Growing Streptocarpus: Light Conditions

Streptocarpus are becoming popular houseplants and for good reason. They are relatively easy to grow, they don’t need high light and they bloom all year long in the right conditions.

How much light do they need?

Light Requirements for Streptocarpus

The common advice when I started growing streptocarpus (streps) was to provide the same amount of light as for African violets. That did not help a lot because I never grew African violets – that was my mom’s thing. The consensus was that they liked fairly low light and no direct sun exposure.

Purdue University confirms that streptocarpus prefers more light than African violets. A report in Introduction to Floriculture, suggests “Streptocarpus plants will produce flowers if they have a light intensity of about 12,900 lux, slightly higher than that for African violets. With sufficient light, hybrids bloom throughout the year.”

Streptocarpus will grow in lower light, but they grow better and bloom more in higher light. But what is the ideal level? That is not really clear and I have found no scientific studies looking at this. Green Fuse® Botanicals recommends 1,000 to 1,500 foot-candles.

Here are some rules to use.

• If they are not flowering, give them more light.
• If the leaves are burning, reduce the light. This will probably only happen in direct sunlight.
• They can take direct sun. They don’t do well with a lot of sun outside, but a bit through a window won’t harm them.
• They will flower at 2,500 lux producing leaves that are thin and large.
• They flower better at 16,000 lux. Their leaves will be smaller and have more texture (i.e. more bumps)

(2,500 lux = 250 foot-candles = 38 PPFD, 16,000 lux = 1,660 foot-candles = 250 PPFD)

The experiments below provide details about some experiments I have carried out to confirm these growing conditions.

PPFD and Foot-candles

The best unit for measuring light for plants is the PPFD but few gardeners have a PAR meter that can measure it. Instead, they use meters measuring lux or foot-candles. The conversion from one of these units to the other is approximate because the conversion factor depends on the type of light being used, but the following will give approximate values that are close enough for home gardening.

Foot-candles X 0.15 (cool white light factor) = PPFD value (example: 700 fc = 105 PPFD)

Lux X 0.1 = foot-candles (example: 2,500 lux = 250 foot-candles)

Daily Light Integral

The above discussion only considered the intensity of light and ignores the duration of light. Providing light to plants is similar to watering. You can water correctly using a trickle of water over a long period of time, or by using a faster flow rate over a shorter period of time. The same applies to light.

You can compensate for a lower intensity of light by leaving the lights on for a longer period of time. There is a limit to this. Plants do need a dark period every 24 hours and this should be at least 10 hours.

The DLI or daily light integral is a way of measuring the cumulative amount of light (photosynthetically active radiation, PAR) a plant experiences in a day. Think of DLI as a rain gauge for light. It accumulates the amount over a 24 hour period and the DLI is that total amount.

In natural daylight, less light is collected in the morning and evening while most is collected at noon. Under artificial lights, plants receive a constant amount of light while the lights are on. The DLI is easily calculated for artificial lights using this equation:

DLI = 0.0036 x PPFD x duration (in hours) (example: a PPFD of 800 for 12 hours = DLI of 34.6)

Casual Observations

I have been growing streptocarpus under lights for several years and use proper LED grow lights that can put out a lot of light. Some of my lights include BESTVA 4000, Spider Farmer SE3000, and Vevor 200, although these are turned down from their maximum output. The following observations are for a variety of complex hybrids including my cultivars.

The lights are about 2 feet above the plants but the overflow plants are farther away from the light, or even on the floor of the plant room, as pictured below. They receive between 2,500 and 16,000 lux, with most of the plants receiving around 6,000 lux.

At 2,500 lux the leaves are larger, thinner, and flat. The surface is quite smooth with little texture except for the veins. The edges of the leaves are also smooth.

At 16,000 lux the leaves grow quite differently. They are shorter and tougher. The surface becomes quite bumpy and textured. The edges undulate and the leaves start to curl under themselves.

Light Requirements for African Violets

Since there is little scientific data about the light requirements for streptocarpus I decided to look at the data for African violets as a proxy. The following recommended values should be considered minimum values because streptocarpus prefers higher light values than African violets.

Light Intensity

The following data is from sources that reported lux or foot-candle values for growing African violets.

Recommended intensity lux (foot-candle)	Source
5,000 – 10,000 (500 – 1,000)	Akbarian B, et al
10,000 – 12,000 (1,000 – 1,200)	African Violet Production Guide
3,000 – 5,000 (300 – 500)	DR. Minh Bui (AVSA)
5,000 – 30,000 (500 – 3,000)	Growing African Violets

The average recommended light intensity for growing African violets is 10,000 lux. This should be considered the minimum value for growing streptocarpus.

DLI Values

The following data is from sources that reported DLI values for growing African violets.

Recommended DLI (Moles/Day)	Source
4 – 10	Purdue University
4 – 6	Daily Light Integral Defined
4 – 14	Plant Light Database
6 – 10	Ball Red Book

The average recommended DLI for African violets is 7. This should be considered the minimum value for growing streptocarpus. How does that compare to the 10,000 lux value (150 PPFD) mentioned above?

DLI = 0.0036 X 150 X 12 = 6.5

Given the errors inherent in the above data, the 6.5 value is surprisingly close to 7. Based on this data, streptocarpus should grow well at or above these values:

• A DLI of over 7
• 10,000 lux for 12 hours
• 1,000 foot-candles for 12 hours

Note that experiment #1 below demonstrates that spreptocarpus will grow and bloom with a DLI between 2 and 11. At the low end, leaves are larger and thinner, plants grow more slowly and flower less.

Experiment #1: Effect of Light Intensity on Leaves

To test the effect of light intensity on leaves I took two numbered seedlings and divided each into equal-sized plants. Each was potted in my standard media which is about 40% perlite and 60% Promix (peat-based media). This report describes the leaves of one of the seedlings.

One of each plant was placed in low light (2,500 lux) and the other in high light (16,000). The light was mostly from the same source. The high light was right under the center of a Spider Farmer SE3000, about 2 feet from the light. The low-light plants were placed on the floor beside the normal tray of plants. The ones on the floor got light from the spider farmer as well as a small amount of light from LED shop lights on the ceiling.

The lights were on for 13 hours and the DLI for the two locations is 1.8 and 11.3.

Low light produces leaves that are thinner, larger, and less textured both on the surface and along the edge of the leaf. High light produces thicker leaves with a lot more texture.

Although not evident from these pictures, leaves grown in high light also curl more.

Flower coloration can also be affected by the light level.

Growing Streptocarpus

Other resources for growing streptocarpus:

Article: An Introduction to Streptocarpus

Article: Streptocarpus Myths

Collection of my videos about growing streptocarpus