Return to the Growing under Lights Forum | Post a Follow-Up

When do I replace T8's used for lettuce

Posted by fred9x z5ct (My Page) on
Sat, Oct 5, 13 at 15:00

I grow lettuce year round under T 8's. Two years ago I put in Sylvania Cool White, FO32/741/ECO. Box says 28,000 lummens and 24,000 hours. Also says 78 CRI and 4100K.

I estimate they have been on about 11,000 hours (365 x2x 15 hours.
Should I replace them now? If so, with same or similar from HD or Lowe's?
Thank you.

Follow-Up Postings:

RE: When do I replace T8's used for lettuce

It's up to you. Some people change their bulbs every year to have the optimum performance as the bulb's output diminishes with time. Are you not getting the results you did when the bulbs were new?

24000 hours is just an estimate for how long the bulb should last before burning out

RE: When do I replace T8's used for lettuce

I would use them until they start flickering.

RE: When do I replace T8's used for lettuce

Results seem the same so I'll save my money for now.
Thank you both.

RE: When do I replace T8's used for lettuce

lifespan is also determined how often they are turned off and on. if you are running on for 16 hours off for 8 hours, they will have more than 24000 hours of life. also, the website should show maintained lumens. you are using a pretty wimpy t8 lamp: I use lamps that are rated at 32,000 lumens - so when you replace them, get yourself some XHL lumen lamps or other similar high output, just search on the internet, and you will see that many companies produce very high output t8 that will grow lettuce faster. only do this according to your budget - pm.

RE: When do I replace T8's used for lettuce

Thanks overdrive,

How often do you replace them?

RE: When do I replace T8's used for lettuce

Hi Fred,

Manufacturers of fluorescent lights usually report two lumen ratings, 'Initial' and 'Means'. Your bulbs are rated at 2800 initial lumens (not 28000) :-) and 2520 means. The means rating is always based on the lumens tested at 40% of the tube's rated hours of life. So, if your bulb is rated at 24000 hours, then the manufacturer is saying that when the bulb is new, it will put out 2800 lumens. When it has been used for 9600 hours (40% of 24000), it should put out about 2520 lumens.

Your bulbs have been in use for 11000 hours so, theoretically, the light output is probably somewhere around 2480 lumens (instead of the initial 2800). That's about 88.5 percent of the original light output. If you don't mind losing a little energy efficiency, that's not really too bad.

I use a light meter and usually replace my T8 fluorescent bulbs when they get down to about 85% of their initial lumens. Not sure if that's right or wrong, it's just what I've always done :-)

Hope something here is helpful,


This post was edited by art33 on Fri, Nov 8, 13 at 16:55

RE: When do I replace T8's used for lettuce


Very helpful! Just what I was looking for. I did buy a box of 30 of the same bulbs at HD and I'll install them on a cabin fever day in a few months.

Thank you,

RE: When do I replace T8's used for lettuce


You're very welcome, glad I could help. Keep in mind that by reporting the mean rating of their bulbs, the manufacturers are not necessarily suggesting that you replace the bulbs at that point. They're simply giving you an idea of how to estimate the lumen depreciation of that bulb.

Personally, I've never heard of a 'proper time' to replace a fluorescent bulb :-) I think its kind of like slimak said, "it's up to you". If your lettuce is growing very well with the old bulbs, you might want to wait awhile before replacing the bulbs.

While on the subject of growing lettuce. I was just wondering about the 4100K color temperature of your bulbs. I know that blue light is supposed to induce vegetative growth and the red part of the spectrum tends to induce flowering. Would you be better off using bulbs rated at 5000K or above, which would provide more blue light to the lettuce seedlings? Just wondering, as I said I'm not a lettuce grower :-)


 o Post a Follow-Up

Please Note: Only registered members are able to post messages to this forum.

    If you are a member, please log in.

    If you aren't yet a member, join now!

Return to the Growing under Lights Forum

Information about Posting

  • You must be logged in to post a message. Once you are logged in, a posting window will appear at the bottom of the messages. If you are not a member, please register for an account.
  • Please review our Rules of Play before posting.
  • Posting is a two-step process. Once you have composed your message, you will be taken to the preview page. You will then have a chance to review your post, make changes and upload photos.
  • After posting your message, you may need to refresh the forum page in order to see it.
  • Before posting copyrighted material, please read about Copyright and Fair Use.
  • We have a strict no-advertising policy!
  • If you would like to practice posting or uploading photos, please visit our Test forum.
  • If you need assistance, please Contact Us and we will be happy to help.

Learn more about in-text links on this page here