How Much Light (PPF) Do You Need for Indoor Plants? 2021 Update

How plants are affected by too little or too much light

Plants like this geranium become leggy when not grown in enough light.
When plants lack light, they don't produce chlorophyll (the green pigment in plants), and plants can turn pale green to yellow to white.
Plant stems become “leggy,” meaning stems become long and thin and appear to be reaching toward the source of light.
A lack of sufficient light causes the plant to grow long spaces on stems between the leaf nodes (the point where a leaf grows out from the stem).
Plants without sufficient light may also drop their leaves, especially older leaves.
You may find that a variegated plant (leaves that are white and green) may revert to being solid green.
Flowering plants may fail to produce flower buds.
Plants exposed to too much light may result in scorched and bleached leaves.

Enter the PPF or Photon Efficiency

In order to calculate efficiency, coverage and harvest potential we need to use Photosynthetic Photon Flux (PPF) data. If you have a light fixture in mind, look for PPF or Photon Efficiency (PPE) data in the manufacturer’s literature or on their website. Manufacturers of high efficiency lights will usually report this data because it is a selling point. For most grow lights, PPF values are between 100 and 1500 µmol. Photon efficiency values are usually between 1.00 and 3.00 µmol/w.

PPF (Not PPFD)

PPF values are often called “PAR Value” or “Total PAR”. They measure the total amount of Photosynthetically Active Radiation (PAR) that a fixture will produce. This data will be reported as “µmol” or “µmol/s”. You can enter this data in the Grow Light Calculator in the field labeled “µmol”.

Unfortunately, many manufacturers of low-quality lights do not report PPF data. They instead try to manipulate Photosynthetic Photon Flux Density (PPFD) data to convince growers that the lights are more powerful than they are. PPFD is a measure of the density rather than the quantity of photons. The data reported in PAR maps is PPFD data. PPFD values are properly reported as µmol/m²/s. If you see the “m²” in the label, then it is PPFD data which cannot be used in the Grow Light Calculator.

Choosing the right plant for your available light

Before getting a plant or starting seeds, determine the quality and hours of natural light in your space. Then choose plants with light requirements that match your indoor environment.

While a plant may tolerate lower light growing conditions, more light may be required to promote dense foliage and flowering.

Low light

(PPF: 50-150 umol m-2s-1 / 50-250 foot-candles / 10-15 watts)

• A low-light plant would be suitable for a north window or a fairly dark corner.
• Low-light plants require little to no direct light. In their native growing environments, these plants are “understory plants” meaning they grow underneath the branches of larger plants.
• Low lighting is not sufficient for starting seeds indoors.
• In environments with less light, plants grow more slowly and use less water. Avoid overwatering by feeling the soil.

Medium light

(PPF: 150-250 umol m-2s-1 / 250-1,000 foot-candles / 15-20 watts)

• A medium-light plant would be suitable for east-facing windows or located near a west-facing window, but out of direct light.
• You would need artificial lighting for starting seeds in medium light.
• Like the low light plants, these plants will not dry out as quickly. Avoid overwatering by feeling the soil.

High light

(PPF: 250-450 umol m-2s-1 / more than 1,000 foot-candles, more than 20 watts)

• A high-light plant would be suitable for brightly lit locations such as south- or southwest-facing windows.
• You may be able to start seeds without artificial lighting, but seeds that need more time indoors, such as tomatoes and peppers, may become leggy without extra light.
• High-light areas can be warm, making plants dry out faster. Check these plants more frequently and water when soil is dry. 

Using Wattage to Estimate the Optimal Amount of Light

With High Intensity Discharge (HID) lights, such as High-Pressure Sodium (HPS) and Ceramic Metal Halide (CMH) it became common to use wattage to determine how much light you need for your grow tent. With HID lighting, the rule is about 40 watts per square foot (430 watts per square meter). This means that a 600W HID grow light is adequate for a 4’ x 4’ grow tent and a 1000-watt HID light is perfect for a 5’ x 5’ tent.

Wattage was never the correct metric to measure the amount of light. It was a reasonable proxy for HID lights only because all HID lights convert electricity into usable light at about the same rate. However, LED grow lights can now be significantly more efficient than HID lights. The wattage rules no longer apply to most home growers.

LED and HPS Equivalency

High Pressure Sodium (HPS) have traditionally been the preferred HID light for flowering cannabis plants. When LED grow lights came into the market, many manufacturers tried to sell their lights by claiming an “HPS equivalency”. Since HPS lights were traditionally measured in terms of watts, LED lights came to be marketed with an “equivalent wattage”. However, there has never been a standardized way to make these “equivalencies”. Each manufacturer comes up with their own metrics to establish them and most are gross exaggerations. We advise you to completely disregard the manufacturer’s claims about “HPS equivalency” or “equivalent wattage”.

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