[Raymond S.]

Depending on who's chart you look at, the 420 nm bulb is in the range of plant use.
On one chart I recently saw, it's better for chlorophil A than the 460 nm.
That having been said, what bulb did you have in there before it ?

 

[Raymond S.]

Anyone feel free to ammend this.
In a "blue" or "red" bulb all the energy is being used to provide light in a spectrum which plants can use best.
Other bulbs often caled "white light bulbs" use energy to provide a wide spectrum of light most of which is a bit harder for the plants to use, except for any blue or red part of them.
I have seen pictures of plant farms where there is no light except red bulbs.