Seedling Growth: Optimum Temperature?

Hello, everyone. I’ve got another of those “newbie” questions for you: what is the optimum temperature for seedling growth (not germination)?

Here’s why I am asking:

In previous years, and until just recently this year, I’ve grown few enough seedlings that I could do it in my kitchen and dining room. This year, however, I’ve run out of room. My husband is putting together a “seedling station” for me right now, where I’ll have oodles of seedling shelf space with lots of grow lights.

Do I want it in my dining room, in front of a nice big bay window, or do I want it in my garage, which is heated but not as warm as the house? The garage appears to be in the low 60’s or upper 50’s (I just put a thermometer in there, but it’s still going down from the house reading).

About a week ago, I put my first batch of germinated seeds out in the garage, under grow lights. The results have not been encouraging. For one thing, I don’t see them to remember to water as well. That might be the biggest thing.

Should I start them in the house and then, once they have a set of true leaves, move them to the garage area? Or should I use only the dining room w/ window and lights or only the garage (cooler than the house but still nice)?

Your advice is appreciated. I don’t want to kill seedlings if you all have some information for me in place of learning the hard way!

And while I’m at it: what about lighting requirements? I believe some of you have your lights on 24 hours a day? I don’t care how long I leave them on, except that I thought plants need a “dark” period each day, where they use the sunlight they’ve processed all day to grow. Am I mis-informed? If 24 hours a day grows the plants better than 12 or 16, I want to do that because I want these guys to have a chance against next winter when I plant them outside. But if keeping the lights on for too long a period was actually counter-productive, I don’t want to do that, either.


Keep the temperature above 60 F, I think, and below 75. One year I tried to grow mine in the garage (cooler than 60), and even with plastic curtains on the lights to hold the heat from the lights, the seedlings just didn’t grow well.

I have used a timer set on 16 hours, and I’ve kept the lights on 24 hrs. The 24-hr lighting gets the seedlings out faster. I can’t tell that having the lights on all the time makes the seedlings less healthy, so I leave the lights on 24 hrs. The sooner I get the seedlings to a point where I can decide whether I’m interested in them, the sooner I have space for other seedlings.


Thanks, Peter! The garage temp is somewhere between 60.5 and 64.8, except for when my husband opens the other door to pull his car in or out. Then it cools into the 50’s but re-warms within 90 minutes or so.

I did go poke around at the seeds I’d potted up and discovered that most ARE growing, just hadn’t popped up above ground yet. I’m still debating starting them inside and then moving the better established seedlings down to the garage.


See the tables and look at what is recommended for the photoperiod for roses.


Below my name are the relevant parts of the document Henry most thoughtfully provided.

Since the light output of a 2-tube fluorescent fixture with 40-watt tubes is, by my calculation, approximately 560 footcandles (at the center of the fixture, less at the ends), it seems that 24-hour lighting is what provides the best growth (shown by what is recommended for growing cut roses). More tubes will produce more light and probably better results to a certain point.

What is recommended for production of cuttings from miniature rose stock plants–300-500 footcandles for 18-24 hrs–does not go against the 24-hr recommendation. The first excerpt suggests that 24 hr lighting will increase production and quality.

I myself have shelves with 2-tube 4-ft fixtures from Lithonia, and consider the light barely adequate. The tubes lose output as they age, but I’m not sure how much the output decreases over a year, or when the low point is reached, or whether the light production stabilizes once that point is reached. There is probably a point of trade-off between the cost of new tubes and the cost of inefficiency of old tubes, but I don’t know what that is. Perhaps someone else can find and provide that information.

At one time a mixture of one warm white and one cool white was thought to give the best light for the least cost, but cool white generally gives OK results. Make sure that you get the 40 watt tubes, since the 34 watt tubes save electricity but give much less light. Also check the rated light output of the tubes you get. Not all cool white tubes give the same amount of light. Probably the same is true of warm white, but there are not as many different kinds of warm white tubes available. Avoid “cool white deluxe” because it gives much less light.

Grow lights also are not created equal. I don’t know what is available in these right now, but in the mid 1970s the choice was mostly between Gro-Lux (Sylvania), Vita-Lite (by Duro-test), and Agro-Lite (Westinghouse). Gro-Lux gives off a purplish light, DuraLite had a natural spectrum, and AgroLite had a pinkish light. I had a chance to get a case of the Agro-Lites for a good price, and found them to be somewhat better than the other two, but in my experience (and according to what I’ve read) none of the special fluorescent grow lights is worth the additional cost. The most economical way to go is with cool white or the cool white/warm white mix. The HID and metal halide lamps and such are beyond my experience. Maybe someone else can shed (dare I say this?) some light on the value of these lights.

Tubes last better in some fixtures than in others, and maybe they also give better light with some fixtures. I had bad experiences with 2 Lighting Corporation of America fixtures–tubes failed in about 6 months with one fixture–and with its replacement under warranty when the first fixture failed.


from “Supplemental Lighting of Greenhouse Crops” by Jim Tsujita at

Duration of lighting: The length of lighting per day is influenced by other factors than photoperiod requirements. Most rose cultivars, for instance, can be lighted 24 hours per day to increase production and quality without undue effects.

Most growers in North America are using an illumination range of 300 to 600 foot-candles or approximately 37 to 75 micro-moles per second per meter photosynthetically active radiation (PAR).

In roses . . . lighting for 12 hours at 800 fc did not accumulate as much net carbon as 24 hours at 400 fc.

Miniature rose stock plants - 300 to 500 fc/18 to 24 hours

Roses (cut) 300 - 800 footcandles continuous

Um… Short answer, Alicia, is… well… do you do any living in your living room?? If not, use the supplemental natural light and (assuming you don’t heat the room obscenely) the warmth to accelerate growth. Your heat may make the room exceptionally dry, so be conscious of that. Methinks you figured that out already, but were hoping for a reason not to monopolize the living room.

Some plants, whose bloom and growth cycles are regulated by day length, should not be left under light 24 hours at certain stages of life, but roses, it is my impression, aren’t amongst them. (I don’t know if recommendations for species roses differ…) Begonia seedlings, for instance, will attempt to bloom too early and commit floral hari-kari (sp?).

(Henry, I’ll read your article when I’m more awake. It sounds like it might be interesting!)

You may want to check out the link I’ve attached. If the link doesn’t work it’s:

One thing I recall from this article is that flourescents

lose the output exponentially. So this recommends you replace the lights annually. I’m too cheap for that, but

I do replace them every two years.