answers, they choose the lamp for you, not the other way around.

1. What will you grow? Fruiting crops need a much bigger daily dose of light than greens and herbs.
2. How big is the area? How far a lamp's light spreads matters as much as how strong it is.
3. How much sun does the spot already get? A lamp can top up daylight or replace it completely.
4. How warm is it, and does air move there? Lamps add heat, and heat changes what plants need.

Why watts and lumens mislead you

Watts measure the electricity going into a lamp. Lumens measure how bright it looks to people. Neither one tells you how much of that light your plants can use.

Plants count something else: the light they use for photosynthesis, in the 400 to 700 nanometer range. Growers call this band PAR, short for photosynthetically active radiation. Our eyes see some colors as brighter than others, so a lamp can score high in lumens and still give plants less usable light than a lamp that looks dimmer to you.

The three numbers your plants care about

PPFD: how much usable light reaches the leaves

PPFD stands for photosynthetic photon flux density. In plain words, it is how much usable light lands on a spot every second, measured in micromoles per square meter per second (µmol/m²/s).

One detail makes all the difference. PPFD is always measured at a set distance from the lamp, and the closer the lamp, the higher the number, but the smaller the area it covers. That is why a good maker doesn't print a single PPFD figure. Instead, it shows a chart, often called a PPFD map, with the light at several heights and the area each height covers.

DLI: the daily dose

PPFD tells you how strong the light is right now, but your plants care about the total they receive over the whole day. That total is the daily light integral, or DLI, measured in moles per square meter per day (mol/m²/day). If you watched our lesson Enough Light?, this is the "daily dose": strength multiplied by time.

As a guide for home growers:

• Leafy greens: roughly 12 to 17 mol/m²/day.
• Fruiting crops such as tomatoes, cucumbers and peppers: at least 15, and 20 or more is better.

Treat these as guide ranges rather than exact targets. With less light, plants simply grow more slowly, and young seedlings need far less than plants in full growth.

Turning one number into the other takes a single calculation: multiply the PPFD at leaf height by the hours the lamp is on, then by 0.0036, which just converts micromoles per second into moles per day.

A lamp that gives 250 µmol/m²/s at leaf height for 16 hours a day delivers 250 × 16 × 0.0036, or about 14 mol/m²/day. That is right in the range for lettuce.

A tomato is a different story. To reach 20 mol/m²/day in the same 16 hours, its leaves would need about 350 µmol/m²/s. It's the same calculation, with a much stronger lamp.

Efficiency: more plant light for every dollar

The third number is efficiency, in micromoles per joule (µmol/J). It tells you how much usable light the lamp makes from each unit of electricity, so a higher number means more light for the same power bill. Current LED grow lights reach values over 2.5 µmol/J, while older high-pressure sodium lamps range from about 0.9 to 1.7.

How high to hang it, and for how long

There is no single right height, because three things change the answer. Lamps differ, so a strong panel and a weak one at the same height give very different doses. Plants differ too: a seedling needs far less light than a plant in full growth. Lettuce, for example, needs around 50 µmol/m²/s when it germinates and 200 to 300 once it is growing. And rooms differ, since a sunny room needs fewer lamp hours than a dark one.

So instead of following a fixed rule, use your lamp's PPFD map. Find the height that gives your crop the PPFD it needs across the area you want to cover. Then put the lamp on a timer for 12 to 16 hours a day, and always leave your plants a dark night.

Reading what your plants tell you

Plants show you when the dose is off. With too little light, stems stretch, leaves turn pale and growth slows down. With too much, leaves turn pale and then brown.

Still, a symptom on a leaf can have more than one cause, so it doesn't prove the light is wrong.

Warm water holds less oxygen, and that can make plants wilt too. Brown, crisp edges on lettuce can be tipburn, which is linked to calcium and heat, not only to light. And a nutrient or pH problem can also turn leaves pale. So before you move the lamp, check water temperature, EC and pH. If they are all where they should be, then adjust the light.

Strong sun and hot balconies

Outside in strong summer sun, the problem is rarely too little light. It's heat. Lettuce prefers days below about 75°F (24°C), and long spells above that can make it bolt and turn bitter. On hot afternoons, plants may also droop for a while.

A shade cloth of around 30% cools the plants and still lets plenty of light through, and tomatoes and peppers can benefit from it too. Keep the reservoir out of direct sun as well. Cover or shade it so the water stays near 64 to 71°F (18 to 22°C). If the heat lasts for weeks and your greens keep bolting, move them to a shadier spot, or grow them indoors under lights for that season.

Which type of grow lamp fits your space?

