What fluorescent bulb is best for my tomato seedlings?

Question

It has been suggested that my seedlings may not be getting enough light. What are the best types of fluorescent lights to get? I have a single bulb, and looking to get another light or two to add to the lighting. I'm not sure where my wife originally got this bulb, but I figured I'd take this chance to get the "best" bulbs for this task.

As a secondary question, do you typically keep your lights on your seedlings 24 hours a day? Or do you try to mimic daylight by turning the lights off at night?

Answer 1

When it comes to growing tomato seedlings, or starting other vegetables indoors from seed the light intensity is more important than the color. Blue light is better for vegetative growth than red light so standard daylight (5000K - 6500k) bulbs work just fine. All the university studies I've read on the subject indicate that they work just as well, sometimes better than more expensive plant grow lights.

I've used daylight fluorescent bulbs to start my vegetable seeds with good results. One thing to keep in mind is that the bulbs dim over time and depending on how long you keep your lights on you should replace the bulbs every season or two to get sufficient light output.

As for how long to leave the lights on, most recommend 12-16 hours a day. I've read accounts from other gardeners that keep their lights on for 24 hours a day and have had dramatically increased yields. If you're mixing daylight and artificial you should time the artificial lights in a way that extends the day and not creating multiple day/night type conditions in a day. Forget where I read it but there was a study that showed doing otherwise could damage the plants.

Personally, I set up my seedlings near a south-facing window so they get natural sunlight but also use daylight fluorescent tubes to provide additional light and I've had good results.

It's important to keep the light as close to the plants as possible. That increases the light intensity. About an inch or two away from the tops of the plants after they've germinated. As the plants grow you'll need to raise the lights. I've used a couple of different solutions to handle raising the light. You may be interested in my DIY plant light stand or even better, this year I built a seed starting bench with adjustable light that I place by a window to get both natural and artificial light. The specialty plant lights are better for plants you plan to grow indoors all the time.

Answer 2

From here:

Shade, in full daylight, is about 10,000 to 25,000 lux (1 lx = 1 lm/m², or 1 lux equals 1 lumen divided by 1 square meter.)

So assuming the lamp will be placed ~18" over the flat, and the luminous efficacy of a typical fluorescence is 50–70 lumens per watt, you will want something in the 250-350 watt range (NOT a 250 watt replacement... which has far less wattage than 250).

Also, designated 'grow lights', although generally far overpriced IME, have a more natural light range for growing plants. The next best are 'daylight' fluorescent bulbs. Also, there are many powerful and good LED grow lights on the market now, and although they are rather pricey for a small operation, they could potentially outlive you, and are energy efficient, and not as hot.

I know seems kind of bright indoors, but it's actually not nearly as intense as direct sunlight. Here's another thread that may be relevant to you:

• Does a luxmeter provide an accurate measurement of usable light?

About turning the lights off at night, that is beneficial, as there are processes that a plant naturally does at night more than in the day, and about all plants are day length sensitive to a degree. I'd say if you can, it'd be a great way to make it more natural for the seedlings.

Answer 3

Lumens is the wrong measure of light for plant growth, plants don't "see" light, they feel it, and what they feel is photosynthetically active radiation (PAR) which drives their metabolism and growth. Fluorescents are an excellent choice for seedlings they have a well balanced spectrum and are very efficient. I would recommend a t5 high output lamp, you can buy one on amazon for about 80.00 dollars for a four foot four tube lamp, which has 4 54 watt bulbs for a total of 216 watts. There are also 2 and 8 bulb lamps in various lengths. The length of daylight I have used in indoor seedlings has either been 18 hours on, or 24 hours on if I want to drive growth. Never less than 18, especially if it's a plant that needs to flower, because you want to be able to induce flowering easily. In spring, if they are going to be placed outside, sync it up with the natural outdoor light cycle.