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Growing under Lights

When do you need to change your grow light bulbs?

Rick (zone 6b, MA)

3 years ago

last modified: 3 years ago

Back in 2012 I bought two Sunlite Garden T5 grow light systems from Gardeners Supply Company to germinate flower seeds and keep the seedlings going until I set them outside. Overall I've been pretty happy with the setup. But I just heard somebody mention that grow light bulbs eventually lose their efficiency and power which is something I honestly never thought about. So now I’m wondering if I should think about replacing mine. I run them 15 hours a day February through April. I have ten of these bulbs each 45” long, so replacing them is not something I would take lightly. What do you use as an indicator that your bulbs are weakening? I do think the seedlings on the edge of the shelf underneath the lights are stretching towards it, and maybe they didn’t do that before.

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Comments (4)

zen_man
3 years ago
last modified: 3 years ago
Hi Rick,

I have the older T8 fluorescent fixtures, and I don't change bulbs in them until they burn out completely, or in rare cases when they start flickering. As long as they are putting out a normal amount of light I keep using them. They are rated for 20,000 hours, so that is a long time. It is not unusual for one of my fluorescent ballasts to burn out before the bulbs burn out. I purchased my beginning light fixtures back in 2005, and many of them are still going fine.

I have made the "overdrive" modification to some of my fixtures, and I am continuing to convert some of my fixtures to the overdriven mode. Over-driving probably does shorten the bulb life, but apparently not seriously, and it nearly doubles the light brightness. I learned to overdrive fluorescent fixtures from the user Zink here in GardenWeb many years ago.

ZM
Rick (zone 6b, MA)
Original Author
3 years ago
Thanks, zen_man. I figured I’ve run my lights about 100 days a year, from February through early May, 15 hours a day so 1500 hours a year. I’ve had it about 9 years so I’m saying 13,500 hours and I’m reading that the typical length of time a T5 bulbs lasts is 10,000-20,000 hours.

The reason I’m concerned is that I’m seeing seedlings located just inches to the side of the bulbs lean diagonally up towards the bulb which is situated only 2” from the soil surface. And one of these looks like it’s getting etiolated already, though it’s hard to tell what’s going on considering I have only cotyledons. These are all seeds of Delphinium cultivars. Any thoughts would be appreciated.
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zen_man
3 years ago
last modified: 3 years ago
Hello Rick,

" The reason I’m concerned is that I’m seeing seedlings located just inches to the side of the bulbs lean diagonally up towards the bulb which is situated only 2” from the soil surface. And one of these looks like it’s getting etiolated already, "

You may actually have your lights too close to the soil surface. Plants do grow toward the light, and that should be at least approximately upward. I place my T8 shoplights as close together as they will go, with their reflectors touching each other. I get four shoplights over a 2-foot by 4-foot shelf. Those eight bulbs approximate a 2' x 4' rectangular planar light source. Click on the photo to see the larger version.

I recommend the book, Gardening Under Lights: The Complete Guide for Indoor Growers

(that is a clickable link) as a source of reasonably up-to-date information. On page 122, in the paragraph titled Distance from Light, Leslie Halleck says "The light source must be very close to your seedlings as they germinate, as near as 3 or 4 inches from the seeds as they emerge from the soil. If the light is further away, young sprouts can quickly stretch beyond their ability to remain intact."

So, at 2 inches, I think you may actually have the lights too close to the soil surface. And you may not have enough tubes. If a seedling can "see" two different tubes, it may not be tempted to grow toward either of them. If a seedling can "see" only one tube, it is going to grow directly toward it.

ZM (not associated with any product or vendor mentioned or linked)
Rick (zone 6b, MA) thanked zen_man
Rick (zone 6b, MA)
Original Author
3 years ago
Thanks, ZM. I just got myself a copy of that book so thanks for the recommendation. As a matter of fact, it was Leslie Halleck who was the person I’d heard interviewed and she mentioned that bulbs do gradually lose their intensity over time. I did get a set of 10 new 54-watt T5 bulbs for $60 total. Maybe I’ll increase the distance to 3” with the new bulbs and see what happens.

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