LEDs are dimmable, yes, just not the way you describe it. They are basically made to blink really fast and the longer they are on vs off the brighter they appear. Putting a resistor in series with them would not only be wasteful but also will make them stop working - they require just the right voltage across the leads to light up. They do make dimmers for LEDs - get one of those. This one looks like it should work and priced reasonably

https://www.amazon.com/Lutron-Creden...dp/B004DZONXI/
(goes up to 100W, so you can control quite a few of those LED grow lights)

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