Once you know your numbers, the type of lamp follows from your space. For a full hydroponic system, and for fruiting crops indoors, the usual choice is an LED panel or bar. It lights a whole area evenly and hangs at an adjustable height. Just check its PPFD map against the area you want to cover.

Where there is little headroom, LED strips and shelf lights make more sense. They are thin and run cool, so they can sit close to seedlings, greens and herbs, but they are not strong enough for large fruiting plants.

A screw-in LED grow bulb fits a lamp you already own and is fine for a single pot of herbs. It is hard to aim, though, and not enough for a whole system. Fluorescent tubes still work for seedlings, but they lose strength over time.

Older sodium and incandescent lamps turn much of their power into heat. That, along with the efficiency gap, is why most home growers have moved to LED.

What to look for when you buy

Garden centers, hydroponic shops and online stores all carry grow lights, and where you buy matters less than the spec sheet. Before you pay, check that the lamp:

• shows a PPFD map with heights and coverage
• lists its efficiency in µmol/J
• fits your area and your crop
• hangs at an adjustable height and runs on a timer

Light as a daily dose

Once you think of light as a daily dose, the whole subject gets simpler. Use the sun when you have it, add a lamp when you don't, and let your plants show you when the dose is right.

Here's to strong stems, deep green leaves and a harvest that doesn't wait for summer.

Questions and answers

Do I need a grow light for hydroponics?

Not always. Leafy greens can grow with as little as three hours of direct sun a day. Fruiting crops struggle with less than six hours and do best with eight to ten. A lamp helps when your spot gets less than that, especially in winter or indoors.

Is a bright window enough for my plants?

Often not. A room can look bright to you and still be dim for a plant, and light fades quickly with distance from the glass. If plants lean toward the window and new leaves come in small and pale, the window isn't enough.

Why don't watts and lumens tell me how good a grow light is?

Watts measure the electricity a lamp uses, and lumens measure how bright it looks to people. Neither measures the usable light, called PAR, that reaches your plants. Look for a PPFD map and efficiency in µmol/J instead.

What is the difference between PPFD and DLI?

PPFD is how much usable light lands on the leaves each second, at a given height. DLI is the total for the day: PPFD × hours × 0.0036. A lamp giving 250 µmol/m²/s for 16 hours delivers about 14 mol/m²/day.

How much light do lettuce and tomatoes need?

As a guide, leafy greens such as lettuce do well with roughly 12 to 17 mol/m²/day. Fruiting crops such as tomatoes need at least 15, and 20 or more is better.

How far should a grow light be from my plants?

There is no universal distance. Lamps differ in strength, and seedlings need far less light than mature plants. Use the lamp's PPFD map to find the height that gives your crop the light it needs.

How many hours a day should a grow light be on?

Most growers run lights 12 to 16 hours a day on a timer, and always leave plants a dark night.

My leaves are pale or brown. Is it the light?

It might be, but check water temperature, EC and pH first. Warm water, nutrient or pH problems and tipburn can all look like a light problem.

Which grow light is best for home hydroponics?

For a full system or fruiting crops indoors, LED panels or bars at an adjustable height. For seedlings, greens and herbs with little headroom, LED strips or shelf lights. A screw-in LED bulb is fine for a single pot.

Keep learning

• Watch first: Lesson 3, Enough Light? How to Tell, the basics this guide builds on.
• More guides in the NatureTech Learn library.

Sources

• University of California Cooperative Extension, Placer County. T. Grenfell, Growing Vegetables in Less Than 6 Hours of Sun.
• University of Missouri Extension. Cabrera-Garcia and Ernst, Understanding Grow Lights (G6987), 2025.
• University of Maryland Extension, Lighting Indoor Plants, updated 2023.
• Purdue Extension. Nemali and Langenhoven, HO-283-W, 2018.
• University of Kentucky. Sharma, Leonberger, Gauthier and Ying, Hydroponic Lettuce Production in Controlled Environments (CCD-CP-63), 2025.
• Michigan State University. E. Runkle, Lighting Greenhouse Vegetables (GPN, 2011) and DLI "Requirements" (GPN, 2019).
• University of New Hampshire Extension, Supplemental Lighting Run Time Worksheet, 2020.
• UConn Home and Garden Education Center. E. Leahy, Shine a Light, But Which One?, 2025.
• Illinois Extension. C. Enroth, A Primer to Supplemental Lighting for Indoor Seed Starting, 2018.
• University of Delaware Cooperative Extension. G. Johnson, Weekly Crop Update, 2013.

Written bySuzyCo-founder, NatureTech

Suzy brings hands-on experience from commercial hydroponics and years of working with home growers, turning practical lessons into clear guidance for growing food at home.

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