I don't know if they all have the same spectrum to start with but they do shift red over time. I would guess they should work well enough either way. I have a 400 watt metal halide that was originally a warehouse light, it came with a bulb of unknown age and my plants (primarily bamboo and veggies) grow very well under it.

IMO Metal Halide lamps are ridiculous for indoor plant propagation and crops. Sure there's many people that will tout it as a
plant lamp but in the final analysis it is the 6500 Kelvin color temperature that gets the work done.

I did some research on various MH bulbs and I have come to the conclusion that the Kelvin Temp and the Lumens Rating seem to be the most important thing to consider when purchasing a bulb for growing seedlings. (vegetable and flowers ).

The bulbs I was looking for replacements for were 250W brand name bulbs sold specifically by Hydrofarm for "growing plants indoors ". These bulbs sold for around $30.00 and up. Initial Lumens: 21000 - Kelvin Temperature: 4200.

I compared the Kevlin Temp and Lumen output of these bulbs with bulbs sold as "warehouse lighting HD bulbs" from Lightingsupply.com. They were rated at: Initial Lumens: 21000 - Kelvin Temperature: 4000. These bulbs sold for $14,00. I am going to use those bulbs this year.

I would like to note that last year I was still using the Hydrofarm MH 250W bulbs. I figure they had about 18000 hours of use. Based on my visual comparison with a new bulb they were putting out about 40% less "light". Even with diminished "light" I had no problem growing onions,three kinds of peppers, tomatoes, petunias, marigolds, salvia, coneflowers, sunflowers and gailardia. I started over 200 plants in all.

I had the lights positioned 24" from the top of the plants and kept raising the lights to maintain that height as the plants grew. The tomatoes ( 18" tall ) and peppers ( 10" tall ) blossomed. I began to get actual peppers. The sunflowers grew to 18" in height. The petunias and marigolds began to blossom. I had to trim the onion stalks so they wouldn't fall over.

But it is also important to note that I start all my plants in plug trays with domes. I get a germination rate of 70% to 90% depending on the plant. The back and two sides of the interior of the growing area is covered with reflective mylar. My grow lights are on 16 hours a day.When the lights are on the temperature is always between 70 and 75 degrees. At "night" temperature drops to 65 to 68 degrees. I purchase new seeds every year, I never use last years seeds. I use Miraclegrow seed starting mix in the plug trays and when the plants are ready I transplanted them into larger containers using "Happy Frog Potting Soil". Using this method I spend about $22.00 on seed starting mix and potting soil for over 200 plants.

In my opinion to do anything else to save a few bucks or try to "reinvent the wheel" by experimenting with my own soil mixtures could create "weak links in the chain".

In essence it's not just the lights...............

Gregg

Lumens are a measure of human perception of light intensity. To meet architectural codes, lumens are required to be on the label of all bulbs sold in the U.S. Plants do not have eyes. The input measure of interest is spectral energy, measured in Watts.

For many crops, the spectral response curve for photo synthesis is very close to 6400-6500 Kelvin blackbody temperature and the optimal intensity is 50-75 Watts/sq.ft.