Growing seedlings is a great way to jump-start your outdoor garden. One of the challenges to indoor growing is making sure your seedlings get the right amount of light they need to thrive.
Seedlings need approximately 32 watts per square foot of growing area for LED grow lights and 100 watts per plant for CFLs. They also require the right color temperature to grow and thrive, so choose a temperature range of 3,500 to 6,500 kelvins.
Read on for tips on how to find the right grow lights to ensure that your seedlings and tomatoes get the light they need to grow strong and how to keep your succulents healthy when sunlight is in short supply.
Grow Light Wattage for Seedlings
Seedlings are, essentially, a recently sprouted plant.
A seedling started indoors needs sufficient light to grow to a size and strength to be transplanted to an outdoor garden or pot.
Indoors however, you typically won’t get enough sunlight to grow healthy seedlings, so you’ll need to supplement indoor light with grow lights to develop into healthy plants.
Two of the most common grow light technologies used by home gardeners are light-emitting diodes (LEDs) and compact fluorescent lights (CFLs).
Both LEDs and CFLs are energy efficient, which means they use less wattage than traditional incandescent bulbs to produce the same amount of light.
When comparing grow lights for seedlings, keep in mind that a watt measures the amount of energy a light uses, but does not indicate how much light is emitted.
So, regardless of the type of bulb you use, many growers recommend using the lumen output when determining the right grow light for your seedlings.
The higher the number of lumens, the brighter the grow light will be.
Without the right amount of light, the plant will fail to thrive.
A seedling that fails to thrive because of inadequate light will look spindly and stretched because, instead of growing thicker and leafier, it will keep growing upward to try to reach more light.
In addition to lumens, you want to ensure that your grow light delivers the right color temperature to promote plant growth.
Light is measured in wavelengths, and the wavelength determines the color of light.
Grow lights have color temperatures, which indicate the color of light it will emit, measured in Kelvins.
Warm (red/yellow) light is 1,000-3,000K, and colors above 5,000K are cool or blue.
Plants cannot use all colors of light equally.
But they also need some wavelengths more when they are seedlings and others to fruit and flower.
While many seedlings rely most on blue light, many grow lights provide full-spectrum light to ensure plants have the right light throughout their growth cycle.
For example, a controlled study on growing Brassica rapa (which has many varieties, including turnips, napa cabbage, and bok choy) found that full-spectrum light produced taller and leafier plants, compared to blue light or red light.
Type of Grow Light
LED and fluorescent lights are commonly used for growing seedlings indoors, and both are available as full-spectrum grow lights.
Choosing a CFL or an LED light is mostly a matter of preference.
CFLs are inexpensive and energy-efficient, though not as efficient or long-lasting as LEDs.
They fit into regular fluorescent light fixtures and can tend to heat up, which means you need to watch how close they are to your seedlings to avoid burning seedlings.
LEDs grow lights are more expensive than CFLs but last longer and use about half the electricity.
LEDs generate less heat than CFLs, which can help smaller spaces.
Plus, LEDs are an environmentally-friendly choice because they don’t contain the mercury of CFLs, which can leak out if CFL bulbs break.
How Much Light Should Seedlings Get?
Seedlings started indoors need bright light to generate healthy roots, stems, and leaf growth to make a healthy transition to an outdoor garden.
In general, seedlings need 16 to 18 hours of light per day.
Typically, grow lights or lamps are positioned 4 to 6 inches from the tops of seedlings and raised as the seedlings grow to maintain a consistent distance.
You can check out our more detailed articles on:
- How far LED grow lights should be from plants
- HPS distance from seedlings (if you’re using high-pressure sodium grow lights)
How many watts do you need for seedlings?
Remember that watts is NOT a measure of light output. It’s better to use lumens to measure light output, regardless of whether you’re using LEDs, flourescents, HID, or HPS grow lights.
Also, wattage can vary widely, between different light types (LED vs. HID) and manufacturers. For example, an LED grow light might use 20 watts, while an HID grow light might use 2,000 watts–that’s 100 times more energy used!
A NASA-funded study found that the grow light watts required could be reduced by over 90% by using 1-watt high-intensity LEDs, compared to conventional high-pressure sodium (HPS) grow lights.
How Many LED Watts for Seedlings?
An LED’s wattage doesn’t determine how much light your seedlings are receiving, which means it may be hard to know if your seedlings are getting enough light based solely on wattage.
However, a rule of thumb is to use lights that are 10 – 30 watts, where you can use fewer watts for low-light plants, and more watts for plants that require high light.
If they are spindly and sparse, widely spaced leaves, they aren’t getting enough light, and you’ll need to do one or more of the following:
- add light,
- shorten the distance between the light and the seedlings, or
- increase the hours per day they spend under the grow light.
How Many CFL Watts for Seedlings?
Gardeners typically use two types of CFLs, soft white and daylight, which cover the entire plant growth cycle. Daylight bulbs with a color temperature of 5,000 to 6,000K are recommended for seedlings.
To select a CFL based on watts instead of lumens, use the true CFL wattage and not the incandescent “replacement wattage” as the watt measurement. For example, if the CFL package indicates the bulb is “23 watts” and a “100-watt replacement,” it is a 23-watt bulb.
A general recommendation is to start with two 32- or 40-watt fluorescent bulbs. As with LED wattage, you’ll need to see how your seedlings fare under the starting wattage to gauge any necessary changes.
If the seedling fails to grow well or particular areas aren’t growing well, you may need to:
- add more light,
- shorten the distance between the plants and the light, or
- Increase the number of hours of light per day.
How Many Watts for Succulent Grow Lights?
Grow lights are a good supplement for succulents, particularly in parts of the country or during seasons when they cannot receive sufficient sunlight.
But remember: watts don’t measure light output. Watts are a measure of energy used. Lumens are a better measure of light output.
Plus, watts can be incredibly variable, depending on the type of light (LED vs. HPS).
Generally, though, since most succulents are high-light plants, 30-40 watt LED or CFL grow lights can be used.
For either LED or CFL grow lights, you’ll want to use full-spectrum lights.
Keep in mind that fluorescent (e.g., CFL) lights produce heat, while LEDs don’t (or at least not much). So, if you use CFL grow lights, keep them 12-14 inches away from plants to avoid burning the plants.
How Many Watts To Grow Tomatoes?
Tomato plants are often started indoors, where they need quite a bit of light to grow well, since tomatoes are high-light plants.
Especially in low-light areas, or during winter, you’ll need more light.
Generally, for CFL grow lights, you’ll need 100-watt-equivalent bulbs. For LED grow lights, usually 30-50 watts provides enough light output.
Remember that CFL bulbs produce heat, which can damage seedlings and plants. LED grow lights produce far less–almost no–heat.
So, with fluorescent lights, you’ll want to keep them 12-14 inches away from plants.
Full-spectrum LED grow lights that emit approximately 2,000 lumens per square foot of grow area are the best to grow tomato seedlings and be placed about 3 inches above the plants.
To summarize, you can use lumens per square foot of growing area to calculate LED and CFL grow lighting.
To estimate light needed based on wattage, you can approximate 32 watts per square foot of growing area for LED grow lights and 100 watts per plant for CFLs.
Be sure also to choose the right color temperature for your seedlings to optimize their growth.