A grow light can help a flowering houseplant when daylight is too weak, but the extension sources read for this guide give general rules, not settings for bloom. Illinois Extension: Lighting says a duration of 14 to 16 hours is sufficient in low light and that plants should never get more than 16 hours. UF/IFAS Gardening Solutions: Light for Houseplants says fluorescents and LEDs are the best options and that plants need eight or more hours under grow lights. The WSU Extension: Houseplants (PNW Gardeners Handbook) says plants should be illuminated for no more than 16 hours and that LEDs are replacing fluorescent bulbs.
None of those pages gives a distance from lamp to plant, and none gives a flowering-specific schedule. This guide explains how to tell whether light is the problem, how the sources compare lamp types, why hours matter less than most owners expect, and where the evidence stops. For the wider question of whether a lamp can make a plant flower, see Will a Grow Light Make Plants Flower?.
First decide whether light is really the problem
The RHS says flowering houseplants usually need more light than foliage plants, and that in winter plants benefit from full sun when daylight is weak. The sources use two measures of light. Illinois Extension: Lighting gives foot-candles by window, and UF/IFAS Gardening Solutions: Light for Houseplants gives ranges.
| Light level | Illinois Extension | UF/IFAS |
|---|---|---|
| Low | 75 foot-candles; a few feet from a north window | 25 to 100 foot-candles; far from windows or heavy shade |
| Medium | 150 foot-candles; directly in front of a north window | 100 to 500 foot-candles; near windows, no direct sun |
| High or bright | 300 foot-candles; directly in front of an east or west window | 500 to 1,000 foot-candles; near windows with some direct light |
| Direct sun | 1,500 foot-candles; directly in front of a south window | Over 1,000 foot-candles; unshaded south or southwest window |
Illinois adds that outdoor sunlight may reach 10,000 to 12,000 foot-candles, which shows how far a windowsill falls short of outdoors. WSU says a south-facing window gives the most intense light, east and west about 60 per cent of that, and north about 20 per cent. If a plant sits in the low or medium rows and wants high light, a lamp is a reasonable fix. If it sits in the high row and still fails, look at temperature, humidity and watering before buying a lamp.
Lamp types: what the sources say
| Lamp | Illinois Extension | UF/IFAS | WSU Extension |
|---|---|---|---|
| Incandescent | Mostly red, some infrared, low blue; suited to blooming plants | Inefficient, hot and costly to run | Produces a lot of heat; not energy efficient |
| Fluorescent | Cool white is mostly blue and works well for foliage plants | Energy efficient; cool white works well; warm white and daylight may also work | One of three main types |
| LED | Not covered | Costs more upfront, lasts longer, uses less electricity | Replacing fluorescents because of efficiency and light quality |
| Grow-light bulbs | Suggested for blooming plants | Broad-spectrum bulbs available | Not specified |
The pages disagree on incandescent bulbs. Illinois, an older-style page, calls them suited to blooming plants because they are red-rich, and says flowering plants need infrared as well as blue and red light. UF/IFAS and WSU both mark incandescent as hot and wasteful, and UF/IFAS names fluorescents and LEDs as the best options. WSU adds that plants primarily use blue and red light and that standard household bulbs are not optimized for growth. The practical reading is to use a fluorescent or LED unit sold for plants, not an ordinary room bulb.
Hours of light and the dark period
The three pages are consistent on the ceiling: no more than 16 hours. Illinois explains why, saying plants need a rest period too. UF/IFAS gives a floor of eight or more hours for grow lights, and Illinois says 14 to 16 hours is sufficient for plants in low light. A timer is the simplest way to hold a steady day length.
Day length matters for a few flowering houseplants only. WSU says most flowering houseplants do not respond to day length, though light intensity still affects flowering and cooler nights can offset day-length problems. The exceptions it names are poinsettia, which needs at least 12 hours of darkness daily from around mid-September for colour by early December, and the holiday cacti, which bloom with short days and cool nights of 55 to 65°F. Illinois likewise says some plants' flowering is influenced by day length. None of the sources read names jasmine, gardenia, hoya or citrus as day-length plants, and Photoperiod and Houseplant Flowering covers what is established.
Distance and intensity: where the sources go quiet
Illinois does not specify lamp distance, UF/IFAS does not either, and WSU says only that intensity rises as a plant gets closer to the source and falls quickly as distance increases. The University of Minnesota, University of Maryland and University of Missouri lighting guides, which are the usual sources for specific inches and foot-candle targets, either returned errors or could not be opened for this page, so no numbers from them are used. Follow the lamp maker's guidance for distance, and watch the plant: UF/IFAS and Illinois both note that light needs vary by species.
Light is only one of the conditions
WSU says most flowering houseplants do best with daytime temperatures of about 70 to 80°F and night temperatures of about 55 to 60°F. A lamp that heats a plant at night works against that. The RHS adds that heating and draughts matter, advising to keep plants away from radiators and cold draughts. Temperature, humidity and watering are the next checks, which Overwatering or Underwatering: Which Drops Flower Buds? and Raise Humidity for Houseplants cover.
What the sources say for plants on this site
| Plant | Light advice | Source |
|---|---|---|
| Gardenia | Warm, bright spot out of direct sun; insufficient light is a listed cause of bud drop | RHS: Gardenia growing guide and Clemson HGIC: Gardenia |
| Hoya | Failure to flower may be due to insufficient light; do not move once buds form | RHS: Hoya growing guide |
| Paperwhites | Sunny, cool location; too little light makes stalks tall and floppy | UF/IFAS: Paperwhite Narcissus |
| Citrus | At least 6 hours direct sun; ideally 8 to 12 hours of light, especially for fruit; supplemental lighting may be needed | Illinois Extension: Growing citrus indoors |
| Stephanotis | Full sun; 6 or more hours direct in summer growth | NC State: Stephanotis floribunda |
The Illinois citrus page is the only one of these that names supplemental lighting for a particular fragrant plant. The gardenia pages describe filtered light and do not suggest a lamp.
What the sources do not say
No opened page gives a lamp distance, a foot-candle target for flowering, or a recommendation of a specific spectrum for scented plants. They also do not test lamps against bloom. A lamp can make a plant grow in a dim room, but none of the sources says it makes a plant flower on its own.
Frequently asked questions
How many hours should a grow light be on for flowering plants?
Illinois says 14 to 16 hours is sufficient in low light and that plants should never get more than 16. UF/IFAS says eight or more hours.
Which bulb is best for flowering houseplants?
UF/IFAS names fluorescents and LEDs as the best options. Illinois suggests special grow-light bulbs for blooming plants.
Do flowering houseplants need darkness?
Yes. Illinois and WSU both cap light at 16 hours so plants get a rest period.
Will a grow light make my plant bloom?
The sources say flowering needs enough light and, for a few species, the right day length. They do not say a lamp alone triggers bloom.
Sources read for this page
- Illinois Extension: Lighting
- UF/IFAS Gardening Solutions: Light for Houseplants
- WSU Extension: Houseplants (PNW Gardeners Handbook)
- RHS: How to grow houseplants
- RHS: Gardenia growing guide
- Clemson HGIC: Gardenia
- RHS: Hoya growing guide
- UF/IFAS: Paperwhite Narcissus
- Illinois Extension: Growing citrus indoors
- NC State: Stephanotis floribunda
Last reviewed 25 October 2026. Written by Md. Firozur Rahman. This page gives general guidance and cannot diagnose a plant from a description; see the Disclaimer. Browse more in Getting Blooms Indoors.
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