Tuberous Begonias in June

Although many plants can be started or overwintered under lights, getting them to flower, thrive, and continue blooming for many months entirely with artificial illumination is a greater challenge. After years of experimenting, here‘s what works for me.

Step 1: Space and Equipment

Cool indoor space

-4-bulb fluorescent light system with 48-inch T5-HO tubes; ViaVolt® T5-44, 4' - 4 Lamp, Atlantis Hydroponics and others

-2 blue (6500K) and 2 red (3000K) fluorescent bulbs; Atlantis Hydroponics and others

-2 narrow-beam halogen spotlights such as Plusrite PAR-3498F; 1000 Bulbs

-2 clamp-on light bulb holders, standard base


-tuberous begonia bulbs

-Rieger or hiemalis begonia hybrid plants

-Streptocarpus plants

-Other small, long-blooming plants

-standard gardening supplies

Step 2: The Lights

A nice figure at https://www.comsol.com/blogs/calculating-the-emis... shows the output of the most common light sources across the visible spectrum. Plants especially like red and near-infrared— of which, compared to sunlight, ordinary fluorescents and white LEDs generally produce too little. Far red is said to be critical for the flowering of many plants, but tends to be omitted from these newer sources because anything outside the visible spectrum is considered wasteful. For now, specialized LED grow-light systems that might contain more red and far red are still very expensive. (Many of them seem designed primarily for growing cannabis.) In contrast, traditional incandescent and halogen bulbs give off far too much red and infrared outside the range of human vision.

So for now I use halogen lights to supplement a 4-bulb fluorescent system with 48-inch T5-HO tubes. The latter are more expensive than regular fluorescents, but the price is coming down, and they produce twice the light per watt. The higher intensities allow a greater spacing between the lights and the plants, which is aesthetically pleasing. I use 2 blue (6500K) and 2 red (3000K).* Some plants, such as young streptocarpus, should be a good 2 feet from the T5-HO’s to avoid scalding.

For the deep red and near-infrared light that blooming lots of flowers requires, the halogen bulbs are a little more efficient than the rapidly-disappearing regular incandescents. To make them less wasteful, and avoid spraying their light all over the room, get the narrow-beam halogen spots—10 degrees or at most 30 degrees. I put a bulb rated 38-39 watts at each end of the setup. Everything stays on for 15 hours a day.


*Please do not start one of those long dogmatic arguments about which fluorescent light temperatures are best. With the halogens I haven’t found it matters that much.

Step 3: The Temperature

Streptocarpus in January

It’s good that winter is cold. T5-HO fluorescents and halogens still generate a high ratio of heat to light. To offset that they work best in spaces that might be a bit chilly for humans. I use the basement, which remains between 60 and 70 degrees year-round. Here in Atlanta, where summers are long, the plants I like to bloom indoors include those that would die outside in the Georgia heat. If your growing space stays at even 75 degrees or more, the heat of artificial lights could be hard on many blooming plants, especially streptocarpus and tuberous begonias. Hybrid begonias and miniature roses might do better.

Step 4: The Plants

Since tuberous begonia bulbs can only be ordered in the spring, and I have been too lazy to raise them from seed, these grow in my basement in the summer. So far they have all gone dormant by Thanksgiving. With proper care they can be resurrected, but I have found that new ones will start sooner the next year. The Amerihybrids have done well for me, although many others might also. The new intergeneric tuberous begonia hybrids are a bit more heat tolerant and seem especially promising. Almost effortless during winter are the Rieger and hiemalis hybrid begonias, which can be bought cheaply at many garden centers and supermarkets from fall to spring. Look for the ones with the biggest flowers.

Streptocarpus also prefer long cool days, but they can be mail-ordered in the fall, and grown right through the winter.

My basement is a lonely and messy place, so on a regular basis the plants rotate upstairs to visit us in the den and bedroom. These are relatively hostile environments, and probably the flowers would be more luxurious if they stayed under the lights forever. But then, what would be the point?

Step 5: Pests

Top: Intimidating the Orchids

Bottom: Last Begonia at Thanksgiving

After years of struggle with assorted mealybugs,scales, poisons, gloves, respirators, etc., I made a surprising discovery: 10 minutes after spraying them with soap solutions, the bugs wash right off, and it takes them a long time to come back. Pick the plants up and turn them over to spray the undersides. Fungi have not been a problem. Warm lights in cool air keep the humidity low; guard instead against plants drying out too fast. Fertilizing: do it.

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